51
|
Warshel A, Sharma PK, Kato M, Xiang Y, Liu H, Olsson MHM. Electrostatic basis for enzyme catalysis. Chem Rev 2007; 106:3210-35. [PMID: 16895325 DOI: 10.1021/cr0503106] [Citation(s) in RCA: 966] [Impact Index Per Article: 56.8] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Arieh Warshel
- Department of Chemistry, University of Southern California, SGM Building 418, 3620 McClintock Avenue, Los Angeles, California 90089-1062, USA.
| | | | | | | | | | | |
Collapse
|
52
|
Dybala-Defratyka A, Paneth P, Banerjee R, Truhlar DG. Coupling of hydrogenic tunneling to active-site motion in the hydrogen radical transfer catalyzed by a coenzyme B12-dependent mutase. Proc Natl Acad Sci U S A 2007; 104:10774-9. [PMID: 17581872 PMCID: PMC1904141 DOI: 10.1073/pnas.0702188104] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Hydrogen transfer reactions catalyzed by coenzyme B(12)-dependent methylmalonyl-CoA mutase have very large kinetic isotope effects, indicating that they proceed by a highly quantal tunneling mechanism. We explain the kinetic isotope effect by using a combined quantum mechanical/molecular mechanical potential and semiclassical quantum dynamics calculations. Multidimensional tunneling increases the magnitude of the calculated intrinsic hydrogen kinetic isotope effect by a factor of 3.6 from 14 to 51, in excellent agreement with experimental results. These calculations confirm that tunneling contributions can be large enough to explain even a kinetic isotope effect >50, not because the barrier is unusually thin but because corner-cutting tunneling decreases the distance over which the system tunnels without a comparable increase in either the effective potential barrier or the effective mass for tunneling.
Collapse
Affiliation(s)
- Agnieszka Dybala-Defratyka
- *Institute of Applied Radiation Chemistry, Department of Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland
- To whom correspondence may be addressed. E-mail: or
| | - Piotr Paneth
- *Institute of Applied Radiation Chemistry, Department of Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland
| | - Ruma Banerjee
- Department of Biochemistry, University of Nebraska, Lincoln, NE 68588-0664; and
| | - Donald G. Truhlar
- Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455-0431
- To whom correspondence may be addressed. E-mail: or
| |
Collapse
|
53
|
Sharma PK, Chu ZT, Olsson MHM, Warshel A. A new paradigm for electrostatic catalysis of radical reactions in vitamin B12 enzymes. Proc Natl Acad Sci U S A 2007; 104:9661-6. [PMID: 17517615 PMCID: PMC1887576 DOI: 10.1073/pnas.0702238104] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2007] [Indexed: 11/18/2022] Open
Abstract
The catalytic power of enzymes containing coenzyme B(12) cofactor has been, in some respects, the "last bastion" for the strain hypothesis. The present work explores the origin of this effect by using simulation methods that overcome the sampling difficulties of previous energy minimization studies. It is found that the major part of the catalytic effect is due to the electrostatic interaction between the ribose and the protein, and that the strain contribution is very small. Remarkably, enzymes can use electrostatic effects even in a radical process, when the charge distribution of the reacting fragments does not change significantly during the reaction. Electrostatic catalysis can, in such cases, be obtained by attaching a polar group to the leaving fragment and designing an active site that interacts more strongly with this group in the product state than in the reactant state. The finding that evolution had to use this trick provides further evidence to the observation that it is extremely hard to catalyze enzymatic reactions by nonelectrostatic factors. The trick used by B(12) enzymes may, in fact, be a very powerful new strategy in enzyme design.
Collapse
Affiliation(s)
- Pankaz K. Sharma
- Department of Chemistry, University of Southern California, 418 SGM Building, 3620 McClintock Avenue, Los Angeles, CA 90089-1062
| | - Zhen T. Chu
- Department of Chemistry, University of Southern California, 418 SGM Building, 3620 McClintock Avenue, Los Angeles, CA 90089-1062
| | - Mats H. M. Olsson
- Department of Chemistry, University of Southern California, 418 SGM Building, 3620 McClintock Avenue, Los Angeles, CA 90089-1062
| | - Arieh Warshel
- Department of Chemistry, University of Southern California, 418 SGM Building, 3620 McClintock Avenue, Los Angeles, CA 90089-1062
| |
Collapse
|
54
|
Harris DA, Stickrath AB, Carroll EC, Sension RJ. Influence of Environment on the Electronic Structure of Cob(III)alamins: Time-Resolved Absorption Studies of the S1 State Spectrum and Dynamics. J Am Chem Soc 2007; 129:7578-85. [PMID: 17530754 DOI: 10.1021/ja066197y] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate state populated following excitation of cob(III)alamin (Cbl(III)) compounds. This state is sensitive both to axial ligation and to solvent polarity. The excited-state lifetime as a function of temperature and solvent environment is used to separate the dynamic and electrostatic influence of the solvent. Two distinct types of excited states are identified, both assigned to pi3d configurations. The spectra of both types of excited states are characterized by a red absorption band (ca. 600 nm) assigned to Co 3d --> 3d or Co 3d --> corrin pi* transitions and by visible absorption bands similar to the corrin pi-->pi* transitions observed for ground state Cbl(III) compounds. The excited state observed following excitation of nonalkyl Cbl(III) compounds has an excited-state spectrum characteristic of Cbl(III) molecules with a weakened bond to the axial ligand (Type I). A similar excited-state spectrum is observed for adenosylcobalamin (AdoCbl) in water and ethylene glycol. The excited-state spectrum of methyl, ethyl, and n-propylcobalamin is characteristic of a Cbl(III) species with a sigma-donating alkyl anion ligand (Type II). This Type II excited-state spectrum is also observed for AdoCbl bound to glutamate mutase. The results are discussed in the context of theoretical calculations of Cbl(III) species reported in the literature and highlight the need for additional calculations exploring the influence of the alkyl ligand on the electronic structure of cobalamins.
Collapse
Affiliation(s)
- D Ahmasi Harris
- FOCUS Center, Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, USA
| | | | | | | |
Collapse
|
55
|
Cheng MC, Marsh ENG. Evidence for coupled motion and hydrogen tunneling of the reaction catalyzed by glutamate mutase. Biochemistry 2007; 46:883-9. [PMID: 17223710 PMCID: PMC2518312 DOI: 10.1021/bi0616908] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Glutamate mutase is one of a group of adenosylcobalamin-dependent enzymes that catalyze unusual isomerizations that proceed through organic radical intermediates generated by homolytic fission of the coenzyme's unique cobalt-carbon bond. These enzymes are part of a larger family of enzymes that catalyze radical chemistry in which a key step is the abstraction of a hydrogen atom from an otherwise inert substrate. To gain insight into the mechanism of hydrogen transfer, we previously used pre-steady-state, rapid-quench techniques to measure the alpha-secondary tritium kinetic and equilibrium isotope effects associated with the formation of 5'-deoxyadenosine when glutamate mutase was reacted with [5'-(3)H]adenosylcobalamin and L-glutamate. We showed that both the kinetic and equilibrium isotope effects are large and inverse, 0.76 and 0.72, respectively. We have now repeated these measurements using glutamate deuterated in the position of hydrogen abstraction. The effect of introducing a primary deuterium kinetic isotope effect on the hydrogen transfer step is to reduce the magnitude of the secondary kinetic isotope effect to a value close to unity, 1.05 +/- 0.08, whereas the equilibrium isotope effect is unchanged. The significant reduction in the secondary kinetic isotope effect is consistent with motions of the 5'-hydrogen atoms being coupled in the transition state to the motion of the hydrogen undergoing transfer, in a reaction that involves a large degree of quantum tunneling.
