UNEXPECTED PLASTICITY OF THE QUATERNARY STRUCTURE OF IRON-MANGANESE SUPEROXIDE DISMUTASES

Protein 3D structure can be remarkably robust to the accumulation of mutations during evolution. On the other hand, sometimes a single amino acid substitution can be sufficient to generate dramatic and completely unpredictable structural consequences. In an attempt to rationally alter the preferences for the metal ion at the active site of a member of the Iron/Manganese superoxide dismutase family, two examples of the latter phenomenon were identified. Site directed mutants of SOD from Trichoderma reesei were generated and studied crystallographically together with the wild type enzyme. Despite being chosen for their potential impact on the redox potential of the metal, two of the mutations (D150G and G73A) in fact resulted in significant alterations to the protein quaternary structure. The D150G mutant presented alternative inter-subunit contacts leading to a loss of symmetry of the wild type tetramer, whereas the G73A mutation transformed the tetramer into an octamer despite not participating directly in any of the inter-subunit interfaces. We conclude that there is considerable intrinsic plasticity in the Fe/MnSOD fold that can be unpredictably affected by single amino acid substitutions. In much the same way as phenotypic defects at the organism level can reveal much about normal function, so too can such mutations teach us much about the subtleties of protein structure.

An Interprotein Co-S Coordination Complex in the B12-Trafficking Pathway

The CblC and CblD chaperones are involved in early steps in the cobalamin trafficking pathway. Cobalamin derivatives entering the cytoplasm are converted by CblC to a common cob(II)alamin intermediate via glutathione-dependent alkyltransferase or reductive elimination activities. Cob(II)alamin is subsequently converted to one of two biologically active alkylcobalamins by downstream chaperones. The function of CblD has been elusive although it is known to form a complex with CblC under certain conditions. Here, we report that CblD provides a sulfur ligand to cob(II)alamin bound to CblC, forming an interprotein coordination complex that rapidly oxidizes to thiolato-cob(III)alamin. Cysteine scanning mutagenesis and EPR spectroscopy identified Cys-261 on CblD as the sulfur donor. The unusual interprotein Co-S bond was characterized by X-ray absorption spectroscopy and visualized in the crystal structure of the human CblD thiolato-cob(III)alamin complex. Our study provides insights into how cobalamin coordination chemistry could be utilized for cofactor translocation in the trafficking pathway.

Non-heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}⁷ in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO}^6-8 complexes using the TMG₃tren coligand. Redox transformations from the hs-{FeNO}⁷ complex [Fe(TMG₃tren)(NO)]²⁺ to its {FeNO}⁶ and {FeNO}⁸ analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO^-, Fe(III)-NO^-, and Fe(IV)-NO^- complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO}^6-8 complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO}⁸ complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

Development of a Rubredoxin-Type Center Embedded in a de Dovo-Designed Three-Helix Bundle

Protein design is a powerful tool for interrogating the basic requirements for the function of a metal site in a way that allows for the selective incorporation of elements that are important for function. Rubredoxins are small electron transfer proteins with a reduction potential centered near 0 mV (vs normal hydrogen electrode). All previous attempts to design a rubredoxin site have focused on incorporating the canonical CXXC motifs in addition to reproducing the peptide fold or using flexible loop regions to define the morphology of the site. We have produced a rubredoxin site in an utterly different fold, a three-helix bundle. The spectra of this construct mimic the ultraviolet-visible, Mössbauer, electron paramagnetic resonance, and magnetic circular dichroism spectra of native rubredoxin. Furthermore, the measured reduction potential suggests that this rubredoxin analogue could function similarly. Thus, we have shown that an α-helical scaffold sustains a rubredoxin site that can cycle with the desired potential between the Fe(II) and Fe(III) states and reproduces the spectroscopic characteristics of this electron transport protein without requiring the classic rubredoxin protein fold.

