Electronic configuration assignment and the importance of low-lying excited states in high-spin imidazole-ligated iron(II) porphyrinates

The synthesis and characterization of six new high-spin deoxymyoglobin models (imidazole(tetraarylporphyrinato)iron(II)) are described. These have been intensively studied by temperature-dependent Mossbauer spectroscopy from 295 to 4.2 K. All complexes show a strong temperature dependence for the quadrupole splitting consistent with low-lying excited states of the same or lower multiplicity. An analysis of the data obtained in applied magnetic fields leads to the assignment of the sign of the quadrupole splitting. All model compounds as well as those of deoxymyoglobin and deoxyhemoglobin, previously studied, have a negative sign for the quadrupole splitting. Although not previously predicted, this experimental observation leads to the assignment of the ground-state electronic configuration for all high-spin imidazole-ligated iron(II) porphyrinates as (d(xz)())(2)(d(yz)())(1)(d(xy)())(1)(d(z)()()2)(1)(d(x)()()2(-)(y)()()2)(1). This is a distinctly different ground-state electronic configuration from other high-spin iron(II) porphyrinates; differences in structural details for the two classes of high-spin complexes are also discussed. The apparent anomaly of differing signs for the zero-field splitting constant between previously studied model complexes and the heme proteins is addressed; the difference appears to result from the fact that the assumptions used in the spin Hamiltonian approach that has been applied to these complexes are not adequately satisfied. Structures of four of the new five-coordinate species have been determined. Core conformations in these derivatives show variation, but these and previously studied compounds reveal a limited number of conformational patterns. The bond lengths and other geometrical parameters such as porphyrin core size and iron out-of-plane displacement support a high-spin state assignment for the iron(II).

Ligand binding properties of myoglobin reconstituted with iron porphycene: unusual O2 binding selectivity against CO binding

Sperm whale myoglobin, an oxygen storage hemoprotein, was successfully reconstituted with the iron porphycene having two propionates, 2,7-diethyl-3,6,12,17-tetramethyl-13,16-bis(carboxyethyl)porphycenatoiron. The physicochemical properties and ligand bindings of the reconstituted myoglobin were investigated. The ferric reconstituted myoglobin shows the remarkable stability against acid denaturation and only a low-spin characteristic in its EPR spectrum. The Fe(III)/Fe(II) redox potential (-190 mV vs NHE) determined by the spectroelectrochemical measurements was much lower than that of the wild-type. These results can be attributed to the strong coordination of His93 to the porphycene iron, which is induced by the nature of the porphycene ring symmetry. The O2 affinity of the ferrous reconstituted myoglobin is 2600-fold higher than that of the wild-type, mainly due to the decrease in the O2 dissociation rate, whereas the CO affinity is not so significantly enhanced. As a result, the O2 affinity of the reconstituted myoglobin exceeds its CO affinity (M' = K(CO)/K(O2) < 1). The ligand binding studies on H64A mutants support the fact that the slow O2 dissociation of the reconstituted myoglobin is primarily caused by the stabilization of the Fe-O2 sigma-bonding. The IR spectra for the carbon monoxide (CO) complex of the reconstituted myoglobin suggest several structural and/or electrostatic conformations of the Fe-C-O bond, but this is not directly correlated with the CO dissociation rate. The high O2 affinity and the unique characteristics of the myoglobin with the iron porphycene indicate that reconstitution with a synthesized heme is a useful method not only to understand the physiological function of myoglobin but also to create a tailor-made function on the protein.

Leg ischemia: assessment with MR angiography and spectroscopy

PURPOSE

To prospectively determine reproducibility of magnetic resonance (MR) angiography and MR spectroscopy of deoxymyoglobin in assessment of collateral vessels and tissue perfusion in patients with critical limb ischemia (CLI) and to follow changes in patients undergoing intramuscular vascular endothelial growth factor (pVEGF)-C gene therapy, percutaneous transluminal angioplasty, supervised exercise training, or no therapy.

MATERIALS AND METHODS

Study and gene therapy protocols were approved, and all patients gave written informed consent. To determine repeatability and reproducibility, seven patients underwent MR angiography and five underwent MR spectroscopy. The techniques were used to judge disease progress in 12 other patients with or without therapy: MR angiography to help determine change in visualization of collateral vessels and MR spectroscopy to help assess change in perfusion at proximal and distal calf levels. MR angiographic results were subjectively analyzed by three blinded readers. Intraobserver variability was expressed as 95% confidence interval (CI) (n=7); interobserver variability, as kappa statistic (n=15). Reexamination variability of MR spectroscopy was given as 95% CI for subsequent recovery times, and correlation with disease extent was calculated with Kendall taub rank correlation. Fisher-Yates test was used to correlate changes with pressure measurements and clinical course.

RESULTS

Intraobserver and interobserver concordance was sensitive for detection of collateral vessels. Intraobserver agreement was 85.7% (95% CI: 42.1%, 99.6%). Interobserver agreement was high for small collateral vessels (kappa=0.74, P <.001) and fair for large collateral vessels (kappa=0.36, P=.002). MR spectroscopy was reproducible (95% CI: +/-26 seconds for proximal, +/-21 seconds for distal) and showed a correlation with disease extent (proximal calf, taub=0.84, P <.001; distal calf, taub=0.68, P=.04). Small collateral vessels increased over time (P=.04) but did not correlate with pressure measurements and clinical course. Recovery time correlated with clinical course (proximal calf, P=.03; distal calf, P=.005).

CONCLUSION

MR angiography and MR spectroscopy of deoxymyoglobin can help document changes in visualization of collateral vessels and tissue perfusion in patients with CLI.

Design of a five-coordinate heme protein maquette: a spectroscopic model of deoxymyoglobin

The substitution of 1-methyl-l-histidine for the histidine heme ligands in a de novo designed four-alpha-helix bundle scaffold results in conversion of a six-coordinate cytochrome maquette into a self-assembled five-coordinate mono-(1-methyl-histidine)-ligated heme as an initial maquette for the dioxygen carrier protein myoglobin. UV-vis, magnetic circular dichroism, and resonance Raman spectroscopies demonstrate the presence of five-coordinate mono-(1-methyl-histidine) ligated ferrous heme spectroscopically similar to deoxymyoglobin. Thermodynamic analysis of the ferric and ferrous heme dissociation constants indicates greater destabilization of the ferric state than the ferrous state. The ferrous heme protein reacts with carbon monoxide to form a (1-methyl-histidine)-Fe(II)(heme)-CO complex; however, reaction with dioxygen leads to autoxidation and ferric heme dissociation. These results indicate that negative protein design can be used to generate a five-coordinate heme within a maquette scaffold.

Bonding in HNO-Myoglobin as Characterized by X-ray Absorption and Resonance Raman Spectroscopies

The EXAFS and resonance Raman spectra on the HNO-myoglobin adduct, 1, are consistent with the presence of HNO bound to a heme center. The three-dimensional structure about the heme center of 1 obtained from multiple-scattering (MS) analysis of the EXAFS of the heme protein yielded an Fe-N-O bond angle of 131 degrees and an Fe-N bond length of 1.82 A, which compare well with published values for model complexes containing RNO ligands. Resonance Raman spectra identified the nu(N=O) stretch at 1385 cm-1 (confirmed by 15N labeling), which corresponds well with those reported for small molecule HNO complexes. The wavelength of the nu(Fe-N) at 636 cm-1 of 1 is significantly higher than those of MbIINO and MbIIINO (554 and 595 cm-1, respectively). The XAFS, XANES, and resonance Raman data are all consistent with the structure deduced from the NMR experiments, providing more detail on the bonding between HNO and the metal center.