Collapse
Affiliation(s)
| | - E. Neil G. Marsh
- Correspondence should be addressed to this author at: Department of Chemistry, University of Michigan, Ann Arbor, MI 48109−1055, USA Tel (734) 763 6096 FAX (734) 764 8815 e-mail
| |
Collapse
|
56
|
Mansoorabadi SO, Magnusson OT, Poyner RR, Frey PA, Reed GH. Analysis of the Cob(II)alamin-5'-deoxy-3',4'-anhydroadenosyl radical triplet spin system in the active site of diol dehydrase. Biochemistry 2007; 45:14362-70. [PMID: 17128975 PMCID: PMC2527747 DOI: 10.1021/bi061586q] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
A triplet spin system (S=1) is detected by low-temperature electron paramagnetic resonance (EPR) spectroscopy in samples of diol dehydrase and the functional adenosylcobalamin (AdoCbl) analogue 5'-deoxy-3',4'-anhydroadenosylcobalamin (anAdoCbl). Different spectra are observed in the presence and absence of the substrate (R,S)-1,2-propanediol. In both cases, the spectra include a prominent half-field transition (DeltaM(S) = 2) that is a hallmark of strongly coupled triplet spin systems. The appearance of 59Co hyperfine splitting in the EPR signals and the positions (g values) of the signals in the spectra show that half of the triplet spin is contributed by the low-spin Co2+ of cob(II)alamin. Line width effects from isotopic labeling (13C and 2H) in the 5'-deoxy-3',4'-anhydroribosyl ring demonstrate that the other half of the spin triplet is from an allylic 5'-deoxy-3',4'-anhydroadenosyl (anhydroadenosyl) radical. The zero-field splitting (ZFS) tensors describing the magnetic dipole-dipole interactions of the component spins of the triplets have rhombic symmetry because of electron spin delocalization within the organic radical component and the proximity of the radical to the low-spin Co2+. The dipole-dipole interaction was modeled as a summation of point-dipole interactions involving the spin-bearing orbitals of the anhydroadenosyl radical and cob(II)alamin. Geometries which are consistent with the ZFS tensors in the presence and absence of the substrate position the 5'-carbon of the anhydroadenosyl radical 3.5 and 4.1 A from Co2+, respectively. Homolytic cleavage of the cobalt-carbon bond of the analogue in the absence of the substrate indicates that, in diol dehydrase, binding of the coenzyme to the protein weakens the bond prior to binding of the substrate.
Collapse
Affiliation(s)
| | | | | | | | - George H. Reed
- Address Correspondence to: George H. Reed, University of Wisconsin, 1710 University Avenue, Madison, WI 53726-4087, Telephone: (608) 262-0509, FAX: (608) 265-2904,
| |
Collapse
|
57
|
Banerjee R, Dybala-Defratyka A, Paneth P. Quantum catalysis in B12-dependent methylmalonyl-CoA mutase: experimental and computational insights. Philos Trans R Soc Lond B Biol Sci 2006; 361:1333-9. [PMID: 16873121 PMCID: PMC1647305 DOI: 10.1098/rstb.2006.1866] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
B12-dependent methylmalonyl-CoA mutase catalyses the interchange of a hydrogen atom and the carbonyl-CoA group on adjacent carbons of methylmalonyl-CoA to give the rearranged product, succinyl-CoA. The first step in this reaction involves the transient generation of cofactor radicals by homolytic rupture of the cobalt-carbon bond to generate the deoxyadenosyl radical and cob(II)alamin. This step exhibits a curious sensitivity to isotopic substitution in the substrate, methylmalonyl-CoA, which has been interpreted as evidence for kinetic coupling. The magnitude of the isotopic discrimination is large and a deuterium isotope effect ranging from 35.6 at 20 degrees C to 49.9 at 5 degrees C has been recorded. Arrhenius analysis of the temperature dependence of this isotope effect provides evidence for quantum tunnelling in this hydrogen transfer step. The mechanistic complexity of the observed rate constant for cobalt-carbon bond homolysis together with the spectroscopically silent nature of many of the component steps limits the insights that can be derived by experimental approaches alone. Computational studies using a newly developed geometry optimization scheme that allows determination of the transition state in the full quantum mechanical/molecular mechanical coordinate space have yielded novel insights into the strategy deployed for labilizing the cobalt-carbon bond and poising the resulting deoxyadenosyl radical for subsequent hydrogen atom abstraction.
Collapse
Affiliation(s)
- Ruma Banerjee
- Biochemistry Department, University of Nebraska, Lincoln, NE 68588-0664, USA.
| | | | | |
Collapse
|
58
|
Pu J, Gao J, Truhlar DG. Multidimensional tunneling, recrossing, and the transmission coefficient for enzymatic reactions. Chem Rev 2006; 106:3140-69. [PMID: 16895322 PMCID: PMC4478620 DOI: 10.1021/cr050308e] [Citation(s) in RCA: 288] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Jingzhi Pu
- Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455-0431
| | - Jiali Gao
- Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455-0431
| | - Donald G. Truhlar
- Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455-0431
| |
Collapse
|
59
|
Kwiecien RA, Khavrutskii IV, Musaev DG, Morokuma K, Banerjee R, Paneth P. Computational insights into the mechanism of radical generation in B12-dependent methylmalonyl-CoA mutase. J Am Chem Soc 2006; 128:1287-92. [PMID: 16433547 DOI: 10.1021/ja056333j] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
ONIOM calculations have provided novel insights into the mechanism of homolytic Co-C5' bond cleavage in the 5'-deoxyadenosylcobalamin cofactor catalyzed by methylmalonyl-CoA mutase. We have shown that it is a stepwise process in which conformational changes in the 5'-deoxyadenosine moiety precede the actual homolysis step. In the transition state structure for homolysis, the Co-C5' bond elongates by approximately 0.5 Angstroms from the value found in the substrate-bound reactant complex. The overall barrier to homolysis is approximately 10 kcal/mol, and the radical products are approximately 2.5 kcal/mol less stable than the initial ternary complex of enzyme, substrate, and cofactor. The movement of the deoxyadenosine moiety during the homolysis step positions the resulting 5'-deoxyadenosyl radical for the subsequent hydrogen atom transfer from the substrate, methylmalonyl-CoA.
Collapse
Affiliation(s)
- Renata A Kwiecien
- Institute of Applied Radiation Chemistry, Technical University of Lodz, Poland
| | | | | | | | | | | |
Collapse
|
60
|
Brooks AJ, Vlasie M, Banerjee R, Brunold TC. Co-C bond activation in methylmalonyl-CoA mutase by stabilization of the post-homolysis product Co2+ cobalamin. J Am Chem Soc 2006; 127:16522-8. [PMID: 16305240 DOI: 10.1021/ja0503736] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Despite decades of research, the mechanism by which coenzyme B12 (adenosylcobalamin, AdoCbl)-dependent enzymes promote homolytic cleavage of the cofactor's Co-C bond to initiate catalysis has continued to elude researchers. In this work, we utilized magnetic circular dichroism spectroscopy to explore how the electronic structure of the reduced B12 cofactor (i.e., the post-homolysis product Co2+ Cbl) is modulated by the enzyme methylmalonyl-CoA mutase. Our data reveal a fairly uniform stabilization of the Co 3d orbitals relative to the corrin pi/pi*-based molecular orbitals when Co2+ Cbl is bound to the enzyme active site, particularly in the presence of substrate. Contrastingly, our previous studies (Brooks, A. J.; Vlasie, M.; Banerjee, R.; Brunold, T. C. J. Am. Chem. Soc. 2004, 126, 8167-8180.) showed that when AdoCbl is bound to the MMCM active site, no enzymatic perturbation of the Co3+ Cbl electronic structure occurs, even in the presence of substrate (analogues). Collectively, these observations provide direct evidence that enzymatic Co-C bond activation involves stabilization of the post-homolysis product, Co2+ Cbl, rather than destabilization of the Co3+ Cbl "ground" state.