Atomic Structure Studies of Zirconia Solid Solutions by EXAFS

We have investigated, using EXAFS, the local atomic structures of four zirconia polymorphs and their solid solutions with Ca2+, Ga3+, Fe3+, y3+, Gd3+, Ge4+, Ti4+, Ce4+ and Nb5+. Structural information up to 9Å, and in one special case up to 10.9Å, from the absorbing atom has been obtained. The characteristic features of local environments of both the host Zr and the dopant cations, and their variations leading to cation ordering and symmetry evolutions, are elucidated in terms of a dopant size effect and an oxygen vacancy effect. The dynamic aspects of the EXAFS Debye-Waller factor are used to shed light on the connection between phase transition, dopant stabilization, and cation-anion correlation.

X-ray absorption spectroscopy and reactivity of thiolate-ligated Fe(III)-OOR complexes

The reaction of a series of thiolate-ligated iron(II) complexes [Fe(II)([15]aneN(4))(SC(6)H(5))]BF(4) (1), [Fe(II)([15]aneN(4))(SC(6)H(4)-p-Cl)]BF(4) (2), and [Fe(II)([15]aneN(4))(SC(6)H(4)-p-NO(2))]BF(4) (3) with alkylhydroperoxides at low temperature (-78 °C or -40 °C) leads to the metastable alkylperoxo-iron(III) species [Fe(III)([15]aneN(4))(SC(6)H(5))(OOtBu)]BF(4) (1a), [Fe(III)([15]aneN(4))(SC(6)H(4)-p-Cl)(OOtBu)]BF(4) (2a), and [Fe(III)([15]aneN(4))(SC(6)H(4)-p-NO(2))(OOtBu)]BF(4) (3a), respectively. X-ray absorption spectroscopy (XAS) studies were conducted on the Fe(III)-OOR complexes and their iron(II) precursors. The edge energy for the iron(II) complexes (∼7118 eV) shifts to higher energy upon oxidation by ROOH, and the resulting edge energies for the Fe(III)-OOR species range from 7121-7125 eV and correlate with the nature of the thiolate donor. Extended X-ray absorption fine structure (EXAFS) analysis of the iron(II) complexes 1-3 in CH(2)Cl(2) show that their solid state structures remain intact in solution. The EXAFS data on 1a-3a confirm their proposed structures as mononuclear, 6-coordinate Fe(III)-OOR complexes with 4N and 1S donors completing the coordination sphere. The Fe-O bond distances obtained from EXAFS for 1a-3a are 1.82-1.85 Å, significantly longer than other low-spin Fe(III)-OOR complexes. The Fe-O distances correlate with the nature of the thiolate donor, in agreement with the previous trends observed for ν(Fe-O) from resonance Raman (RR) spectroscopy, and supported by optimized geometries obtained from density functional theory (DFT) calculations. Reactivity and kinetic studies on 1a- 3a show an important influence of the thiolate donor.

Competing ferromagnetism and superconductivity on FeAs layers in EuFe2(As0.73P0.27)2

We have measured the spin-polarized electron momentum density distributions of EuFe2(As0.73P0.27)2 by magnetic Compton scattering (MCS) measurements. For the first time, we show direct evidence of competing ferromagnetism and superconductivity (SC) on FeAs layers in this iron pnictide system. The MCS orbitalwise decomposition of the density distributions reveals that between 16 and 19 K, the spin-polarized Fe-3d character is enhanced (as the ferromagnetic character supersedes superconducting character), where the resistivity shows a maximum, reentrant SC-like peak, at 18 K. The spin polarization of the Fe-3d orbital, enhanced by ferromagnetic Eu ions, suppresses the SC around 18 K, while at other temperatures the system indeed exhibits SC where the Fe-3d spin polarization is suppressed or collapses.