Mechanism of nitrosylmyoglobin autoxidation: temperature and oxygen pressure effects on the two consecutive reactions

As shown by singular value decomposition and global analysis of the absorption spectra, oxidation of nitrosylmyoglobin, MbFe(II)NO, by oxygen occurs in two consecutive (pseudo) first-order reactions in aqueous air- saturated solutions at physiological conditions (pH 7.0, I=0.16 m (NaCl)). Both reaction steps have a large temperature dependence with the following activation parameters: DeltaS++(1) = 121+/-7 and DeltaS++(1) = 23+/-29; and DeltaS++(2) = 88+/-14 kJ mol(-1) and DeltaS++(2)-63+/-51 J(-1) K(-1) mol(-1) at 25 degrees C for the first and second step, respectively. At physiological temperature, the initial reaction is faster, while at lower temperatures, the first reaction is slower and rate-determining. The rate of the first reaction is linearly dependent on oxygen pressure at lower pressures, while for oxygen pressures above atmospheric, the rate exhibits saturation behaviour. The second reaction is independent of oxygen pressure. The rate of the second reaction increases when oxymyoglobin is added. In contrast, the rate of the first reaction is independent of the presence of oxymyoglobin. The observed kinetics are in agreement with a reaction mechanism in which the nitric oxide that is initially bound to the Fe(II) centre of myoglobin is displaced by oxygen in a reversible ligand-exchange reaction prior to an irreversible electron transfer. The ligand-exchange process is dissociative in nature and depends bond breaking, and nitric oxide is suggested to be trapped in a protein cavity. The absorption spectrum of the intermediate, as resolved from the global analysis, is in agreement with a peroxynitrite complex, and the initial process must involve partial electron transfer.

Different relaxations in myoglobin after photolysis

To clarify the interplay of kinetic hole-burning (KHB), structural relaxation, and ligand migration in myoglobin (Mb), we measured time-resolved absorption spectra in the Soret region after photolysis of carbon monoxide Mb (MbCO) in the temperature interval 120-260 K and in the time window 350 ns to 200 ms. The spectral contributions of both photolyzed (Mb*) and liganded Mb (MbCO) have been analyzed by taking into account homogeneous bandwidth, coupling to vibrational modes, and static conformational heterogeneity. We succeeded in separating the "time-dependent" spectral changes, and this work provides possibilities to identify the events in the process of ligand rebinding. KHB is dominant at T <190 K in both the Mb* and the MbCO components. For MbCO, conformational substates interconversion at higher temperatures tends to average out the KHB effect. At 230-260 K, whereas almost no shift is observed in the MbCO spectrum, a shift of the order of approximately 80 cm(-1) is observed in Mb*. We attribute this shift to protein relaxation coupled to ligand migration. The time dependence of the Mb* spectral shift is interpreted with a model that enables us to calculate the highly nonexponential relaxation kinetics. Fits of stretched exponentials to this kinetics yield Kohlrausch parameter values of 0.25, confirming the analogy between proteins and glasses.

Human heme oxygenase oxidation of 5- and 15-phenylhemes

Human heme oxygenase-1 (hHO-1) catalyzes the O2-dependent oxidation of heme to biliverdin, CO, and free iron. Previous work indicated that electrophilic addition of the terminal oxygen of the ferric hydroperoxo complex to the alpha-meso-carbon gives 5-hydroxyheme. Earlier efforts to block this reaction with a 5-methyl substituent failed, as the reaction still gave biliverdin IXalpha. Surprisingly, a 15-methyl substituent caused exclusive cleavage at the gamma-meso-rather than at the normal, unsubstituted alpha-meso-carbon. No CO was formed in these reactions, but the fragment cleaved from the porphyrin eluded identification. We report here that hHO-1 cleaves 5-phenylheme to biliverdin IXalpha and oxidizes 15-phenylheme at the alpha-meso position to give 10-phenylbiliverdin IXalpha. The fragment extruded in the oxidation of 5-phenylheme is benzoic acid, one oxygen of which comes from O2 and the other from water. The 2.29- and 2.11-A crystal structures of the hHO-1 complexes with 1- and 15-phenylheme, respectively, show clear electron density for both the 5- and 15-phenyl rings in both molecules of the asymmetric unit. The overall structure of 15-phenylheme-hHO-1 is similar to that of heme-hHO-1 except for small changes in distal residues 141-150 and in the proximal Lys18 and Lys22. In the 5-phenylheme-hHO-1 structure, the phenyl-substituted heme occupies the same position as heme in the heme-HO-1 complex but the 5-phenyl substituent disrupts the rigid hydrophobic wall of residues Met34, Phe214, and residues 26-42 near the alpha-meso carbon. The results provide independent support for an electrophilic oxidation mechanism and support a role for stereochemical control of the reaction regiospecificity.

Efficient trapping of HNO by deoxymyoglobin

Nitrosyl hydride, HNO, also commonly termed nitroxyl, is a transient species that has been implicated in the biological activity of nitric oxide, NO. Herein, we report the first generation of a stable HNO-metal complex by direct trapping of free HNO. Deoxymyoglobin (Mb-Fe(II)) rapidly reacts with HNO produced from the decomposition of methylsulfonylhydroxylamine (MSHA) or Angeli's salt (AS) in aqueous solutions from pH 7 to pH 10, forming an adduct, Mb-HNO. The unique 1H NMR signal of the Fe-bound HNO at 14.8 ppm allows definitive proof of its formation. The generation of Mb-HNO and quantification of various myoglobin byproducts were accomplished by correlation of 1H NMR, UV-vis, and EPR spectroscopies. Typically, the maximum Mb-HNO yield obtained is 60-80%; competitive side reactions with byproducts as well as the further reactivity of the Mb-HNO decrease the overall yield. At pH 10, the observed rate of Mb-HNO generation by trapping HNO from MSHA is close to that for MSHA decomposition; kinetic simulations give a lower limit to the bimolecular rate of trapping as 1.4 x 10(4) M(-1) s(-1). The binding of HNO to deoxymyoglobin is rapid and essentially irreversible, which suggests that the biological activity of nitroxyl may be mediated by its reactivity with ferrous heme proteins such as myoglobin and hemoglobin.

Engineering peroxidase activity in myoglobin: the haem cavity structure and peroxide activation in the T67R/S92D mutant and its derivative reconstituted with protohaemin-l-histidine

Atomic co-ordinates and structure factors for the T67R/S92D metMbCN mutant have been deposited with the Protein Data Bank, under accession codes 1h1x and r1h1xsf, respectively. Protein engineering and cofactor replacement have been employed as tools to introduce/modulate peroxidase activity in sperm whale Mb (myoglobin). Based on the rationale that haem peroxidase active sites are characterized by specific charged residues, the Mb haem crevice has been modified to host a haem-distalpropionate Arg residue and a proximal Asp, yielding the T67R/S92D Mb mutant. To code extra conformational mobility around the haem, and to increase the peroxidase catalytic efficiency, the T67R/S92D Mb mutant has been subsequently reconstituted with protohaem-L-histidine methyl ester, yielding a stable derivative, T67R/S92D Mb-H. The crystal structure of T67R/S92D cyano-metMb (1.4 A resolution; R factor, 0.12) highlights a regular haem-cyanide binding mode, and the role for the mutated residues in affecting the haem propionates as well as the neighbouring water structure. The conformational disorder of the haem propionate-7 is evidenced by the NMR spectrum of the mutant. Ligand-binding studies show that the iron(III) centres of T67R/S92D Mb, and especially of T67R/S92D Mb-H, exhibit higher affinity for azide and imidazole than wild-type Mb. In addition, both protein derivatives react faster than wild-type Mb with hydrogen peroxide, showing higher peroxidase-like activity towards phenolic substrates. The catalytic efficiency of T67R/S92D Mb-H in these reactions is the highest so far reported for Mb derivatives. A model for the protein-substrate interaction is deduced based on the crystal structure and on the NMR spectra of protein-phenol complexes.