Collapse
Affiliation(s)
- Amanda J Brooks
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | | | | | | |
Collapse
|
61
|
Jensen KP, Ryde U. How the Co-C bond is cleaved in coenzyme B12 enzymes: a theoretical study. J Am Chem Soc 2005; 127:9117-28. [PMID: 15969590 DOI: 10.1021/ja050744i] [Citation(s) in RCA: 98] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The homolytic cleavage of the organometallic Co-C bond in vitamin B12-dependent enzymes is accelerated by a factor of approximately 10(12) in the protein compared to that of the isolated cofactor in aqueous solution. To understand this much debated effect, we have studied the Co-C bond cleavage in the enzyme glutamate mutase with combined quantum and molecular mechanics methods. We show that the calculated bond dissociation energy (BDE) of the Co-C bond in adenosyl cobalamin is reduced by 135 kJ/mol in the enzyme. This catalytic effect can be divided into four terms. First, the adenosine radical is kept within 4.2 angstroms of the Co ion in the enzyme, which decreases the BDE by 20 kJ/mol. Second, the surrounding enzyme stabilizes the dissociated state by 42 kJ/mol using electrostatic and van der Waals interactions. Third, the protein itself is stabilized by 11 kJ/mol in the dissociated state. Finally, the coenzyme is geometrically distorted by the protein, and this distortion is 61 kJ/mol larger in the Co(III) state. This deformation of the coenzyme is caused mainly by steric interactions, and it is especially the ribose moiety and the Co-C5'-C4' angle that are distorted. Without the polar ribose group, the catalytic effect is much smaller, e.g. only 42 kJ/mol for methyl cobalamin. The deformation of the coenzyme is caused mainly by the substrate, a side chain of the coenzyme itself, and a few residues around the adenosine part of the coenzyme.
Collapse
Affiliation(s)
- Kasper P Jensen
- Department of Theoretical Chemistry, Lund University, Chemical Center, P.O. Box 124, S-221 00 Lund, Sweden
| | | |
Collapse
|
62
|
Affiliation(s)
- Kenneth L Brown
- Department of Chemistry and Biochemistry, Ohio University, Athens, 45701, USA.
| |
Collapse
|
63
|
Ohkubo K, Fukuzumi S. Electron-Transfer Oxidation of Coenzyme B12 Model Compounds and Facile Cleavage of the Cobalt(IV)−Carbon Bond via Charge-Transfer Complexes with Bases. A Negative Temperature Dependence of the Rates. J Phys Chem A 2005; 109:1105-13. [PMID: 16833419 DOI: 10.1021/jp0453008] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The electron-transfer oxidation and subsequent cobalt-carbon bond cleavage of vitamin B12 model complexes were investigated using cobaloximes, (DH)2Co(III)(R)(L), where DH- = the anion of dimethylglyoxime, R = Me, Et, Ph, PhCH2, and PhCH(CH3), and L = a substituted pyridine, as coenzyme B12 model complexes and [Fe(bpy)3](PF6)3 or [Ru(bpy)3](PF6)3 (bpy = 2,2'-bipyridine) as a one-electron oxidant. The rapid one-electron oxidation of (DH)2Co(III)(Me)(py) (py = pyridine) with the oxidant gives the corresponding Co(IV) complexes, [(DH)2Co(IV)(Me)(py)]+, which were well identified by the ESR spectra. The reorganization energy (lambda) for the electron-transfer oxidation of (DH)2Co(Me)(py) was determined from the ESR line broadening of [(DH)2Co(Me)(py)]+ caused by the electron exchange with (DH)2Co(Me)(py). The lambda value is applied to evaluate the rate constants of photoinduced electron transfer from (DH)2Co(Me)(py) to photosensitizers in light of the Marcus theory of electron transfer. The Co(IV)-C bond cleavage of [(DH)2Co(Me)(py)]+ is accelerated significantly by the reaction with a base. The overall activation energy for the second-order rate constants of Co(IV)-C bond cleavage of [(DH)2Co(IV)(Me)(py)]+ in the presence of a base is decreased by charge-transfer complex formation with a base, which leads to a negative activation energy for the Co(IV)-C cleavage when either 2-methoxypyridine or 2,6-dimethoxypyridine is used as the base.
Collapse
Affiliation(s)
- Kei Ohkubo
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
| | | |
Collapse
|
64
|
Sension RJ, Cole AG, Harris AD, Fox CC, Woodbury NW, Lin S, Marsh ENG. Photolysis and recombination of adenosylcobalamin bound to glutamate mutase. J Am Chem Soc 2004; 126:1598-9. [PMID: 14871067 DOI: 10.1021/ja0396910] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Ultrafast spectroscopic measurements are used to determine the kinetics of homolysis and recombination for adenosylcobalamin bound in the active site of glutamate mutase. These are the first such measurements on an adenosylcobalamin-dependent enzyme. A short-lived intermediate is formed prior to formation of the cob(II)alamin radical. This intermediate was not observed upon photolysis of adenosylcobalamin in free solution. The intrinsic rate constant for geminate recombination for adenosylcobalamin bound to glutamate mutase is 1.08 +/- 0.10 ns-1, only 16% smaller than the rate constant measured in free solution, 1.39 +/- 0.06 ns-1, suggesting the protein does not greatly perturb the stability of the cobalt-carbon bond upon binding the coenzyme.
Collapse
Affiliation(s)
- Roseanne J Sension
- Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA.
| | | | | | | | | | | | | |
Collapse
|
65
|
Dybala-Defratyka A, Paneth P, Pu J, Truhlar DG. Benchmark Results for Hydrogen Atom Transfer between Carbon Centers and Validation of Electronic Structure Methods for Bond Energies and Barrier Heights. J Phys Chem A 2004. [DOI: 10.1021/jp037312j] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Agnieszka Dybala-Defratyka
- Institute of Applied Radiation Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland, and Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431
| | - Piotr Paneth
- Institute of Applied Radiation Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland, and Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431
| | - Jingzhi Pu
- Institute of Applied Radiation Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland, and Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431
| | - Donald G. Truhlar
- Institute of Applied Radiation Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland, and Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431
| |
Collapse
|
66
|
Tang KH, Casarez AD, Wu W, Frey PA. Kinetic and biochemical analysis of the mechanism of action of lysine 5,6-aminomutase. Arch Biochem Biophys 2003; 418:49-54. [PMID: 13679082 DOI: 10.1016/s0003-9861(03)00346-1] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Lysine 5,6-aminomutase (5,6-LAM) catalyzes the reversible and nearly isoenergetic transformations of D-lysine into 2,5-diaminohexanoate (2,5-DAH) and of L-beta-lysine into 3,5-diaminohexanoate (3,5-DAH). The activity of 5,6-LAM depends on pyridoxal-5(')-phosphate (PLP) and adenosylcobalamin. The currently postulated multistep mechanism involves at least 12 steps, two of which involve hydrogen transfer. The deuterium kinetic isotope effects on k(cat) and k(cat)/K(m) have been found to be 10.4+/-0.3 and 8.3+/-1.9, respectively, in the reaction of DL-lysine-3,3,4,4,5,5,6,6-d(8). The corresponding isotope effects for reaction of DL-lysine-4,4,5,5-d(4) are 8.5+/-0.7 and 7.1+/-1.2, respectively. Neither cob(II)alamin nor a free radical can be detected in the steady state by UV-Vis spectrophotometry or electron paramagnetic resonance (EPR) spectroscopy. Therefore, hydrogen abstraction from carbon-5 of the substrate side chain is rate limiting in the mechanism. DL-4-Oxalysine is an alternative substrate for 5,6-LAM. DL-4-Oxalysine reacts irreversibly because the product breaks down into ammonia, acetaldehyde, and DL-serine. The value of K(m) for the reaction of DL-4-oxalysine is lower than that for DL-lysine and that of k(cat) for DL-4-oxalysine is slightly lower than that for DL-lysine. As measured by values of k(cat)/K(m), 5,6-LAM uses DL-4-oxalysine essentially as efficiently as the best substrates, D-lysine and L-beta-lysine, and more efficiently than DL-lysine. DL-4-Oxalysine induces the same suicide inactivation by electron transfer as do the biological substrates. The putative substrate-related radical intermediate is not sufficiently stabilized by the nonbonding 4-oxa electrons to be detectable by EPR spectroscopy.
