Identification of the titrating group in the heme cavity of myoglobin. Evidence for the heme-protein pi-pi interaction

1H and 13C NMR investigation of the influence of nonligated residue contacts on the heme electronic structure in cyanometmyoglobin complexes reconstituted with centro- and pseudocentrosymmetric hemins

The 1H and 13C chemical shifts for the heme methyls of low-spin, ferric sperm whale cyanometmyoglobin reconstituted with a variety of centrosymmetric and pseudocentrosymmetric hemins have been recorded and analyzed to shed light on the nature of heme-protein contacts, other than that of the axial His, that modulate the rhombic perturbation to the heme's in-plane electronic asymmetry. The very similar 1H dipolar shifts for heme pocket residues in all complexes yield essentially the same magnetic axes as in wild type, and the resultant dipolar shifts allow the direct determination of the heme methyl proton and 13C contact shifts in all complexes. It is demonstrated that, even when the magnetic axes and anisotropies are known, the intrinsic uncertainties in the orientational parameters lead to a sufficiently large uncertainty in dipolar shift that the methyl proton contact shifts are inherently significantly less reliable indicators of the unpaired electron spin distribution than the methyl 13C contact shifts. The pattern of the noninversion symmetry in 13C contact shifts in the centro- or pseudocentrosymmetric hemes is shown to correlate with the positions of aromatic rings of Phe43(CD1) and His97(FG3) parallel to, and in contact with, the heme. These results indicate that such pi-pi interactions significantly perturb the in-plane asymmetry of the heme pi spin distribution and cannot be ignored in a quantitative interpretation of the heme methyl 13C contact shifts in terms of the axial His orientation in b-type hemoproteins.

Salt effects on ionization equilibria of histidines in myoglobin

The salt dependence of histidine pK(a) values in sperm whale and horse myoglobin and in histidine-containing peptides was measured by (1)H-NMR spectroscopy. Structure-based pK(a) calculations were performed with continuum methods to test their ability to capture the effects of solution conditions on pK(a) values. The measured pK(a) of most histidines, whether in the protein or in model compounds, increased by 0.3 pH units or more between 0.02 M and 1.5 M NaCl. In myoglobin two histidines (His(48) and His(36)) exhibited a shallower dependence than the average, and one (His(113)) showed a steeper dependence. The (1)H-NMR data suggested that the salt dependence of histidine pK(a) values in the protein was determined primarily by the preferential stabilization of the charged form of histidine with increasing salt concentrations rather than by screening of electrostatic interactions. The magnitude and salt dependence of interactions between ionizable groups were exaggerated in pK(a) calculations with the finite-difference Poisson-Boltzmann method applied to a static structure, even when the protein interior was treated with arbitrarily high dielectric constants. Improvements in continuum methods for calculating salt effects on pK(a) values will require explicit consideration of the salt dependence of model compound pK(a) values used for reference in the calculations.

Influence of proximal side mutations on the molecular and electronic structure of cyanomet myoglobin: an 1H NMR study

A series of proximal side mutants of sperm whale metmyoglobin (metMb) that involves residues which provide hydrogen bonds to the axial His and heme have been prepared, and the CO binding and solution molecular and electronic structure has been investigated by 1H NMR. These include Ser92(F7), whose O gamma serves as a hydrogen-bond acceptor to the axial His ring NdeltaH and whose O gamma H serves as hydrogen-bond donor to the 7-propionate carboxylate, and His97(FG3) whose ring provides the other hydrogen-bond donor to the 7-propionate carboxylate. 2D NMR data on the S92A-metMbCN, S92P-metMbCN and H97F-metMbCN show that the distal structure is completely conserved and that proximal side structural changes are highly localized. For the S92A-metMbCN, altered dipolar contacts to the F-helix backbone show that the axial His imidazole has rotated clockwise by approximately 10 degrees relative to a stationary heme, while in H97F-metMbCN, the altered heme-E helix backbone contacts reveal that the heme has rotated counterclockwise by approximately 3 degrees relative to a conserved axial His. The pattern of axial His rotation was qualitatively predicted by energy minimization calculations. The assignments and conserved structural elements allow the determination of a set of magnetic axes whose major magnetic axis is unchanged with respect to WT and confirms that local distal, and not proximal, interactions control the orientation of the major magnetic axis and, by inference, the degree and direction of tilt of the Fe-CN from the heme normal. The rhombic magnetic axes in S92A-metMbCN are rotated approximately 10 degrees in the opposite direction from the established approximately 10 degrees rotation for the axial His ring as expected. It is shown, moreover, that the pairwise alpha-, gamma-meso vs beta-, delta-meso-H hyperfine shift differences are well predicted by the change in the location of the rhombic magnetic axes. Carbon monoxide ligation rates experience minor but systematic perturbation for the S92A substitutions which confirms an influence (albeit very small) for axial His orientation on ligand affinity.