Residues in the Distal Heme Pocket of Neuroglobin

Amino acid residues in the ligand binding pocket of human neuroglobin have been identified by site-directed mutagenesis and their properties investigated by resonance Raman and flash photolysis methods. Wild-type neuroglobin has been shown to have six-coordinate heme in both ferric and ferrous states. Substitution of His96 by alanine leads to complete loss of heme, indicating that His96 is the proximal ligand. The resonance Raman spectra of M69L and K67T mutants were similar to those of wild-type (WT) neuroglobin in both ferric and ferrous states. By contrast, H64V was six-coordinate high-spin and five-coordinate high-spin in the ferric and ferrous states, respectively, at acidic pH. The spectra were pH-dependent and six-coordinate with the low-spin component dominating at alkaline pH. In a double mutant H64V/K67T, the high-spin component alone was detected in the both ferric and the ferrous states. This implies that His64 is the endogenous ligand and that Lys67 is situated nearby in the distal pocket. In the ferrous H64V and H64V/K67T mutants, the nu(Fe-His) stretching frequency appears at 221 cm(-1), which is similar to that of deoxymyoglobin. In the ferrous CO-bound state, the nu(Fe-CO) stretching frequency was detected at 521 and 494 cm(-1) in WT, M69L, and K67T, while only the 494 cm(-1) component was detected in the H64V and H64V/K67T mutants. Thus, the 521 cm(-1) component is attributed to the presence of polar His64. The CO binding kinetics were biphasic for WT, H64V, and K67T and monophasic for H64V/K67T. Thus, His64 and Lys67 comprise a unique distal heme pocket in neuroglobin.

NO-Bound Myoglobin: Structural Diversity and Dynamics of the NO Ligand

We used femtosecond infrared polarization spectroscopy and density functional theory in a study on the key signaling molecule nitric oxide (NO) bound to myoglobin. Our results show that after photolysis, a substantial fraction of NO recombines within the first few picoseconds. We discovered that the diatomic ligand is severely tilted in the protein and present evidence that the Fe-NO moiety can sample a wide range of off-axis tilting and bending conformations.

Effects of systematic peripheral group deuteration on the low-frequency resonance Raman spectra of myoglobin derivatives

Resonance Raman spectra are reported for a series of systematically deuterated analogues of myoglobin in its deoxy state as well as for its CO and O(2) adducts. Specifically, the myoglobin samples studied are those that have been reconstituted with deuterated protoheme analogues. These include the methine deuterated, protoheme-d4; analogue bearing C(2)H(3) groups at the 1, 3, 5, and 8 positions, protoheme-d12; the species bearing C(2)H(3) groups at the 1 and 3 positions only, 1,3-protoheme-d6; and the species bearing C(2)H(3) groups at the 5 and 8 positions only, 5,8-protoheme-d6. While the results are generally consistent with previously reported data for synthetic metalloporphyrin models and previous studies of labeled heme proteins, the high-quality low-frequency RR data reported here reveal several important aspects of these low-frequency modes. Of special interest is the fact that, using the two d6-protoheme analogues, it is shown that certain modes are apparently localized on particular pyrrole rings, while others are localized on different rings; i.e., several of these low-frequency modes are localized on one side of the heme.

Hybridization of Modified-Heme Reconstitution and Distal Histidine Mutation to Functionalize Sperm Whale Myoglobin

To modulate the physiological function of a hemoprotein, most approaches have been demonstrated by site-directed mutagenesis. Replacement of the native heme with an artificial prosthetic group is another way to modify a hemoprotein. However, an alternate method, mutation or heme reconstitution, does not always demonstrate sufficient improvement compared with the native heme enzyme. In the present study, to convert a simple oxygen storage hemoprotein, myoglobin, into an active peroxidase, we applied both methods at the same time. The native heme of myoglobin was replaced with a chemically modified heme 2 having two aromatic rings at the heme-propionate termini. The constructed myoglobins were examined for 2-methoxyphenol (guaiacol) oxidation in the presence of H2O2. Compared with native myoglobin, rMb(H64D.2) showed a 430-fold higher kcat/Km value, which is significantly higher than that of cytochrome c peroxidase and only 3-fold less than that of horseradish peroxidase. In addition, myoglobin-catalyzed degradation of bisphenol A was examined by HPLC analysis. The rMb(H64D.2) showed drastic acceleration (>35-fold) of bisphenol A degradation compared with the native myoglobin. In this system, a highly oxidized heme reactive species is smoothly generated and a substrate is effectively bound in the heme pocket, while native myoglobin only reversibly binds dioxygen. The present results indicate that the combination of a modified-heme reconstitution and an amino acid mutation should offer interesting perspectives toward developing a useful biomolecule catalyst from a hemoprotein.

NMR investigation of the heme electronic structure in deoxymyoglobin possessing a fluorinated heme

The heme electronic structures of deoxymyoglobins (deoxy-Mbs) reconstituted with 13,17-bis(2-carboxylatoethyl)-3,8-diethyl-2,12,18-trimethyl-7-(trifluoromethyl)porphyrinatoiron(III) (7-PF), 13,17-bis(2-carboxylatoethyl)-3,7-difluoro-2,8,12,18-tetramethylporphyrinatoiron(III) (3,7-DF), and 13,17-bis(2-carboxylatoethyl)-3,8-diethyl-2-fluoro-7,12,18-trimethylporphyrinatoiron(III) (2-MF) have been characterized by (1)H and (19)F NMR. The analysis of heme methyl proton shift patterns of the hemes in their bis-cyano forms demonstrated that, owing to the substitution of a strongly electron-withdrawing perfluoromethyl group, CF(3), to porphyrin, the porphyrin pi-system of 7-PF is more significantly distorted from four-fold symmetry than those of the ring-fluorinated hemes, 3,7-DF and 2-MF. The presence of the heme orientation disorder resulted in the observation of the two well-resolved (19)F signals in the spectra of deoxy-Mbs possessing 7-PF and 2-MF. The (19)F signals of deoxy-Mb possessing 7-PF exhibited a relatively large difference in paramagnetic shift (approximately 30 ppm), despite their small paramagnetic shifts (approximately 30 ppm), supporting the significant contribution of a pi spin delocalization mechanism in this Mb due to the d-electron configuration derived from the (5)E ground state. On the other hand, (19)F signals of deoxy-Mbs with 3,7-DF as well as 2-MF exhibited large paramagnetic shifts (approximately 250 ppm) with a relatively small difference in the paramagnetic shift (approximately 20 ppm), indicating the predominant contribution of spin delocalization, due to a d-electron configuration derived from the (5)B(2) ground state. These results demonstrate for the first time that the relative contributions of the orbital ground states derived from (5)E and (5)B(2) states to the heme electronic structure in deoxy-Mb are affected by the distortion of the porphyrin pi-system exerted by chemical properties of the heme peripheral side-chains.

Myocardial oxygenation and high-energy phosphate levels during KATP channel blockade

Inhibition of ATP-sensitive K+ (KATP) channel activity has previously been demonstrated to result in coronary vasoconstriction with decreased myocardial blood flow and loss of phosphocreatine (PCr). This study was performed to determine whether the high-energy phosphate abnormality during KATP channel blockade can be ascribed to oxygen insufficiency. Myocardial blood flow and oxygen extraction were measured in open-chest dogs during KATP channel blockade with intracoronary glibenclamide, whereas high-energy phosphates were examined with 31P magnetic resonance spectroscopy (MRS), and myocardial deoxymyoglobin (Mb-delta) was determined with 1H MRS. Glibenclamide resulted in a 20 +/- 8% decrease of myocardial blood flow that was associated with a loss of phosphocreatine (PCr) and accumulation of inorganic phosphate. Mb-delta was undetectable during basal conditions but increased to 58 +/- 5% of total myoglobin during glibenclamide administration. This degree of myoglobin desaturation during glibenclamide was far greater than we previously observed during a similar reduction of blood flow produced by a coronary stenosis (22% of myoglobin deoxygenated during stenosis). The findings suggest that reduction of coronary blood flow with an arterial stenosis was associated with a decrease of myocardial energy demands and that this response to hypoperfusion was inhibited by KATP channel blockade.