Collapse
Affiliation(s)
- Kuo-Hsiang Tang
- Department of Biochemistry, University of Wisconsin-Madison, 53726, USA
| | | | | | | |
Collapse
|
67
|
Doll KM, Bender BR, Finke RG. The first experimental test of the hypothesis that enzymes have evolved to enhance hydrogen tunneling. J Am Chem Soc 2003; 125:10877-84. [PMID: 12952467 DOI: 10.1021/ja030120h] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The literature hypothesis that "the optimization of enzyme catalysis may entail the evolutionary implementation of chemical strategies that increase the probability of quantum-mechanical tunneling" is experimentally tested herein for the first time. The system employed is the key to being able to provide this first experimental test of the "enhanced hydrogen tunneling" hypothesis, one that requires a comparison of the three criteria diagnostic of tunneling (vide infra) for the same, or nearly the same, reaction with and without the enzyme. Specifically, studied herein are the adenosylcobalamin (AdoCbl, also known as coenzyme B(12))-dependent diol dehydratase model reactions of (i). H(D)(*) atom abstraction from ethylene glycol-d(0) and ethylene glycol-d(4) solvent by 5'-deoxyadenosyl radical (Ado(*)) and (ii.) the same H(*) abstraction reactions by the 8-methoxy-5'-deoxyadenosyl radical (8-MeOAdo(*)). The Ado(*) and 8-MeOAdo(*) radicals are generated by Co-C thermolysis of their respective precursors, AdoCbl and 8-MeOAdoCbl. Deuterium kinetic isotope effects (KIEs) of the H(*)(D(*)) abstraction reactions from ethylene glycol have been measured over a temperature range of 80-120 degrees C: KIE = 12.4 +/- 1.1 at 80 degrees C for Ado(*) and KIE = 12.5 +/- 0.9 at 80 degrees C for 8-MeOAdo(*) (values ca. 2-fold that of the predicted maximum primary times secondary ground-state zero-point energy (GS-ZPE) KIE of 6.4 at 80 degrees C). From the temperature dependence of the KIEs, zero-point activation energy differences ([E(D) - E(H)]) of 3.0 +/- 0.3 kcal mol(-)(1) for Ado(*) and 2.1 +/- 0.6 kcal mol(-)(1) for 8-MeOAdo(*) have been obtained, both of which are significantly larger than the nontunneling, zero-point energy only maximum of 1.2 kcal mol(-)(1). Pre-exponential factor ratios (A(H)/A(D)) of 0.16 +/- 0.07 for Ado(*) and 0.5 +/- 0.4 for 8-MeOAdo(*) are observed, both of which are significantly less than the 0.7 minimum for nontunneling behavior. The data provide strong evidence for the expected quantum mechanical tunneling in the Ado(*) and 8-MeOAdo(*)-mediated H(*) abstraction reactions from ethylene glycol. More importantly, a comparison of these enzyme-free tunneling data to the same KIE, (E(D) - E(H)) and A(H)/A(D) data for a closely related, Ado(*)-mediated H(*) abstraction reaction from a primary CH(3)- group in AdoCbl-dependent methylmalonyl-CoA mutase shows the enzymic and enzyme-free data sets are identical within experimental error. The Occam's Razor conclusion is that at least this adenosylcobalamin-dependent enzyme has not evolved to enhance quantum mechanical tunneling, at least within the present error bars. Instead, this B(12)-dependent enzyme simply exploits the identical level of quantum mechanical tunneling that is available in the enzyme-free, solution-based H(*) abstraction reaction. The results also require a similar, if not identical, barrier width and height within experimental error for the H(*) abstraction both within, and outside of, the enzyme.
Collapse
Affiliation(s)
- Kenneth M Doll
- Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA
| | | | | |
Collapse
|
68
|
Toraya T. Radical catalysis in coenzyme B12-dependent isomerization (eliminating) reactions. Chem Rev 2003; 103:2095-127. [PMID: 12797825 DOI: 10.1021/cr020428b] [Citation(s) in RCA: 312] [Impact Index Per Article: 14.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Tetsuo Toraya
- Department of Bioscience and Biotechnology, Faculty of Engineering, Okayama University, Tsushima-naka, Okayama 700-8530, Japan.
| |
Collapse
|
69
|
Banerjee R. Radical carbon skeleton rearrangements: catalysis by coenzyme B12-dependent mutases. Chem Rev 2003; 103:2083-94. [PMID: 12797824 DOI: 10.1021/cr0204395] [Citation(s) in RCA: 252] [Impact Index Per Article: 12.0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Ruma Banerjee
- Biochemistry Department, University of Nebraska, Lincoln, Nebraska 68588-0664, USA.
| |
Collapse
|
70
|
Abstract
Vitamin B12 is a complex organometallic cofactor associated with three subfamilies of enzymes: the adenosylcobalamin-dependent isomerases, the methylcobalamin-dependent methyltransferases, and the dehalogenases. Different chemical aspects of the cofactor are exploited during catalysis by the isomerases and the methyltransferases. Thus, the cobalt-carbon bond ruptures homolytically in the isomerases, whereas it is cleaved heterolytically in the methyltransferases. The reaction mechanism of the dehalogenases, the most recently discovered class of B12 enzymes, is poorly understood. Over the past decade our understanding of the reaction mechanisms of B12 enzymes has been greatly enhanced by the availability of large amounts of enzyme that have afforded detailed structure-function studies, and these recent advances are the subject of this review.
Collapse
Affiliation(s)
- Ruma Banerjee
- Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664, USA. ;
| | | |
Collapse
|
71
|
Vlasie MD, Banerjee R. Tyrosine 89 accelerates Co-carbon bond homolysis in methylmalonyl-CoA mutase. J Am Chem Soc 2003; 125:5431-5. [PMID: 12720457 DOI: 10.1021/ja029420+] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The contribution of the active-site residue, Y89, to the trillion-fold acceleration of Co-carbon bond homolysis rate in the methylmalonyl-CoA mutase-catalyzed reaction has been evaluated by site-directed mutagenesis. Conversion of Y89 to phenylalanine or alanine results in a 10(3)-fold diminution of k(cat) and suppression of the overall kinetic isotope effect. The spectrum of the enzyme under steady-state conditions reveals the presence of AdoCbl but no cob(II)alamin. Together, these results are consistent with homolysis becoming completely rate determining in the forward direction in the two mutants and points to the role of Y89 as a molecular wedge in accelerating Co-carbon bond cleavage.
Collapse
Affiliation(s)
- Monica D Vlasie
- Biochemistry Department, University of Nebraska, Lincoln, Nebraska 68588-0664, USA
| | | |
Collapse
|
72
|
Abstract
A peculiar function resides in a peculiar structure. Coenzyme B(12) or adenosylcobalamin, a naturally occurring organometallic compound, serves as a cofactor for enzymatic radical reactions. How do the enzymes form catalytic radicals at the active sites? How do the enzymes utilize and control the high reactivity of the radicals for catalysis? Recently, three-dimensional structures of several radical-containing or radical-forming enzymes including B(12) enzymes have been reported, enabling the analysis of the fine mechanisms of the action of these interesting enzymes. Our biochemical, mutational, and crystallographic studies as well as theoretical calculations on diol dehydratase, an adenosylcobalamin-dependent enzyme, revealed that its structure is adapted for its function-that is, activation of the Cobond;C bond toward homolysis, abstraction of a specific hydrogen atom from the substrate and its recombination to a particular product, and transition state stabilization in the hydroxyl group migration of a substrate-derived radical. The functions of K(+) and the active-site amino acid residues in enzyme catalysis are also investigated. Based on the results, the fine mechanism of the enzyme and the energetic feasibility of enzymatic radical catalysis are described here.