Electrostatic modification of the active site of myoglobin: characterization of the proximal Ser92Asp variant

The structural and functional consequences of the introduction of a negatively charged amino acid into the active site of horse heart myoglobin have been investigated by replacement of the proximal Ser92 residue (F7) with an aspartyl residue (Ser92Asp). UV-visible absorption maxima of various ferrous and ferric derivatives and low-temperature EPR spectra of the metaquo (metMb) derivative indicate that the active site coordination geometry has not been perturbed significantly in the variant. 1H-NMR spectroscopy provides direct evidence for the existence of a distal water molecule as the sixth ligand in the oxidized form of the variant at pD 5.7. Spectrophotometric pH titration of the Ser92Asp variant is consistent with this finding and with a pKa = 8.90 +/- 0.02 [25.0 degrees C, mu = 0.10 M (NaCl)] for titration of the distal water molecule, identical to the value reported for the wild-type protein. X-ray crystallography of the metMb derivative indicates that the heme substituents conserve their orientations in the variant protein, except for a slight reorientation of the pyrrole A propionate group to which Ser92 normally hydrogen bonds and reorientation of the carboxyl end of the pyrrole D propionate group. No change is observed in conformation of the proximal (His93) or distal (Wat156) heme ligands. 1H-NMR spectroscopy of the metMbCN form of the protein indicates that a slight rotation of the proximal His93 ligand has occurred in this derivative. Resonance Raman experiments indicate increased conformational heterogeneity in the proximal pocket of the variant. Failure to detect electron density for the Asp residue in the X-ray diffraction map of the variant protein and high average thermal factors for the pyrrole A propionate substituent are consistent with this observation. The variant exhibits novel pH-dependent behavior in the metMb form, as shown by 1H-NMR spectroscopy, and provides evidence for a heme-linked titratable group with a pKa of 5.4 in this derivative. The metMbCN and deoxyMb derivatives also exhibit pH-dependent behavior, with pKas of 5.60 +/- 0.07 and 6.60 +/- 0.07, respectively, compared to the wild-type values of 5.4 +/- 0.04 and 5.8 +/- 0.1. The heme-linked ionizable group is proposed to be His97 in all three derivatives. The reduction potential of the variant is 72 +/- 2 mV vs SHE [25.0 degrees C, mu = 0.10 M (phosphate), pH 6.0], an increase of 8 mV over the wild-type value. The possible influence of a number of variables on the magnitude of the reduction potential in myoglobin and other heme proteins is discussed.

Ligand binding to heme proteins: III. FTIR studies of His-E7 and Val-E11 mutants of carbonmonoxymyoglobin

Fouier-transform infrared (FTIR) difference spectra of several His-E7 and Val-E11 mutants of sperm whale carbonmonoxymyoglobin were obtained by photodissociation at cryogenic temperatures. The IR absorption of the CO ligand shows characteristic features for each of the mutants, both in the ligand-bound (A) state and in the photodissociated (B) state. For most of the mutants, a single A substate band is observed, which points to the crucial role of the His-E7 residue in determining the A substrate spectrum of the bound CO in the native structure. The fact that some of the mutants show more than one stretch band of the bound CO indicates that the appearance of multiple A substates is not exclusively connected to the presence of His-E7. In all but one mutant, multiple stretch bands of the CO in the photodissociated state are observed; these B substates are thought to arise from discrete positions and/or orientations of the photodissociated ligand in the heme pocket. The red shifts of the B bands with respect to the free-gas frequency indicate weak binding in the heme pocket. The observation of similar red shifts in microperoxidase (MP-8), where there is no residue on the distal side, suggests that the photodissociated ligand is still associated with the heme iron. Photoselection experiments were performed to determine the orientation of the bound ligand with respect to the heme normal by photolyzing small fractions of the sample with linearly polarized light at 540 nm. The resulting linear dichroism in the CO stretch spectrum yielded angles alpha > 20 degrees between the CO molecular axis and the heme normal for all of the mutants. We conclude that the off-axis position of the CO ligand in the native structure does not arise from steric constraints imposed by the distal histidine. There is no clear correlation between the size of the distal residue and the alpha of the CO ligand.