Solution NMR characterization of the electronic structure and magnetic properties of high-spin ferrous heme in deoxy myoglobin from Aplysia limacina

Solution (1)H NMR has been used to elucidate the magnetic properties and electronic structure of the prosthetic group in high-spin, ferrous deoxy myoglobin from the sea hare Aplysia limacina. A sufficient number of dipolar shifted residue signals were assigned to allow the robust determination of the orientation and anisotropy of the paramagnetic susceptibility tensor, chi. The resulting quantitative description of dipolar shifts allows a determination of the contact shifts for the heme. Chi was found to be axial, with Deltachi(ax) = -2.07 x 10(-8) m(3)/mol, with the major axis tilted (approximately 76 degrees) almost into the heme plane and in the general direction of the orientation of the axial HisF8 imidazole plane which coincides approximately with the beta-,delta-meso axis. The factored contact shifts for the heme are shown to be consistent with the transfer of positive pi spin density into one of the two components of the highest filled pi molecular orbital, 3e(pi), and the transfer of negative pi-spin density, via spin-spin correlation, into the orthogonal excited-state component of the 3e(pi) molecular orbital. The thermal population of the excited state leads to strong deviation from the Curie law for the heme substituents experiencing primarily the negative pi-spin density. The much larger transfer of negative spin density via the spin-paired dpi orbital into the excited state 3e(pi) in high-spin iron(II) than in low-spin iron(III) hemoproteins is attributed to the much stronger correlation exerted by the four unpaired spin on the iron in the former, as compared to the single unpaired spins on iron in the latter.

The magnetic properties of myoglobin as studied by NMR spectroscopy

Deoxymyoglobin has been investigated by NMR spectroscopy to determine the magnetic anisotropy through pseudocontact shifts and the total magnetic susceptibility through Evans measurements. The magnetic anisotropy values were found to be Deltachi(ax)=-2.03+/-0.08 x 10(-32) m(3) and Deltachi(rh)=-1.02+/-0.09 x 10(-32) m(3). The negative value of the axial susceptibility anisotropy originates from the z tensor axis lying in the heme plane, unlike all other heme systems investigated so far. This magnetic axis is almost exactly orthogonal to the axial histidine plane. The other two axes lie essentially in the histidine plane, the closest to the heme normal being tilted by about 36 degrees from it, towards pyrrole A on the side of the proximal histidine. From the comparison with cytochrome c' it clearly appears that the position of the one axis lying in the heme plane is related to the axial histidine orientation. Irrespective of the directions, the magnetic anisotropy is smaller than that of the analogous reduced cytochrome c' and of the order of that of low-spin iron(III). The magnetic anisotropy of the system permits the measurement of residual dipolar couplings, which, together with pseudocontact shifts, prove that the solution structure is very similar to that in the crystalline state. Magnetic measurements, at variance with previous data, demonstrate that there is an orbital contribution to the magnetic moment, micro(eff)=5.5 micro(B). Finally, from the magnetic anisotropy data, the hyperfine shifts of iron ligands could be separated in pseudocontact and contact components, and hints are provided to understand the spin-delocalisation mechanism in S=2 systems by keeping in mind the delocalisation patterns in low-spin S=1/2 and high-spin S= 5/2 iron(III) systems.

Oxygen equilibrium properties of myoglobin locked in the liganded and unliganded conformations

A comparison of the O(2) equilibrium curves of sperm-whale myoglobin locked in the liganded (CO-bound) and unliganded (deoxy) conformations by encapsulation in a wet porous sol-gel silica reveals a marked difference between them. The CO-bound state-locked myoglobin showed a nearly monophasic (hyperbolic) O(2) equilibrium curve with a dissociation constant of 0.2 Torr, which is smaller than that of myoglobin in solution (0.5 Torr). On the other hand, the deoxy state-locked myoglobin exhibited a multiphasic O(2) equilibrium curve that can be represented by a sum of three independent components with dissociation constants of 0.19, 0.90, and 44 Torr, respectively, indicating that deoxymyoglobin exists in multiple conformations. These results show that myoglobin can be frozen into ligand-dependent conformational populations at room temperature in the wet sol-gel and suggest that the overall O(2) equilibrium properties of myoglobin in solution are generated by a redistribution of protein conformational populations in response to ligand binding.

Equilibrium O2 distribution in the Zn2+-protoporphyrin IX deoxymyoglobin mimic: application to oxygen migration pathway analysis

Proton spin relaxation induced by the triplet ground state of O(2) in the zinc-containing diamagnetic analogue of sperm whale deoxymyoglobin has been measured as a function of oxygen concentration. As no covalent binding of oxygen to the metal occurs in the zinc species, the relaxation effects of O(2) on the protein (1)H resonances arise exclusively via much weaker noncovalent interactions. The relaxation effects at the amide proton sites are found to be highly localized and are derived almost exclusively from O(2) binding at the four previously identified xenon binding sites. Relative binding constants of 1.0, 0.08, 0.07, and 0.23 were determined for the Xe 1, Xe 2, Xe 3, and Xe 4 sites, respectively. In combination with earlier measurements of the kinetics of the heme binding of oxygen, these equilibria measurements enable a more detailed analysis of models characterizing O(2) entry and egress. A correlation is established between the fraction of O(2) which enters the Fe(2+)-binding site via rotation of the distal histidine side chain (so-called "histidine gate") and the experimentally observable O(2) (or CO) lifetime in the Xe 1 site. A physiological role for these secondary oxygen binding sites is proposed in enhancing the efficiency of the O(2) association reaction by rendering more favorable its competition with water binding in the distal heme pocket.

A water network within a protein: temperature-dependent water ligation in H64V-metmyoglobin and relaxation to deoxymyoglobin

The sperm whale myoglobin mutant H64V, where the distal histidine is mutated to valine, is known to be five coordinated in the ferric state at room temperature and physiological pH. A change of the ligation in this H64V-Mbmet has been observed by optical absorption spectroscopy as a function of temperature from 20 K to 300 K. Above the dynamical transition at about 180 K one observes the temperature-dependent equilibrium between five- and six-ligated heme. Below the dynamical transition the equilibrium is frozen-in at about 50% of six-coordinate molecules. The water ligation of the iron occurs at temperatures where protein-specific motions are present, as monitored by Mössbauer spectroscopy. The X-ray structures of H64V-Mbmet at 300 K and 110 K are reported with a resolution of 1.5 A and 1.3 A, respectively. The measurements at high resolutions are possible owing to crystallization in the space group P2(1), whereas all mutant myoglobins studies up to now have been carried out with crystals in the space group P6. The overall structure at both temperatures is very close to the native myoglobin. The binding of water at the sixth coordination site at lower temperatures is possible owing to a stabilizing water network extending from the protein surface to the active centre. The reduction of the H64V-Mbmet by electrons obtained by X-ray irradiation of the water-glycerol solvent at 85 K produces an intermediate low-spin state of the water-ligated molecules where Fe(II) retains the six-fold coordination. Mössbauer spectroscopy shows that the relaxation of the metastable low-spin state to high-spin H64V-Mbdeoxy with dissociation of the Fe(II)-H(2)O bond starts at about 115 K and is completed at about 170 K. Differences in the dynamics properties of the native and mutant myoglobin and the connection to the dynamical transition around 180 K are discussed.