Collapse
Affiliation(s)
- Tetsuo Toraya
- Department of Bioscience and Biotechnology, Faculty of Engineering; Okayama University, Tsushima-naka, Okayama 700-8530, Japan.
| |
Collapse
|
73
|
Loferer MJ, Webb BM, Grant GH, Liedl KR. Energetic and stereochemical effects of the protein environment on substrate: a theoretical study of methylmalonyl-CoA mutase. J Am Chem Soc 2003; 125:1072-8. [PMID: 12537507 DOI: 10.1021/ja028906n] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
QM/MM methods were used to study the isomerization step from (2R)-methylmalonyl-CoA to succinyl-CoA. A pathway via a "fragmentation-recombination" mechanism is ruled out on energetic grounds. For the other radicalic pathway, involving an addition recombination step, geometries and vibrational contributions have been determined, and a barrier height of 11.70 kcal/mol was found. The effect of adjacent hydrogen-donating groups was found to reduce the energy barrier by 1-2 kcal/mol each and thus to provide a significant catalytic effect for this reaction. By means of molecular dynamics studies, the stereochemistry of the methylmalonyl-CoA mutase catalyzed reaction was examined. It is shown that TYR89 is essential for maintaining stereoselectivity of the abstraction of a hydrogen in the backreaction. The subsequent selective formation of one isomer of methylmalonyl-CoA is probably due to the presence of a bulky side chain.
Collapse
Affiliation(s)
- Markus J Loferer
- Institute of General, Inorganic, and Theoretical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria
| | | | | | | |
Collapse
|
74
|
Doll KM, Fleming PE, Finke RG. The synthesis and characterization of 8-methoxy-5'-deoxyadenosylcobalamin: a coenzyme B(12) analog which, following Co-C bond homolysis, avoids cyclization of the 8-methoxy-5'-deoxyadenosyl radical. J Inorg Biochem 2002; 91:388-97. [PMID: 12161308 DOI: 10.1016/s0162-0134(02)00401-4] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
The compound 8-methoxy-5'-deoxyadenosylcobalamin (8-MeOAdoCbl), has been synthesized in 37% yield and > or = 95% purity by HPLC, monitored at both 254 and 525 nm, or 90+/-2% purity as judged by the (1)H NMR spectrum of the aromatic cobalamin region. This is the first synthesis of this complex in which sufficient details are reported, where a yield and purity are reported, and where key problems in the synthesis and purification are overcome, so that 8-MeOAdoCbl can actually be obtained for use in other studies. Also demonstrated is the clean Co-C bond homolysis of 8-MeOAdoCbl to give initially 8-MeOAdoCbl and Co(II)Cbl in a UV-visible thermolysis experiment at 110 degrees C, results which show that the 8-MeO moiety suppresses the cyclization to the 8,5'-anhydro-adenosine otherwise seen for the adenosyl radical (Ado)*. Suppression of this cyclization pathway makes 8-MeOAdoCbl invaluable for studying the kinetic isotope effect (KIE) of the Ado* plus substrate H* abstraction reaction, a component of the first definitive test of Klinman's hypothesis that the optimization of enzyme catalysis may entail strategies that increase the probability of tunneling and thereby accelerate H* atom abstraction reaction rates.
Collapse
Affiliation(s)
- Kenneth M Doll
- Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
| | | | | |
Collapse
|
75
|
Vlasie M, Chowdhury S, Banerjee R. Importance of the histidine ligand to coenzyme B12 in the reaction catalyzed by methylmalonyl-CoA mutase. J Biol Chem 2002; 277:18523-7. [PMID: 11893736 DOI: 10.1074/jbc.m111809200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Methylmalonyl-CoA mutase is an adenosylcobalamin (AdoCbl)-dependent enzyme that catalyzes the rearrangement of methylmalonyl-CoA to succinyl-CoA. The crystal structure of this protein revealed that binding of the cofactor is accompanied by a significant conformational change in which dimethylbenzimidazole, the lower axial ligand to the cobalt in solution, is replaced by His-610 donated by the active site. The contribution of the lower axial base to the approximately 10(12)-fold rate acceleration of the homolytic cleavage of the upper axial cobalt-carbon bond has been the subject of intense scrutiny in the model inorganic literature. In contrast, trans ligand effects in methylmalonyl-CoA mutase and indeed the significance of the ligand replacement are poorly understood. In this study, we have used site-directed mutagenesis to create the H610A and H610N variants of methylmalonyl-CoA mutase and report that both mutations exhibit both diminished activity (5,000- and 40,000-fold, respectively) and profoundly weakened affinity for the native cofactor, AdoCbl. In contrast, binding of the truncated cofactor analog, adenosylcobinamide, lacking the nucleotide tail, is less impaired. The catalytic failure of the His-610 mutants is in marked contrast to the phenotype of the adenosylcobinamide-GDP reconstituted wild type enzyme that exhibits only a 4-fold decrease in activity, although His-610 fails to coordinate when this cofactor analog is bound. Together, these studies suggest that His-610 may: (i) play a structural role in organizing a high affinity cofactor binding site possibly via electrostatic interactions with Asp-608 and Lys-604, as suggested by the crystal structure and (ii) play a role in catalyzing the displacement of dimethylbenzimidazole thereby facilitating the conformational change that must precede cofactor docking to the mutase active site.
Collapse
Affiliation(s)
- Monica Vlasie
- Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664, USA
| | | | | |
Collapse
|
76
|
Jensen KP, Ryde U. The axial N -base has minor influence on Co–C bond cleavage in cobalamins. ACTA ACUST UNITED AC 2002. [DOI: 10.1016/s0166-1280(02)00049-0] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
|
77
|
Fedosov SN, Berglund L, Fedosova NU, Nexo E, Petersen TE. Comparative analysis of cobalamin binding kinetics and ligand protection for intrinsic factor, transcobalamin, and haptocorrin. J Biol Chem 2002; 277:9989-96. [PMID: 11788601 DOI: 10.1074/jbc.m111399200] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Changes in the absorbance spectrum of aquo-cobalamin (Cbl x OH(2)) revealed that its binding to transcobalamin (TC) is followed by slow conformational reorganization of the protein-ligand complex (Fedosov, S. N., Fedosova, N. U., Nexø, E., and Petersen, T. E. (2000) J. Biol. Chem. 275, 11791-11798). Two phases were also observed for TC when interacting with a Cbl-analogue cobinamide (Cbi), but not with other cobalamins. The slow phase had no relation to the ligand recognition, since both Cbl and Cbi bound rapidly and in one step to intrinsic factor (IF) and haptocorrin (HC), namely the proteins with different Cbl specificity. Spectral transformations observed for TC in the slow phase were similar to those upon histidine complexation with Cbl x OH(2) and Cbi. In contrast to a closed structure of TC x Cbl x OH(2), the analogous IF and HC complexes revealed accessibility of Cbl's upper face to the external reagents. The binders decreased sensitivity of adenosyl-Cbl (Cbl x Ado) to light in the range: free ligand, IF x, HC x, TC x Cbl x Ado. The spectrum of TC x Cbl small middle dotAdo differed from those of IF and HC and mimicked Cbl x Ado participating in catalysis. The above data suggest presence of a histidine-containing cap shielding the Cbl-binding site in TC. The cap coordinates to certain corrinoids and, possibly, produces an incapsulated Ado-radical when Cbl small middle dotAdo is bound.