Modulation of the heme electronic structure and cystathionine beta-synthase activity by second coordination sphere ligands: The role of heme ligand switching in redox regulation

In humans, cystathionine beta-synthase (CBS) is a hemeprotein, which catalyzes a pyridoxal phosphate (PLP)-dependent condensation reaction. Changes in the heme environment are communicated to the active site, which is approximately 20A away. In this study, we have examined the role of H67 and R266, which are in the second coordination sphere of the heme ligands, H65 and C52, respectively, in modulating the heme's electronic properties and in transmitting information between the heme and active sites. While the H67A mutation is comparable to wild-type CBS, interesting differences are revealed by mutations at the R266 site. The pathogenic mutant, R266K, is moderately PLP-responsive while the R266M mutation shows dramatic differences in the ferrous state. The electrostatic interaction between C52 and R266 is critical for stabilizing the ferrous heme and its disruption leads to the facile formation of a 424nm (C-424) absorbing ferrous species, which is inactive, compared to the active 449nm ferrous species for wild-type CBS. Resonance Raman studies on the R266M mutant reveal that the kinetics of C52 rebinding after Fe-CO photolysis are comparable to that of wild-type CBS. EXAFS studies on C-424 CBS are consistent with the presence of two axial N/O low Z scatters with only one being a rigid unit of a histidine residue while the other could be a solvent molecule, an oxygen atom from the peptide backbone or a side chain nitrogen. The redox potential for the heme in full-length CBS is -350+/-4mV and is substantially lower than the value of -287+/-2mV determined for truncated CBS. A redox-regulated ligand change has the potential to serve as an allosteric on/off switch in human CBS and the second sphere ligand, R266, plays an important role in this transition.

Zinc-promoted alkyl transfer: a new role for zinc

The roles of zinc in biology are often thought to be limited to activating water, as in hydrolytic enzymes, and conferring structure, as in the zinc finger proteins. Over the past 15 years, it has been shown that there are many zinc-containing proteins that have 'structural-like' zinc sites with multiple cysteine ligands but in which the site promotes the alkylation of a zinc-bound thiolate. Recent work continues to extend the range of proteins showing zinc-promoted alkytransfer activity, and has refined the structural details of these sites. Of particular interest are recent crystal structures suggesting that in most cases the endogenous ligand that is displaced when the substrate thiol bind is an endogenous amino acid and not water, as had been previously thought. Despite extensive study, it remains unclear whether these enzymes function via an associative mechanism (direct alkylation of a zinc-bound thiolate) or a dissociate mechanism (nucleophilic attack by a free thiolate that has dissociated from the zinc).

Speciation of the Catalytic Oxygen Evolution System: [MnIII/IV2(μ-O)2(terpy)2(H2O)2](NO3)3 + HSO5-

[MnIII/IV2(-O)2(terpy)2(OH2)2](NO3)3 (1, where terpy = 2,2':6'2' '-terpyridine) + oxone (2KHSO5 x KHSO4 x K2SO4) provides a functional model system for the oxygen-evolving complex of photosystem II that is based on a structurally relevant Mn-(-O)2-Mn moiety (Limburg, J.; et al. J. Am. Chem. Soc. 2001, 123, 423-430). In this study, electron paramagnetic resonance, ultraviolet-visible spectroscopy, electrospray ionization mass spectrometry, X-ray absorption spectroscopy, and gas-phase stable isotope ratio mass spectrometry were utilized to identify the title compounds in the catalytic solution. We find that (a) O2 evolution does not proceed through heterogeneous catalysis by MnO2 or other decomposition products, that (b) O atoms from solvent water are incorporated into the evolved O2 to a significant extent but not into oxone, that (c) the MnIII/IV2 title compound 1 is an active precatalyst in the catalytic cycle of O2 evolution with oxone, while the MnIV/IV2 oxidation state is not, and that (d) the isotope label incorporation in the evolved O2, together with points a-c above, is consistent with a mechanism involving competing reactions of oxone and water with a "MnV=O" intermediate in the O-O bond-forming step.