Nuclear magnetic resonance shifts in paramagnetic metalloporphyrins and metalloproteins

We report the first detailed investigation of the (1)H, (13)C, (15)N, and (19)F nuclear magnetic resonance (NMR) spectroscopic shifts in paramagnetic metalloprotein and metalloporphyrin systems. The >3500 ppm range in experimentally observed hyperfine shifts can be well predicted by using density functional theory (DFT) methods. Using spin-unrestricted methods together with large, locally dense basis sets, we obtain very good correlations between experimental and theoretical results: R(2) = 0.941 (N = 37, p < 0.0001) when using the pure BPW91 functional and R(2) = 0.981 (N = 37, p < 0.0001) when using the hybrid functional, B3LYP. The correlations are even better for C(alpha) and C(beta) shifts alone: C(alpha), R(2) = 0.996 (N = 8, p < 0.0001, B3LYP); C(beta), R(2) = 0.995 (N = 8, p < 0.0001, B3LYP), but are worse for C(meso), in part because of the small range in C(meso) shifts. The results of these theoretical calculations also lead to a revision of previous heme and proximal histidine residue (13)C NMR assignments in deoxymyoglobin which are confirmed by new quantitative NMR measurements. Molecular orbital (MO) analyses of the resulting wave functions provide a graphical representation of the spin density distribution in the [Fe(TPP)(CN)(2)](-) (TPP = 5,10,15,20-tetraphenylporphyrinato) system (S = (1)/(2)), where the spin density is shown to be localized primarily in the d(xz) (or d(yz)) orbital, together with an analysis of the frontier MOs in Fe(TPP)Cl (S = (5)/(2)), Mn(TPP)Cl (S = 2), and a deoxymyoglobin model (S = 2). The ability to now begin to predict essentially all heavy atom NMR hyperfine shifts in paramagnetic metalloporphyrins and metalloproteins using quantum chemical methods should open up new areas of research aimed at structure prediction and refinement in paramagnetic systems in much the same way that DFT methods have been used successfully in the past to predict/refine elements of diamagnetic heme protein structures.

Effect of mild carboxy-hemoglobin on exercising skeletal muscle: intravascular and intracellular evidence

We studied muscle blood flow, muscle oxygen uptake (VO(2)), net muscle CO uptake, Mb saturation, and intracellular bioenergetics during incremental single leg knee-extensor exercise in five healthy young subjects in conditions of normoxia, hypoxia (H; 11% O(2)), normoxia + CO (CO(norm)), and 100% O(2) + CO (CO(hyper)). Maximum work rates and maximal oxygen uptake (VO(2 max)) were equally reduced by approximately 14% in H, CO(norm), and CO(hyper). The reduction in arterial oxygen content (Ca(O(2))) (approximately 20%) resulted in an elevated blood flow (Q) in the CO and H trials. Net muscle CO uptake was attenuated in the CO trials. Suprasystolic cuff measurements of the deoxy-Mb signal were not different in terms of the rate of signal rise or maximum signal attained with and without CO. At maximal exercise, calculated mean capillary PO(2) was most reduced in H and resulted in the lowest Mb-associated PO(2). Reductions in ATP, PCr, and pH during H, CO(norm), and CO(hyper) occurred earlier during progressive exercise than in normoxia. Thus the effects of reduced Ca(O(2)) due to mild CO poisoning are similar to H.

Measurements of the photodissociation quantum yields of MbNO and MbO(2) and the vibrational relaxation of the six-coordinate heme species

The (t approximately 0) photodissociation quantum yields (Y(0)) of MbNO and MbO(2) are measured to be 50 +/- 5 and 28 +/- 6%, respectively, using MbCO (Y(0) = 100%) as a reference. When photolysis does not take place, we find that a significant portion of the photon energy contributes to heating of the residual six-coordinate heme (MbNO and MbO(2)). The time constant for vibrational relaxation of the six-coordinate ligand-bound heme is found to be close to 1 ps for both samples. The MbO(2) sample also shows a approximately 4-ps optical response that is assigned to a rapid phase (25-30% amplitude) of O(2) geminate rebinding. We observe no additional geminate recombination in the MbO(2) sample out to 120 ps. In contrast, the MbNO sample displays significant geminate recombination over the first 120 ps, which can be adequately fit with two exponentials whose amplitudes and time constants appear to depend weakly on the pump wavelength. This more complex kinetic behavior conceivably arises due to heating of the photodissociated heme and its effect on the geminate recombination as the system cools. Overall, the data are consistent with a hypothesis that distortions along the iron-ligand bending coordinate play a key role in the photodissociation process. The transient formation of an unphotolyzable FeO(2) side-on binding geometry is suggested to be responsible for the lowered quantum yield of MbO(2) relative to MbNO.

Effect of nitrosylmyoglobin and saturated fatty acid anions on metmyoglobin-catalyzed oxidation of aqueous methyl linoleate emulsions

In aqueous methyl linoleate emulsions (pH 7.4, 25 degrees C, air-saturated), nitrosylmyoglobin and saturated fatty acid anions (palmitate and stearate investigated) each showed antioxidant effect on metmyoglobin-induced peroxidation as measured by oxygen depletion rate. For equimolar concentration of nitrosylmyoglobin and metmyoglobin and for metmyoglobin in moderate excess, a reduction in oxygen consumption rate of approximately 70% was observed. Fatty acid anions reduced oxygen consumption rate most significantly for palmitate (up to 60% for a fatty acid:heme protein ratio of 90:1). No further antioxidative effect was seen for fatty acid anions in the presence of nitrosylmyoglobin, whereas nitrosylmyoglobin showed a further antioxidant effect in presence of fatty acid anions in the metmyoglobin-catalyzed process. The antioxidative mechanism of nitrosylmyoglobin and fatty acid anions is different, and while the fatty acid anions seem active in inhibiting initiation of oxidation through protection against metmyoglobin activation into perferrylmyoglobin, as shown by freeze-quench Electron Spin Resonance (ESR) spectroscopy, nitrosylmyoglobin is rather active in the oxygen consuming (propagation) phase.

Observation of an isotope-sensitive low-frequency Raman band specific to metmyoglobin

A resonance Raman band involving significantly the iron(III)-histidine stretching (upsilonFe-His) character is identified for metmyoglobin (metMb) through isotope sensitivity of its low-frequency resonance Raman bands, but the identification was not successful for methemoglobin (metHb) and its isolated alpha and beta subunits. A band at 218 cm-1 of natural abundance metMb exhibited a low-frequency shift for 15N-His-labeled metMb (-1.4 cm-1 shift), while the strong porphyrin bands at 248 and 271 cm-1 did not shift significantly. The frequency of the 218-cm-1 band of metMb decreased by 1.6 cm-1 in D2O, probably due to Ndelta-deuteration of the proximal His, in a similar manner to that of the upsilonFe-His band of deoxyMb in D2O. This 218-cm-1 band shifted slightly to a lower frequency in H2(18)O, whereas it did little upon 54Fe isotopic substitution (<0.3 cm-1), presumably because of the six-coordinate structure. The lack of the 54Fe-isotope shift shows that the 218-cm-1 band is specific to metMb and not due to the deoxy species. The intensity of this band decreased for hydroxymetMb and was indiscernible for cyanometMb. For metHb and its alpha and beta subunits, however, the frequencies of the band around 220 cm-1 were not D2O sensitive. These results suggest an assignment of the band around 220 cm-1 to a pyrrole tilting mode, which significantly contains the Fe-His stretching character for metMb but scarcely for metHb and its subunits. The differences in the isotope sensitivity of this band in different proteins are considered to reflect the heme distortion from the planarity and the Fe-His geometry specific to individual proteins.