NMR study of the molecular and electronic structure of the heme cavity in Dolabella met-cyano myoglobin

The molecular and electronic structure of the active site of the cyanide-ligated ferric complex of the myoglobin from the mollusc Dolabella auricularia has been investigated using NMR. Analysis of nuclear Overhauser effects has revealed that the correlation times for the internal motion of the heme propionate alpha-CH2 and beta-CH2 groups at ambient temperature are about 5 and 4 ns, respectively. These correlation times indicate that the terminal carboxylate groups of both the heme propionates are not bound to the protein via salt bridges. Although the absence of the propionate-protein interaction does not influence the equilibrium population of the two heme orientational isomers involving rotation about the alpha,gamma-meso axis, it allows the heme to rotate about the iron-His bond in the active site of the myoglobin. Such rotational motion of the heme resulted in an anomalous temperature-dependence of the heme methyl-proton hyperfine shift. Thus the present myoglobin studies provide the first example demonstrating the rotation of the heme about the iron-His bond in native myoglobin.

Reactivity patterns for redox reactions of monomer forms of myoglobin, hemocyanin and hemerythrin

Electron-transfer reactions of myoglobin, hemocyanin and hemerythrin with the inorganic complexes [Fe(CN)6]3- (oxidant) and [Co(sep)]2+ (reductant) are considered. Rate constants kFe (25 degrees C) have been determined for the [Fe(CN)6]3- (410 mV) oxidation of horse deoxyMb, I = 0.100 M (NaCl). From the decrease in kFe over the range pH 5.5 to 9.0 a pKa of less than 6.2 is obtained, most likely due to the involvement of the heme propionate(s). At the higher pH values the rate constant is 1.2 x 10(6) M-1 s-1. Rate constants kCo (25 degrees C) for the [Co(sep)]2+ (-260 mV) reduction of metMb are also pH-dependent, pKa = 8.82, corresponding to acid dissociation of the H2O axially coordinated to the Fe(III). The rate constant for the aqua-met form is 2.8 x 10(3) M-1 s-1 at pH values less than 7.0. In contrast, no reaction is observed for the deoxy and met forms of P. interruptus hemocyanin monomer subunit a with the same two complexes (k less than 10(2) M-1 s-1). Comparisons are made with rate constants for hemerythrin, also as the monomer, which have been determined previously. Rate constants for the reactions of deoxy forms with the neutral small molecules, here O2 and H2O2, are also considered. Whereas the reactions of [Fe(CN)6]3- and [Co(sep)]2+ are at the protein surface, those of O2 and H2O2 are at the active site. Hemocyanin with the more buried (approximately 20 A) active site compared with myoglobin (3.8 A) and hemerythrin (6.3 A), does not readily undergo electron transfer with reagents at the surface. However, with the small molecules O2 and H2O2 penetration of the surrounding peptide occurs, with reaction at the active site. Rate constants for the three proteins are now of similar magnitude, and in the range (2.3-7.8) x 10(7) M-1 s-1 for O2, and 10.9 to 3600 M-1 s-1 for H2O2.

Fluorescence study of the conformational properties of myoglobin structure. 3. pH-dependent changes in porphyrin and tryptophan fluorescence of the complex of sperm whale apomyoglobin with protoporphyrin IX; the role of the porphyrin macrocycle and iron in formation of native myoglobin structure

The porphyrin and tryptophan fluorescence of sperm whale apomyoglobin complexed with protoporphyrin IX has been studied in the pH range 2-13. It has been shown that the fluorescence and absorption spectra of protoporphyrin incorporated into the heme crevice remain constant in the pH range 5.5-10.8 but change significantly at pH less than 5.5 and pH greater than 10.8, due to the acid and alkaline denaturation, respectively, of the complex accompanied by dissociation of protoporphyrin IX. At the same pH ranges, the quantum yield of tryptophanyl fluorescence increases sharply as a result of removal of protoporphyrin, acting as a quencher, from the complex. Other parameters of tryptophanyl fluorescence (maximum position, halfwidth and spectrum shape) change in the alkaline region as well. In the acidic pH range, these parameters change only at pH less than 4.3, indicating that the Trp surroundings are more stable to denaturation than the heme crevice region. Between pH 5.5 and 10.9, where the complex of apomyoglobin with protoporphyrin IX is in its native state, the main parameters of tryptophan fluorescence remain unchanged except for the ratio I325/I350 which diminishes at pH greater than 9.5. Its alteration precedes the alkaline denaturation of the complex and can be explained by a local conformational change induced by the break of the 'salt bridges' essential for the maintenance of the native Mb structure in the N-terminal region. The fluorescence data obtained for apomyoglobin, myoglobin and the complex between protoporphyrin IX and apomyoglobin enable one to compare their structures and to evaluate the role of the porphyrin macrocycle and the iron atom in the formation of the native myoglobin structure and its functioning.