Synthesis, Characterization, and Physicochemical Properties of Manganese(III) and Manganese(V)−Oxo Corrolazines

The structural and physicochemical properties of the manganese-corrolazine (Cz) complexes (TBP8Cz)Mn(V)O (1) and (TBP8Cz)Mn(III) (2) (TBP = p-tert-butylphenyl) have been determined. Recrystallization of 2 from toluene/MeOH resulted in the crystal structure of (TBP8Cz)Mn(III)(CH3OH) (2 x MeOH). The packing diagram of 2 x MeOH reveals hydrogen bonds between MeOH axial ligands and meso N atoms of adjacent molecules. Solution binding studies of 2 with different axial ligands (Cl-, Et3PO, and Ph3PO) reveal strong binding, corroborating the preference of the Mn(III) ion for a five-coordinate environment. High-frequency and field electron paramagnetic resonance (HFEPR) spectroscopy of solid 2 x MeOH shows that 2 x MeOH is best described as a high-spin (S = 2) Mn(III) complex with zero-field splitting parameters typical of corroles. Structural information on 1 was obtained through an X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) study and compared to XANES/EXAFS data for 2 x MeOH. The XANES data for 1 shows an intense pre-edge transition characteristic of a high-valent metal-oxo species, and a best fit of the EXAFS data gives a short Mn-O bond distance of 1.56 A, confirming the structure of the metal-oxo unit in 1. Detailed spectroelectrochemical studies of 1 and 2 were performed revealing multiple reversible redox processes for both complexes, including a relatively low potential for the Mn(V) --> Mn(IV) process in 1 (near 0.0 V vs saturated calomel reference electrode). Chemical reduction of 1 results in the formation of a Mn(III)Mn(IV)(mu-O) dimer as characterized by electron paramagnetic resonance spectroscopy.

Metal Cation−Methyl Interactions in CB11Me12- Salts of Me3Ge+, Me3Sn+, and Me3Pb+

Oxidation of Me(6)M(2) (M = Ge, Sn) and Me(4)Pb with the CB(11)Me(12)(*) radical in alkane solvents produced the insoluble salts Me(3)M(+)CB(11)Me(12)(-), characterized by CP-MAS NMR and EXAFS. The cations interact with methyl groups of CB(11)Me(12)(-) with coordination strength increasing from Pb to Ge. Density functional theory (DFT) calculations for the isolated ion pairs, Me(3)M(+)CB(11)Me(12)(-) (M = Ge, Sn), revealed three isomers with the cation above methyl 2, 7, or 12, and not above a BB edge or a BBB triangle. The interaction has a considerable covalent component, with the cation attempting to perform a backside S(E)2 substitution on the methyl carbon. In a fourth less favorable isomer the cation is near methyl 1, inclined toward methyl 2, and interacts with hydrogens. DFT atomic charge distributions and plots of the electrostatic potential on the surface of spheres centered at the CB(11)H(12)(-) and CB(11)Me(12)(-) icosahedra display the effects of uneven charge distribution within the anion and contradict the common belief that the negative charge of the cage anion is concentrated primarily on the cage boron atoms 7-12; in CB(11)Me(12)(-), roughly half is on the cage carbon and the rest on methyls 7-12.

Is the Allylpalladium Structure Altered between Solid and Solutions?

Recent EXAFS measurements on [(Ph(2)PCH(2)CH(2)PPh(2))Pd(H(2)CCHCMe(2))]O(3)SCF(3) (Tromp et al. J. Am. Chem. Soc. 2002, 124, 14814) were interpreted as evidence that, when the complex is dissolved in THF, the allyl ligand adopts an eta(2) structure with a dangling allyl CH(2) substituent. DFT calculations of the Pd complex using H(2)P-CH(2)CH(2)-PH(2) as a model for Ph(2)P-CH(2)CH(2)-PPh(2) (dppe), in the absence or the presence of the triflate counteranion, and modeling the THF solvent by explicit Me(2)O molecules or by a continuum model give always a conventional eta(3)-H(2)CCHCMe(2) structure with equal Pd-C bonds to the terminal carbon centers of the allyl. QM/MM calculations using the dppe ligand also fail to support an eta(2)-allyl structure as a global minimum. The EXAFS parameter space is shown to have multiple minima. These have very similar overall EXAFS, but have very different structural parameters. The minimum that was the basis for the previous structural conclusion gives a slightly better fit but has unrealistic Debye-Waller factors and threshold energies.