Skeletal muscle intracellular PO(2) assessed by myoglobin desaturation: response to graded exercise

The relationship between skeletal muscle intracellular PO(2) (iPO(2)) and progressive muscular work has important implications for the understanding of O(2) transport and utilization. Presently there is debate as to whether iPO(2) falls progressively with increasing O(2) demand or reaches a plateau from moderate to maximal metabolic demand. Thus, using (1)H magnetic resonance spectroscopy of myoglobin (Mb), we studied cellular oxygenation during progressive single-leg knee extensor exercise from unweighted to 100% of maximal work rate in six active human subjects. In all subjects, the Mb peak at 73 ppm was not visible at rest, whereas the peak was small or indistinguishable from the noise in the majority of subjects during progressive exercise from unweighted to 50-60% of maximum work rate. In contrast, beyond this exercise intensity, a Mb peak of consistent magnitude was discernible in all subjects. When a Mb half saturation of 3.2 Torr was used, the calculated skeletal muscle PO(2) was variable before 60% of maximum work rate but in general was relatively high (>18 Torr, the measurable PO(2) with the poorest signal-to-noise ratio, in the majority of cases), whereas beyond this exercise intensity iPO(2) fell to a relatively uniform and invariant level of 3.8 +/- 0.5 Torr across all subjects. These results do not support the concept of a progressive linear fall in iPO(2) across increasing work rates. Instead, this study documents variable but relatively high iPO(2) from rest to moderate exercise and again confirms that from 50-60% of maximum work rate iPO(2) reaches a plateau that is then invariant with increasing work rate.

The Fe(2+)-His(F8) Raman band shape of deoxymyoglobin reveals taxonomic conformational substates of the proximal linkage

The band shape of the Raman line attributed to the Fe(2+)-N(epsilon)(His(F8)) stretching mode in deoxymyoglobin contains significant information on the nature of the Fe-His proximal linkage. Raman lines appearing close to it, however, obscure the true line profile. To isolate this from its accompanying lines we use its isotopic shift of approximately 1 cm(-1) when (56)Fe in natural-abundance deoxymyoglobin is substituted by (54)Fe. This enables us to isolate the true line shape. We have measured this line shape in sperm whale myoglobin dissolved in a 66% vol/vol glycerol/water solution for nine temperatures from 10 K to 300 K. The nu(Fe-His) band shows a complex temperature-dependent profile, with a shoulder on its high-frequency wing, which becomes more prominent with increasing temperature. Detailed analysis reveals that the band is composed of five distinct lines attributable to taxonomic conformational substates of the nu(Fe-His) linkage. These are in thermodynamic equilibrium above the glass transition temperature T(f) but freeze in into the thermodynamic distribution at T(f) for lower temperatures. Alternative models that try to explain the nu(Fe-His) band shape by either an anharmonic coupling of the nu(Fe-His) to a low-frequency heme doming mode or by conformational substates related to a Gaussian distribution of iron out-of-plane displacements are at variance with the distinct features observed experimentally.

Limited acid hydrolysis as a means of fragmenting proteins isolated upon ProteinChip array surfaces

ProteinChip array technology enables protein purification, protein profiling, and biomarker discovery on a convenient biochip platform. Traditional proteomic approaches towards protein identification rely upon the generation of peptides through the use of specific proteases. However, for a variety of reasons, the digestion of proteins bound to planar arrays by specific proteases, such as trypsin, has proven to be difficult, at times providing little or no protein digestion at all. Additionally, should more than one protein be present on the array surface, the digestion product consists of peptides from different proteins, adding another dimension of complexity to database mining approaches. These factors have driven our group to explore alternative means of on-chip protein digestion. In this article, we describe an approach to generate peptide maps by limited acid hydrolysis. Depending upon the adsorbed protein, this method requires between 500 femtomole to 5 picomole of protein for on-chip hydrolysis. Besides generating several internal peptide fragments, limited acid hydrolysis also has the advantage of generating peptide ladders from the N- or C-terminus of the protein. From these ladders, partial primary sequence of the protein can be directly derived when analyzed by a simple laser desorption/ionization mass spectrometer. Furthermore, tandem mass spectrometry can be performed on several internal peptide fragments, thus facilitating the identification of several proteins within a mixture. Based upon the preliminary results of this work, we continue to explore the possibility of using limited acid hydrolysis to identify unknown proteins captured on ProteinChip array surfaces.

Quantitative (1)H magnetic resonance spectroscopy of myoglobin de- and reoxygenation in skeletal muscle: reproducibility and effects of location and disease

1H-magnetic resonance spectroscopy ((1)H-MRS) of deoxymyoglobin (DMb) provides a means to noninvasively monitor the oxygenation state of human skeletal muscle in work and disease. As shown in this work, it also offers the opportunity to measure the absolute tissue content of DMb, the basic oxygen consumption of resting muscle, and the reperfusion characteristics after release of a pressure cuff. The methodology to determine these tissue properties simultaneously at two positions along the calf is presented. The obtained values are in agreement with invasive determinations. The reproducibility of the (1)H-MRS measurements is established for healthy controls and patients with peripheral arterial disease (PAD). A location dependence in axial direction, as well as differences between controls and patients are demonstrated for all parameters. The reoxygenation time in particular is expected to provide a means to quantitatively monitor therapies aimed at improving muscular perfusion in these patients.

Protein conformation change of myoglobin upon ligand binding probed by ultraviolet resonance Raman spectroscopy

Conformational change of myoglobin (Mb) accompanied by binding of a ligand was investigated with 244 nm excited ultraviolet resonance Raman Spectroscopy (UVRR). The UVRR spectra of native sperm whale (sw) and horse (h) Mbs and W7F and W14F swMb mutants for the deoxy and CO-bound states enabled us to reveal the UVRR spectra of Trp7, Trp14, and Tyr151 residues, separately. The difference spectra between the deoxy and CO-bound states reflected the environmental or structural changes of Trp and Tyr residues upon CO binding. The W3 band of Trp7 near the N-terminus exhibited a change upon CO binding, while Trp14 did not. Tyr151 in the C-terminus also exhibited a definite change upon CO binding, but Tyr103 and Tyr146 did not. The spectral change of Tyr residues was characterized through solvent effects of a model compound. The corresponding spectral differences between CO- and n-butyl isocyanide-bound forms were much smaller than those between the deoxy and CO-bound forms, suggesting that the conformation change in the C- and N-terminal regions is induced by the proximal side of the heme through the movement of iron. Although the swinging up of His64 upon binding of a bulky ligand is noted by X-ray crystallographic analysis, UVRR spectra of His for the n-butyl isocyanide-bound form did not detect the exposure of His64 to solvent.

[Catalytic effect of ferricyanide on the rate of electron transfer between myoglobin and cytochrome c]

The influence of small amounts of low-molecular electron acceptor, potassium ferricyanide, 1 to 20% relative to the cytohrome c concentration, on the rate of electron transfer in the sperm whale oxymyoglobin--horse heart cytochrome c and deoxymyoglobin--cytochrome c systems (under aerobic and anaerobic conditions, respectively) was studied. At low ionic strength, the redox reaction rate was found to increase proportionally to the concentration of ferricyanide in both redox systems. The effect depends on pH in the pH range 5-8, increasing sharply at pH < 6. It was shown that the enhancing of electron transfer is caused by the complexing of [Fe(CN)6]3- with cytohrome c in the Lys72 region, where one of the two strong binding sites for this anion is determined by NMR. Both the high ionic strength and the chemical modification of Lys72 residue inhibit this effect at low ionic strength, markedly decreasing the rate of reaction with myoglobin. Under the same conditions, the effect of ferricyanide in the reaction of oxy-Mb with yeast cytohrome c, which is isopotential to animal cytochromes c but possesses trimethylated Lys72, was several times smaller. In turn, the chemical modification of His residues in myoglobin and the complexing of zinc ion to His119(GH1) almost completely inhibit electron transfer in the systems. Thus, electron transfer between the proteins must proceed through the formation of the Mb.[Fe(CN)6]3-.Cyt c ternary complex, the contacting sites being localized in the His119(GH1) region of myoglobin and near Lys72 of cytohrome c. The increased electron transfer rate in the presence of [Fe(CN)6]3- can be explained by that its binding near Lys72, firstly, provides better electrostatic interactions in the electron transfer complex and, besides, decreases significantly (about 2-fold) the tunneling distance between the two hemes (two lengths of 1.7 and 1.2 nm instead of one of 2.9 nm).