Collapse
Affiliation(s)
- Sergey N Fedosov
- Protein Chemistry Laboratory, Department of Molecular and Structural Biology, University of Aarhus, Science Park, Gustav Wieds Vej 10, 8000 Aarhus C, Denmark.
| | | | | | | | | |
Collapse
|
78
|
Abstract
Two classes of enzymatic mechanisms that proceed by free radical chemistry initiated by the 5'-deoxyadenosyl radical are discussed. In the first class, the mechanism of the interconversion of L-lysine and L-beta-lysine catalyzed by lysine 2,3-aminomutase (LAM) involves four radicals, three of which have been spectroscopically characterized. The reversible formation of the 5'-deoxyadenosyl radical takes place by the chemical cleavage of S-adenosylmethionine (SAM) reacting with the [4Fe-4S]+ center in LAM. In other reactions of SAM with iron-sulfur proteins, SAM is irreversibly consumed to generate the 5'-deoxyadenosyl radical, which activates an enzyme by abstracting a hydrogen atom from an enzymatic glycyl residue to form a glycyl radical. The glycyl radical enzymes include pyruvate formate-lyase, anaerobic ribonucleotide reductase from Escherichia coli, and benzylsuccinate synthase. Biotin synthase and lipoate synthase are SAM-dependent [4Fe-4S] proteins that catalyze the insertion of sulfur into unactivated C-H bonds, which are cleaved by the 5'-deoxyadenosyl radical from SAM. In the second class of enzymatic mechanisms using free radicals, adenosylcobalamin-dependent reactions, the 5'-deoxyadenosyl radical arises from homolytic cleavage of the cobalt-carbon bond, and it initiates radical reactions by abstracting hydrogen atoms from substrates. Three examples are described of suicide inactivation through the formation of exceptionally stable free radicals at enzymatic active sites.
Collapse
Affiliation(s)
- P A Frey
- Department of Biochemistry, University of Wisconsin-Madison, 1710 University Avenue, Madison, Wisconsin 53705, USA.
| |
Collapse
|
79
|
Madhavapeddi P, Marsh EN. The role of the active site glutamate in the rearrangement of glutamate to 3-methylaspartate catalyzed by adenosylcobalamin-dependent glutamate mutase. CHEMISTRY & BIOLOGY 2001; 8:1143-9. [PMID: 11755393 DOI: 10.1016/s1074-5521(01)00081-3] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
BACKGROUND Adenosylcobalamin (coenzyme B(12))-dependent enzymes catalyze a variety of chemically difficult reactions that proceed through the generation of free radical intermediates. A long-standing question is how proteins stabilize what are normally regarded as highly reactive organic radicals and direct them towards productive reactions. In glutamate mutase the carboxylate of Glu171 hydrogen bonds with the amino group of the substrate. We have investigated the role of this residue in the enzyme mechanism. RESULTS Several sterically and functionally conservative mutations were introduced at position 171. In the most impaired mutant, Glu171Gln, k(cat) is reduced 50-fold, although the K(m) for glutamate is little affected. In the wild-type enzyme activity was pH-dependent and the acidic limb of the activity curve titrated with an apparent pK(a) of 6.6 on V(max), whereas for the sluggish Glu171Gln mutant activity is independent of pH. The steady state deuterium kinetic isotope effect is reduced in the mutant enzyme, but the steady state concentration of free radical species on the enzyme (as measured by the steady state concentration of cob(II)alamin) is unaffected by the mutation. CONCLUSIONS The properties of the mutant proteins are consistent with the hypothesis that Glu171 acts as a general base that serves to deprotonate the amino group of the substrate during catalysis. Deprotonation is expected to facilitate the formation of the glycyl radical intermediate formed during the inter-conversion of substrate and product radicals, but to have little effect on the stability of product or substrate radicals themselves.
Collapse
Affiliation(s)
- P Madhavapeddi
- Department of Chemistry and Division of Biophysics, University of Michigan, Ann Arbor, MI 48109-1055, USA
| | | |
Collapse
|
80
|
Abstract
Recent progress in computational modeling of the catalytic activation of cobalt-carbon bond cleavage shows that quantum chemical calculations could be an important part of coenzyme B(12) research. Particular emphasis has been placed on density functional theory, which is now emerging as a powerful tool to elucidate the electronic structure and spectroscopic properties of the active sites of metalloenzymes.
Collapse
Affiliation(s)
- P M Kozlowski
- Department of Chemistry, University of Louisville, Kentucky 40292, USA.
| |
Collapse
|
81
|
Dybala-Defratyka A, Paneth P. Theoretical evaluation of the hydrogen kinetic isotope effect on the first step of the methylmalonyl-CoA mutase reaction. J Inorg Biochem 2001; 86:681-9. [PMID: 11583786 DOI: 10.1016/s0162-0134(01)00230-6] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
We have calculated hydrogen kinetic isotope effects (KIEs) for the first step of the methylmalonyl-CoA mutase reaction, including multidimensional tunneling correction at the zero curvature (ZCT) level, and compared them with the experimental values. Both alternative mechanisms of this step, concerted and stepwise, can be accommodated. It turned out to be essential to include Arg207 hydrogen-bonded to the reactant in the mechanism predicting simultaneous breaking of the Co-C bond of AdoCbl and hydrogen atom transfer. The consequence of the stepwise mechanism is a much larger facilitation of the homolytic dissociation of the carbon-cobalt bond by the enzyme than currently appreciated; our results suggest lowering of the activation energy by about 23 kcal mol(-1). We have also shown that large hydrogen KIEs of tunneling origin do not necessarily break the Swain-Schaad equation. Furthermore, when this equation does not hold, the exponent may be smaller in the presence of tunneling than it is at the semi-classical limit, indicating that nonclassical behavior may be a more common phenomenon than expected.
Collapse
Affiliation(s)
- A Dybala-Defratyka
- Institute of Applied Radiation Chemistry, Department of Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924, Lodz, Poland
| | | |
Collapse
|
82
|
Jensen KP, Mikkelsen KV. Semi-empirical studies of cobalamins, corrin models, and cobaloximes. The nucleotide loop does not strain the corrin ring in cobalamins. Inorganica Chim Acta 2001. [DOI: 10.1016/s0020-1693(01)00525-4] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
|
83
|
Marsh EN, Drennan CL. Adenosylcobalamin-dependent isomerases: new insights into structure and mechanism. Curr Opin Chem Biol 2001; 5:499-505. [PMID: 11578922 DOI: 10.1016/s1367-5931(00)00238-6] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Adenosylcobalamin-dependent isomerases catalyze a variety of chemically difficult 1,2-rearrangements that proceed through a mechanism involving free radical intermediates. These radicals are initially generated by homolysis of the cobalt-carbon bond of the coenzyme. Recently, the crystal structures of several of these enzymes have been solved, revealing two modes of coenzyme binding and highlighting the role of the protein in controlling the rearrangement of reactive substrate radical intermediates. Complementary data from kinetic, spectroscopic and theoretical studies have produced insights into the mechanism by which substrate radicals are generated at the active site, and the pathways by which they rearrange.