Element-specific detection in capillary electrophoresis using X-ray fluorescence spectroscopy

X-ray fluorescence spectroscopy is demonstrated here as a novel, element-specific detector for capillary electrophoresis. Monochromatic 10 keV X-rays from a synchrotron light source are used to excite core electrons, causing emission of characteristic Kalpha X-ray fluorescence (XRF) lines. Using this technique, XRF energies provide elemental identification, while XRF intensities can be used to quantitate the metal composition of each eluent. An X-ray transparent polymer coupling is used to create a window for the on-line, X-ray detection. This coupling contributes no measurable extra-column variance, and electrophoretic mobilities for the metal complexes used as model solutes are highly reproducible. The combination of XRF detection with capillary electrophoresis (CE-XRF) creates the first on-line detection system that is element-specific, nondestructive, and directly applicable to a broad range of applications including nonelectroactive species. CE-XRF is successfully demonstrated here for high binding-constant complexes of Fe(III), Co(II), Cu(II), and Zn(II). Within a single injection, electropherograms are obtained for each element of interest, with the element identity obtained directly from the emission energy. In contrast with ICPMS, this detection technique is directly on-line and does not require volatilization of the eluent. As a result, element-specific detection is not limited by the sample or the buffer volatility or atomization efficiency. Simultaneous XRF and UV absorbance detection can be used to provide an on-line determination of metal/chelate ratios. Although XRF detection limits are presently only in the 0.1 mM (0.5 ng) range, both collection geometry and incident intensity have yet to be optimized. Further optimization is expected to enhance this detection limit by another 2-3 orders of magnitude. As a result, the advent of XRF detection combined with the separating power of CE presents new possibilities for on-line, element-specific analysis.

The zinc coordination site of the bacteriophage Mu translational activator protein, Com

The bacteriophage Mu Com protein is a small "zinc finger-like" protein that binds a specific site in com-mom operon mRNA and activates translation of the mom open-reading-frame. Com contains six cysteine and five histidine residues that have the potential to form several alternative zinc-finger-like motifs. We have used oligonucleotide site-directed mutagenesis to individually alter each of these amino acids (Cys to Ser, and His to Asn or Gln) and tested the various forms of Com for their ability to function in vivo. We observed that mutation of any one of the four N-terminal cysteine residues (Cys-6, 9, 26 or 29) resulted in loss of Com activity. The Com protein requires zinc in order to fold into its functional tertiary structure, as demonstrated by characteristic 1H nuclear magnetic resonance (NMR) chemical shifts. 1H chemical shifts revert to random coil values in the presence of the metal chelator EDTA. The metal-binding specificity and thermal stability of Com also has been investigated using 1H NMR. We report the use of 113Cd NMR, 1H-113Cd heteronuclear spin-echo difference spectroscopy HSED and Zn extended X-ray absorption fine structure spectroscopy EXAFS to determine the zinc/protein stoichiometry as 1:1 and the ligand environment as tetrathiolate. Comparative NMR spectra of Com mutants C6S and C39S suggest position 6 is involved in zinc coordination, while position 39 is not metal-liganded. These studies indicate that the metal coordination, site of Com is a four-cysteine complex, involving residues 6, 9, 26 and 29.

Pharmaceutical prices, quantities and innovation. Comparing Japan with the US

Per capita expenditure on pharmaceuticals is higher in Japan than in the US, despite a series of drug price reductions instigated by the Japanese Ministry of Health and Welfare that began in 1981. For some individual products, these price reductions cumulatively totalled more than 50%. This article argues that although the price of individual drugs is lower in Japan than in the US, aggregate expenditure is higher because of the greater use of newly-introduced original drugs and lower use of generics. Providers and consumers also tend to use drugs in larger quantities in Japan, because of polypharmacy and greater use of vitamins and nutrients, antihypertensives, cerebral metabolic activators (e.g. idebenone) and milder-acting drugs (i.e. drugs with low toxicity but unproven clinical efficacy). The level of expenditure is unlikely to decline, despite changes to pricing policy and ongoing efforts to improve the pharmaceutical distribution system and to discourage physician dispensing activities.