A photolysis-triggered heme ligand switch in H93G myoglobin

Resonance Raman spectroscopy and step-scan Fourier transform infrared (FTIR) spectroscopy have been used to identify the ligation state of ferrous heme iron for the H93G proximal cavity mutant of myoglobin in the absence of exogenous ligand on the proximal side. Preparation of the H93G mutant of myoglobin has been previously reported for a variety of axial ligands to the heme iron (e.g., substituted pyridines and imidazoles) [DePillis, G., Decatur, S. M., Barrick, D., and Boxer, S. G. (1994) J. Am. Chem. Soc. 116, 6981-6982]. The present study examines the ligation states of heme in preparations of the H93G myoglobin with no exogenous ligand. In the deoxy form of H93G, resonance Raman spectroscopic evidence shows water to be the axial (fifth) ligand to the deoxy heme iron. Analysis of the infrared C-O and Raman Fe-C stretching frequencies for the CO adduct indicates that it is six-coordinate with a histidine trans ligand. Following photolysis of CO, a time-dependent change in ligation is evident in both step-scan FTIR and saturation resonance Raman spectra, leading to the conclusion that a conformationally driven ligand switch exists in the H93G protein. In the absence of exogenous nitrogenous ligands, the CO trans effect stabilizes endogenous histidine ligation, while conformational strain favors the dissociation of histidine following photolysis of CO. The replacement of histidine by water in the five-coordinate complex is estimated to occur in < 5 micros. The results demonstrate that the H93G myoglobin cavity mutant has potential utility as a model system for studying the conformational energetics of ligand switching in heme proteins such as those observed in nitrite reductase, guanylyl cyclase, and possibly cytochrome c oxidase.

The energy level scheme for the ferryl heme in compound II of the peroxidase-catalase family as determined from analysis of low-temperature magnetic circular dichroism

The expressions for temperature-dependent magnetic circular dichroism (MCD) of the ferryl heme (Fe(4+)Por, S=1), which is a model of an intermediate product of the catalytic cycle of heme enzymes (compound II), have been derived in the framework of a two-term model. Theoretical predictions for the temperature and magnetic field dependence of MCD intensity of the ferryl heme are compared with those of the high-spin and low-spin ferric heme. Analysis of reported MCD spectra of myoglobin peroxide [Foot et al., Biochem. J. 2651 (1989) 515-522] and compound II of horseradish peroxidase [Browett et al., J. Am. Chem. Soc. 110 (1987) 3633-3640] has shown the presence in the samples of approximately 1% of a low-spin ferric component, which, however, should be taken into account in simulating observed temperature dependences of MCD intensity. The values of two adjustable parameters are estimated from the fit of the observed and simulated plots of MCD intensity against the reciprocal of the absolute temperature. One of them, the energy gap between the ground and excited terms, predetermines the axial zero-field splitting. The other parameter is correlated with the energy of splitting of excited quartets arising from either the porphyrin pi-->pi* transition or the spin-allowed charge-transfer transition.

Vibrational energy transfer in a protein molecule

Mode coupling in a protein molecule was studied by a molecular dynamics simulation of the intramolecular vibrational energy transfer in myoglobin at near zero temperature. It was found that the vibrational energy is transferred from a given normal mode to a very few number of selective normal modes. These modes are selected by the relation between their frequencies, like Fermi resonance, governed by the third order mode coupling term. It was also confirmed that the coupling coefficients had high correlation with how much the coupled modes geometrically overlapped with each other.

Identification of histidine 77 as the axial heme ligand of carbonmonoxy CooA by picosecond time-resolved resonance Raman spectroscopy

The heme proximal ligand of carbonmonoxy CooA, a CO-sensing transcriptional activator, in the CO-bound form was identified to be His77 by using picosecond time-resolved resonance Raman spectroscopy. On the basis of the inverse correlation between Fe-CO and C-O stretching frequencies, we proposed previously that His77 is the axial ligand trans to CO [Uchida et al. (1998) J. Biol. Chem. 273, 19988-19992], whereas later a possibility of displacement of His77 by CO with retention of another unidentified axial ligand was reported [Vogel et al. (1999) Biochemistry 38, 2679-2687]. Although our previous resonance Raman study failed to detect the Fe-His stretching [nu(Fe-His)] mode of CO-photodissociated CooA of the carbonmonoxy adduct due to the rapid recombination, application of the picosecond time-resolved resonance Raman technique enabled us to observe a new intense line assignable to nu(Fe-His) at 211 cm(-)(1) immediately after photolysis, while it became nondiscernible after 100-ps delay. The low nu(Fe-His) frequency of photodissociated CooA indicates the presence of some strain in the Fe-His bond in CO-bound CooA. This and the rapid recombination of CO characterize the heme pocket of CooA. The 211 cm(-)(1) band was completely absent in the spectrum of the CO-photodissociated form of the His77-substituted mutant but the Fe-Im stretching band was observed in the presence of exogenous imidazole (Im). Thus, we conclude that His77 is the axial ligand of CO-bound CooA and CO displaces the axial ligand trans to His77 with retention of ligated His77 to activate CooA as the transcriptional activator.

Nuclear forward scattering of synchrotron radiation by deoxymyoglobin

Nuclear forward scattering of synchrotron radiation is used to determine the quadrupole splitting and the mean square displacement of the iron atom in deoxymyoglobin in the temperature range between 50 K and 243 K. Above 200 K an abnormally fast decay of the forward scattered intensity at short times after the synchrotron flash is observed, which is caused by protein-specific motions. The results strongly support the picture that protein dynamics seen at the position of the iron can be understood by harmonic motions in the low temperature regime while in the physiological regime diffusive motions in limited space are present. The shape of the resonance broadening function is investigated. An inhomogeneous broadening with a Lorentzian distribution indicating dipole interactions results in a better agreement with the experimental data than the common Gaussian distribution.

From metmyoglobin to deoxy myoglobin: relaxations of an intermediate state

Metmyoglobin has been reduced at low temperature (below 100 K) using x-rays or by excitation of tris(2,2,bipyridine)ruthenium(II) chloride with visible light. Upon reduction, an intermediate state is formed where the structure of the protein is very similar to that of metmyoglobin with the water molecule still bound to the heme iron, but the iron is II low spin. The nature of the intermediate state has been investigated with optical spectroscopy. The Q(O) and Q(V) bands of the intermediate state are split, suggesting that the protoporphyrin is distorted. The intermediate state undergoes a relaxation observed by a shifting of the Soret band at temperatures above 80 K. Above 140 K, the protein begins to relax to the deoxy conformation. The relaxation kinetics of the protein have been monitored optically as a function of time and temperature from minutes to several hours and from 150 K to 190 K. By measuring the entire visible spectrum, we are able to distinguish between electron transfer processes and the protein relaxation from the intermediate state to deoxy myoglobin. The relaxation has been measured in both horse myoglobin and sperm whale myoglobin with the relaxation occurring on faster time scales in horse myoglobin. Both the reduction kinetics and the relaxation show non-exponential behavior. The reduction kinetics can be fit well to a stretched exponential. The structural relaxation from the intermediate state to the deoxy conformation shows a more complex, dynamical behavior and the reaction is most likely affected by the relaxation of the protein within the intermediate state.