Collapse
Affiliation(s)
- E N Marsh
- Department of Chemistry and Division of Biophysics, University of Michigan, Ann Arbor, MI 48109, USA.
| | | |
Collapse
|
84
|
Gruber K, Reitzer R, Kratky C. Radical Shuttling in a Protein: Ribose Pseudorotation Controls Alkyl-Radical Transfer in the Coenzyme B(12) Dependent Enzyme Glutamate Mutase This work was supported by the Österreichische Akademie der Wissenschaften (APART fellowship 614), the Österreichische Fonds zur Förderung der wissenschaftlichen Forschung (FWF-project 11599), and the European Commission (TMR project number ERB 4061 PL 95-0307). Crystallographic data were collected at the EMBL-beamline BW7B at DESY in Hamburg, Germany. We thank the beamline scientists for their assistance, and Ingrid Dreveny, Günter Gartler, Gerwald Jogl, and Oliver Sauer for their help during data collection. This research emerged from a collaboration with Prof. W. Buckel (Marburg) who supplied us with clones of the glutamate mutase proteins. Angew Chem Int Ed Engl 2001; 40:3377-3380. [PMID: 11592143 DOI: 10.1002/1521-3773(20010917)40:18<3377::aid-anie3377>3.0.co;2-8] [Citation(s) in RCA: 115] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Affiliation(s)
- Karl Gruber
- Institut für Chemie Universität Graz Heinrichstrasse 28, 8010 Graz (Austria)
| | | | | |
Collapse
|
85
|
Gruber K, Reitzer R, Kratky C. Radikalpendelmechanismus in einem Protein: Kontrolle des Alkylradikaltransfers durch Ribose-Pseudorotation in der Coenzym-B12-abhängigen Glutamatmutase. Angew Chem Int Ed Engl 2001. [DOI: 10.1002/1521-3757(20010917)113:18<3481::aid-ange3481>3.0.co;2-w] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
|
86
|
Gruber K, Reitzer R, Kratky C. Radical Shuttling in a Protein: Ribose Pseudorotation Controls Alkyl-Radical Transfer in the Coenzyme B12 Dependent Enzyme Glutamate Mutase. Angew Chem Int Ed Engl 2001. [DOI: 10.1002/1521-3773(20010917)40:18%3c3377::aid-anie3377%3e3.0.co;2-8] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
|
87
|
Warncke K, Utada AS. Interaction of the substrate radical and the 5'-deoxyadenosine-5'-methyl group in vitamin B(12) coenzyme-dependent ethanolamine deaminase. J Am Chem Soc 2001; 123:8564-72. [PMID: 11525664 DOI: 10.1021/ja003658l] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The distance and relative orientation of the C5' methyl group of 5'-deoxyadenosine and the substrate radical in vitamin B(12) coenzyme-dependent ethanolamine deaminase from Salmonella typhimurium have been characterized by using X-band two-pulse electron spin-echo envelope modulation (ESEEM) spectroscopy in the disordered solid state. The (S)-2-aminopropanol-generated substrate radical catalytic intermediate was prepared by cryotrapping steady-state mixtures of enzyme in which catalytically exchangeable hydrogen sites in the active site had been labeled by previous turnover on (2)H(4)-ethanolamine. Simulation of the time- and frequency-domain ESEEM requires two types of coupled (2)H. The strongly coupled (2)H has an effective dipole distance (r(eff)) of 2.2 A, and isotropic coupling constant (A(iso)) of -0.35 MHz. The weakly coupled (2)H has r(eff) = 3.8 A and A(iso) = 0 MHz. The best (2)H ESEEM time- and frequency-domain simulations are achieved with a model in which the hyperfine couplings arise from one strongly coupled hydrogen site and two equivalent weakly coupled hydrogen sites located on the C5' methyl group of 5'-deoxyadenosine. This model indicates that the unpaired electron on C1 of the substrate radical and C5' are separated by 3.2 A and are thus at closest contact. The close proximity of C1 and C5' indicates that C5' of the 5'-deoxyadenosyl moiety directly mediates radical migration between cobalt in cobalamin and the substrate/product site over a distance of 5-7 A in the active site of ethanolamine deaminase.
Collapse
Affiliation(s)
- K Warncke
- Department of Physics, Emory University, 1001 Rollins Research Center, 1510 Clifton Road, Atlanta, GA 30322, USA.
| | | |
Collapse
|
88
|
Fontecave M, Pierre JL. Mechanisms of formation of free radicals in biological catalysis. ACTA ACUST UNITED AC 2001. [DOI: 10.1016/s1387-1609(01)01283-x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
|
89
|
Huhta MS, Chen HP, Hemann C, Hille CR, Marsh EN. Protein-coenzyme interactions in adenosylcobalamin-dependent glutamate mutase. Biochem J 2001; 355:131-7. [PMID: 11256957 PMCID: PMC1221720 DOI: 10.1042/0264-6021:3550131] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Glutamate mutase catalyses an unusual isomerization involving free-radical intermediates that are generated by homolysis of the cobalt-carbon bond of the coenzyme adenosylcobalamin (coenzyme B(12)). A variety of techniques have been used to examine the interaction between the protein and adenosylcobalamin, and between the protein and the products of coenzyme homolysis, cob(II)alamin and 5'-deoxyadenosine. These include equilibrium gel filtration, isothermal titration calorimetry, and resonance Raman, UV-visible and EPR spectroscopies. The thermodynamics of adenosylcobalamin binding to the protein have been examined and appear to be entirely entropy-driven, with DeltaS=109 J.mol(-1).K(-1). The cobalt-carbon bond stretching frequency is unchanged upon coenzyme binding to the protein, arguing against a ground-state destabilization of the cobalt-carbon bond of adenosylcobalamin by the protein. However, reconstitution of the enzyme with cob(II)alamin and 5'-deoxyadenosine, the two stable intermediates formed subsequent to homolysis, results in the blue-shifting of two of the bands comprising the UV-visible spectrum of the corrin ring. The most plausible interpretation of this result is that an interaction between the protein, 5'-deoxyadenosine and cob(II)alamin introduces a distortion into the ring corrin that perturbs its electronic properties.
Collapse
Affiliation(s)
- M S Huhta
- Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, U.S.A
| | | | | | | | | |
Collapse
|
90
|
Affiliation(s)
- E N Marsh
- Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA
| | | |
Collapse
|
91
|
Chowdhury S, Thomas MG, Escalante-Semerena JC, Banerjee R. The coenzyme b12 analog 5'-deoxyadenosylcobinamide-gdp supports catalysis by methylmalonyl-coa mutase in the absence of trans-ligand coordination. J Biol Chem 2001; 276:1015-9. [PMID: 11031263 DOI: 10.1074/jbc.m006842200] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Methylmalonyl-CoA mutase is an 5'-adenosylcobalamin (AdoCbl)-dependent enzyme that catalyzes the rearrangement of methylmalonyl-CoA to succinyl-CoA. The crystal structure of this protein revealed that binding of the cofactor is accompanied by a significant conformational change in which dimethylbenzimidazole, the lower axial ligand to cobalt in solution, is replaced by His(610) donated by the active site. The role of the lower axial ligand in the trillion-fold labilization of the upper axial cobalt-carbon bond has been the subject of enduring debate in the model inorganic literature. In this study, we have used a cofactor analog, 5'deoxyadenosylcobinamide GDP (AdoCbi-GDP), which reconstitutes the enzyme in a "histidine-off" form and which allows us to evaluate the contribution of the lower axial ligand to catalysis. The k(cat) for the enzyme in the presence of AdoCbi-GDP is reduced by a factor of 4 compared with the native cofactor AdoCbl. The overall deuterium isotope effect in the presence of AdoCbi-GDP ((D)V = 7.2 +/- 0.8) is comparable with that observed in the presence of AdoCbl (5.0 +/- 0.6) and indicates that the hydrogen transfer steps in this reaction are not significantly affected by the change in coordination state of the bound cofactor. These surprising results are in marked contrast to the effects ascribed to the corresponding lower axial histidine ligands in the cobalamin-dependent enzymes glutamate mutase and methionine synthase.
Collapse
Affiliation(s)
- S Chowdhury
- Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664 , USA
| | | | | | | |
Collapse
|
92
|
Jensen KP, Sauer SPA, Liljefors T, Norrby PO. Theoretical Investigation of Steric and Electronic Effects in Coenzyme B12 Models. Organometallics 2001. [DOI: 10.1021/om000260p] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Kasper P. Jensen
- Royal Danish School of Pharmacy, Department of Medicinal Chemistry, Universitetsparken 2, DK-2100 Copenhagen, Denmark, and Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
| | - Stephan P. A. Sauer
- Royal Danish School of Pharmacy, Department of Medicinal Chemistry, Universitetsparken 2, DK-2100 Copenhagen, Denmark, and Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
| | - Tommy Liljefors
- Royal Danish School of Pharmacy, Department of Medicinal Chemistry, Universitetsparken 2, DK-2100 Copenhagen, Denmark, and Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
| | - Per-Ola Norrby
- Royal Danish School of Pharmacy, Department of Medicinal Chemistry, Universitetsparken 2, DK-2100 Copenhagen, Denmark, and Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
| |
Collapse
|
93
|
Chih HW, Marsh ENG. Mechanism of Glutamate Mutase: Identification and Kinetic Competence of Acrylate and Glycyl Radical as Intermediates in the Rearrangement of Glutamate to Methylaspartate. J Am Chem Soc 2000. [DOI: 10.1021/ja002488+] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
|
94
|
Taoka S, Padmakumar R, Grissom CB, Banerjee R. Magnetic field effects on coenzyme B12-dependent enzymes: validation of ethanolamine ammonia lyase results and extension to human methylmalonyl CoA mutase. Bioelectromagnetics 2000; 18:506-13. [PMID: 9338632 DOI: 10.1002/(sici)1521-186x(1997)18:7<506::aid-bem6>3.0.co;2-6] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Enzymes with radical-pair intermediates have been considered as a likely target for purported magnetic field effects in humans. The bacterial enzyme ethanolamine ammonia lyase and the human enzyme methylmalonyl-CoA mutase catalyze coenzyme B12-dependent rearrangement reactions. A common step in the mechanism of these two enzymes is postulated to be homolysis of the cobalt-carbon bond of the cofactor to generate a spin-correlated radical pair consisting of the 5'-deoxyadenosyl radical and cob(II)alamin [Ado. Cbl(II)]. Thus, the reactions catalyzed by these enzymes are expected to be sensitive to an applied magnetic field according to the same principles that control radical pair chemical reactions. The magnetic field effect on ethanolamine ammonia lyase reported previously has been corroborated independently in one of the authors' laboratory. However, neither the human nor the bacterial mutase from Propionibacterium shermanii exhibits a magnetic field effect that could be greater than about 15%, considering the error limit imposed by the uncertainty of the coupled assay. Our studies suggest that putative magnetic field effects on physiological processes are not likely to be mediated by methylmalonyl-CoA mutase.
Collapse
Affiliation(s)
- S Taoka
- Biochemistry Department, University of Nebraska, Lincoln 68588-0664, USA
| | | | | | | |
Collapse
|
95
|
Abstract
Adenosylcobalamin (coenzyme B12)-dependent glutamate mutase catalyzes a most unusual carbon skeleton rearrangement involving the isomerization of l-glutamate to L-threo-methylaspartate, a reaction that is without precedent in organic chemistry. This reaction proceeds through a mechanism involving free radical intermediates that are initiated by homolysis of the cobalt-carbon bond of the coenzyme. The enzyme serves as a paradigm for adenosylcobalamin-dependent catalysis and, more generally, provides insights into how enzymes generate and control reactive free radical species. This review describes how recent studies on the mechanism and structure of glutamate mutase have contributed to our understanding of adenosylcobalamin-mediated catalysis. Copyright 2000 Academic Press.
Collapse
Affiliation(s)
- EN Marsh
- Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109-1055
| |
Collapse
|
96
|
Chowdhury S, Banerjee R. Evidence for Quantum Mechanical Tunneling in the Coupled Cobalt−Carbon Bond Homolysis−Substrate Radical Generation Reaction Catalyzed by Methylmalonyl-CoA Mutase. J Am Chem Soc 2000. [DOI: 10.1021/ja994302g] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Shantanu Chowdhury
- Biochemistry Department, University of Nebraska Lincoln, Nebraska 68588-0664
| | - Ruma Banerjee
- Biochemistry Department, University of Nebraska Lincoln, Nebraska 68588-0664
| |
Collapse
|
97
|
Sirovatka JM, Rappé AK, Finke RG. Molecular mechanics studies of coenzyme B12 complexes with constrained CoN(axial-base) bond lengths: introduction of the universal force field (UFF) to coenzyme B12 chemistry and its use to probe the plausibility of an axial-base-induced, ground-state corrin butterfly conformational steric effect. Inorganica Chim Acta 2000. [DOI: 10.1016/s0020-1693(00)00025-6] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
|
98
|
Ribonucleoside Triphosphate Reductase from Lactobacillus leichmannii: Kinetic Evaluation of a Series of Adenosylcobalamin Competitive Inhibitors, [ω-(Adenosin-5′-O-yl)alkyl]cobalamins, Which Mimic the Post Co-C Homolysis Intermediate. Bioorg Chem 1999. [DOI: 10.1006/bioo.1999.1149] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
|
99
|
Maiti N, Widjaja L, Banerjee R. Proton transfer from histidine 244 may facilitate the 1,2 rearrangement reaction in coenzyme B(12)-dependent methylmalonyl-CoA mutase. J Biol Chem 1999; 274:32733-7. [PMID: 10551831 DOI: 10.1074/jbc.274.46.32733] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Methylmalonyl-CoA mutase is an adenosylcobalamin-dependent enzyme that catalyzes the 1,2 rearrangement of methylmalonyl-CoA to succinyl-CoA. This reaction results in the interchange of a carbonyl-CoA group and a hydrogen atom on vicinal carbons. The crystal structure of the enzyme reveals the presence of an aromatic cluster of residues in the active site that includes His-244, Tyr-243, and Tyr-89 in the large subunit. Of these, His-244 is within hydrogen bonding distance to the carbonyl oxygen of the carbonyl-CoA moiety of the substrate. The location of these aromatic residues suggests a possible role for them in catalysis either in radical stabilization and/or by direct participation in one or more steps in the reaction. The mechanism by which the initially formed substrate radical isomerizes to the product radical during the rearrangement of methylmalonyl-CoA to succinyl-CoA is unknown. Ab initio molecular orbital theory calculations predict that partial proton transfer can contribute significantly to the lowering of the barrier for the rearrangement reaction. In this study, we report the kinetic characterization of the H244G mutant, which results in an acute sensitivity of the enzyme to oxygen, indicating the important role of this residue in radical stabilization. Mutation of His-244 leads to an approximately 300-fold lowering in the catalytic efficiency of the enzyme and loss of one of the two titratable pK(a) values that govern the activity of the wild type enzyme. These data suggest that protonation of His-244 increases the reaction rate in wild type enzyme and provides experimental support for ab initio molecular orbital theory calculations that predict rate enhancement of the rearrangement reaction by the interaction of the migrating group with a general acid. However, the magnitude of the rate enhancement is significantly lower than that predicted by the theoretical studies.
Collapse
Affiliation(s)
- N Maiti
- Biochemistry Department, University of Nebraska, Lincoln, Nebraska 68588-0664, USA
| | | | | |
Collapse
|
100
|
Warncke K, Schmidt JC, Ke SC. Identification of a Rearranged-Substrate, Product Radical Intermediate and the Contribution of a Product Radical Trap in Vitamin B12 Coenzyme-Dependent Ethanolamine Deaminase Catalysis. J Am Chem Soc 1999. [DOI: 10.1021/ja984005w] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Kurt Warncke
- Contribution from the Department of Physics, Emory University, Atlanta, Georgia 30322
| | - Jennifer C. Schmidt
- Contribution from the Department of Physics, Emory University, Atlanta, Georgia 30322
| | - Shyue-Chu Ke
- Contribution from the Department of Physics, Emory University, Atlanta, Georgia 30322
| |
Collapse
|