Heme methyl 1H chemical shifts as structural parameters in some low-spin ferriheme proteins

The different paramagnetic shifts of the four methyl groups in ferriheme proteins have been described as being due to the effect of the axial ligand nodal plane orientation. An equation, heuristically found and theoretically explained, describing the relation between contact and pseudocontact shifts and the position of the axial ligand(s) has been derived for bis-histidine ferriheme proteins and for cyanide-histidine ferriheme proteins. The values of the heuristic parameters contained in the equations were found by fitting the shifts of bovine cytochrome b5 and several bis-histidine cytochromes c3 and histidine-cyanide systems. The agreement between the observed and the calculated shifts was found to be good. Therefore, by taking advantage of this study, information on the position of the axial ligands, that can be used as a constraint for structure determination, can be obtained from the shifts of the methyl protons.

Oxidation of deoxy myoglobin by [Fe(CN)6]3-

The ability of myoglobin (Mb) to reversibly bind O2 and other ligands has been well characterized. Mb also participates with a variety of redox metals to form metmyoglobin (metMb). By using an anaerobic stopped-flow device we have measured outer-sphere oxidation by [Fe(CN)6]3 of native sperm whale myoglobin, recombinant wild-type Mb, and a series of mutant Mb proteins in which the distal His-64 was changed to Gly, Phe, Leu or Val. Second-order rate constants for oxidation of mutant proteins are 10-15 times greater than for recombinant or native (kox approximately 10(6) M-1 s-1). We attribute the reduced rate of oxidation of wild-type protein to a higher reorganization energy imposed by the presence of the unique water/His-64/heme interaction, which is absent in the mutant proteins.

In vitro and in vivo studies of 1H NMR visibility to detect deoxyhemoglobin and deoxymyoglobin signals in myocardium

1H nuclear magnetic resonance (NMR) spectroscopy can be used noninvasively to detect the proximal histidyl N delta proton signals of deoxymyoglobin in the myocardium. However, the quantification of deoxymyoglobin is based on the assumption that the deoxymyoglobin signal detected is not contaminated by the deoxyhemoglobin signals contributed from the blood. The purpose of this study was to conduct in vitro and in vivo 1H NMR studies to examine the in vivo NMR visibility of deoxyhemoglobin in the myocardium. The results demonstrate that the NMR visibility of alpha and beta subunits of deoxyhemoglobin is sensitive to the pulse width for spin excitation because of short T2 relaxation times, and they are not NMR visible in the canine myocardium in vivo at 4.7 T when a 0.5-1.0 msec long Gaussian excitation pulse is used. Therefore, the resonance peak detected at approximately 72 ppm (relative to the water resonance) in the ischemic canine myocardium in vivo is dominated by deoxymyoglobin.

Comparative analysis of NMR and NIRS measurements of intracellular PO2 in human skeletal muscle

1H NMR has detected both the deoxygenated proximal histidyl NdeltaH signals of myoglobin (deoxyMb) and deoxygenated Hb (deoxyHb) from human gastrocnemius muscle. Exercising the muscle or pressure cuffing the leg to reduce blood flow elicits the appearance of the deoxyMb signal, which increases in intensity as cellular PO2 decreases. The deoxyMb signal is detected with a 45-s time resolution and reaches a steady-state level within 5 min of pressure cuffing. Its desaturation kinetics match those observed in the near-infrared spectroscopy (NIRS) experiments, implying that the NIRS signals are actually monitoring Mb desaturation. That interpretation is consistent with the signal intensity and desaturation of the deoxyHb proximal histidyl NdeltaH signal from the beta-subunit at 73 parts per million. The experimental results establish the feasibility and methodology to observe the deoxyMb and Hb signals in skeletal muscle, help clarify the origin of the NIRS signal, and set a stage for continuing study of O2 regulation in skeletal muscle.

Heme protein dynamics revealed by geminate nitric oxide recombination in mutants of iron and cobalt myoglobin

Nitric oxide myoglobin (MbNO) at 300 K was photodissociated with 405 nm pulses. The NO recombination in several mutants of iron and cobalt myoglobins was investigated at a time resolution of ca. 70 fs. The geminate recombination of NO was nonexponential on sub-nanosecond time scales. For both metals, the change of the detailed structure of the heme pocket (position 68 mutations) caused significant changes in the rates of recombination; however, the metal substitution influenced the recombination much less than did amino acid substitution. The results indicate a primary role of the heme pocket structure in the dynamics, and they suggest that proximal protein relaxation is not the limiting factor in the geminate recombination process. Recombination in cobalt derivatives is somewhat more efficient on the sub-nanosecond time scales than in corresponding iron myoglobins, consistent with other results that show a greater intrinsic reactivity toward the NO of cobalt compared with the iron heme. A comparison of results using Soret band excitation with previous Q-state excitation studies demonstrates that the ligand dissociates with a similar kinetic energy in both cases, suggesting fast intramolecular energy redistribution before dissociation.

A steric mechanism for inhibition of CO binding to heme proteins

The crystal structures of myoglobin in the deoxy- and carbon monoxide-ligated states at a resolution of 1.15 angstroms show that carbon monoxide binding at ambient temperatures requires concerted motions of the heme, the iron, and helices E and F for relief of steric inhibition. These steps constitute the main mechanism by which heme proteins lower the affinity of the heme group for the toxic ligand carbon monoxide.

Reversible and irreversible hemichrome generation by the oxygenation of nitrosylmyoglobin

The repeated oxygenation/reduction/nitrosylation of nitrosylmyoglobin produces low-spin ferric heme hemichromes which have been characterized by electron spin resonance spectroscopy. The predominant myoglobin hemichrome is a chemically reversible dihistidyl complex identified by the g values 1.53, 2.21, and 2.97. Also present is a low-spin ferric hydroxide derivative which is represented by the g values 1.83, 2.18, and 2.59. The formation of these species goes undetected by UV-vis spectroscopy, but the oxygenation of myoglobin to metmyoglobin is correlated with complete conversion of nitric oxide to nitrate which is released following a clear induction period. These results are interpreted in terms of the intermediates generated during the MbNO oxygenation reaction.

Spatial distribution of deoxymyoglobin in human muscle: an index of local tissue oxygenation

The proximal histidyl NdeltaH signal of myoglobin is detectable in 1H NMR spectra of myocardial and skeletal muscle, and its intensity reflects the intracellular oxygenation. At 1.5 Tesla (T), the typical field strength of clinical magnetic resonance imaging (MRI) magnets, the paramagnetic relaxation contribution decreases sufficiently to permit the implementation of chemical shift imaging technique to map the spatial distribution of the deoxy Mb NdeltaH signal from human gastrocnemius muscle. One and two-dimensional chemical shift imaging experiments reveal clearly the localized deoxy Mb signal in muscle and consequently the spatial distribution of the cellular oxygenation. The results indicate the feasibility to assess the pO2 in tissue regions and to directly study the regulation of oxidative metabolism in human tissue.

Spectroscopic evidence for nanosecond protein relaxation after photodissociation of myoglobin-CO

Nanosecond time-resolved absorption and magnetic optical rotatory dispersion (MORD) measurements of photolyzed myoglobin-CO visible bands (500-650 nm) are presented. These measurements reveal a 400 ns process, spectrally distinct from ligand recombination, that accounts for 7% of the observed spectral evolution in the visible absorption bands and 4% in the MORD. The time-resolved MORD, more sensitive to heme coordination geometry than absorption, suggests that this process is most likely associated with protein relaxation on the distal side of the heme pocket, perhaps accompanying rehydration of the deoxymyoglobin photoproduct or accommodation of protein side chains to ligand escape.