1H NMR study of the role of heme carboxylate side chains in modulating heme pocket structure and the mechanism of reconstitution of cytochrome b5

1H nuclear magnetic resonance spectroscopy was used to assign the hyperfine-shifted resonances and determine the position of a side chain in the heme cavity of wild-type rat apocytochrome b5 reconstituted with a series of synthetic hemins possessing systematically perturbed carboxylate side chains. The hemins included protohemin derivatives with individually removed or pairwise shortened and lengthened carboxylate side chains, as well as (propionate)n(methyl)8-nporphine-iron(III) isomers with n = 1-3 designed to force occupation of nonnative propionate sites. The resonance assignments were effected on the basis of available empirical heme contact shift correlations and steady-state nuclear Overhauser effect measurements in the low-spin oxidized proteins. The failure to detect holoproteins with certain hemins dictates that the stable holoproteins, unlike the case of myoglobin, demand the axial iron-His bonds and cannot accommodate carboxylate side chains at interior positions in the binding pocket. Hence, the heme pocket interior in cytochrome b5 is judged much less polar and less sterically accommodating than that of myoglobin. The propionate occupational preference was greatest as the native 7-propionate site, but also possible at the nonnative crystallographic 5-methyl or 8-methyl positions. Only for a propionate at the crystallographic 8-methyl position was a significant perturbation of the native molecular/electronic structure observed, and this was attributed to an alternative propionate-protein hydrogen bond at the crystallographic 8-methyl position. The structures of the transient protein complexes detected only shortly after reconstitution reveal that the initial encounter complexes during assembly of holoprotein from apoprotein and hemin involve one of the two alternate propionate-protein links at either the 7-propionate or native 8-methyl position. In a monopropionate hemin, this leads to the characterization of a new type of heme orientational disorder involving rotation about a N-Fe-N axis.

1H-NMR study of the mechanism of assembly and equilibrium heme orientation of sperm whale myoglobin reconstituted with protohemin type-isomers

The products of the incorporation of various protohemin type-isomers into the heme pocket of sperm whale myoglobin were investigated by 1H-NMR in the met-cyano complexes, both immediately after reconstitution as well as at equilibrium. The type-isomers studied include those involving all possible interchanges of the two substituents on a given pyrrole. The protohemin-III and -XIII isomers, with true 2-fold symmetry, yielded only homogeneous products. Protohemins-XI, -XIV both exhibited two species after reconstitution, with one disappearing with time. Protohemin-I was the only asymmetric hemin that failed to exhibit two isomers initially. The orientation of the hemin within the pocket was established by nuclear Overhauser detected dipolar connectivities among heme substituents and between heme substituents and assigned heme pocket residues. At equilibrium, the heme orientations were dominated by the asymmetric propionate rather than vinyl dispositions on the hemin, with a clear preference for placing a propionate at the 8- vs. 5-methyl position of native myoglobin. For protohemin-XI, the propionates were found in the unexpected positions of the 7-propionate and 2-vinyl groups of native myoglobin, indicating that propionates can occupy positions well within the hydrophobic interior. The alternate heme orientation for the metastable intermediates detected for protohemin-XI and -XIV involved rotational isomerism about the alpha,gamma-meso axes bisecting the vinyl positions, but these two axes are at right angles to each other in the protein matrix. The fact that protohemin-XIV, but not protohemin-I, exhibits a reversed orientation as a reconstitution intermediate provides direct evidence that vinyl contacts, as well as propionate links, modulate the relative stabilities of the initial encounter complexes between hemin and apomyoglobin. The heme cavity molecular/electronic structure was found largely unperturbed for the complexes of the various protohemin type-isomers.

1H NMR spectroscopic studies of calcium-binding proteins. 2. Histidine microenvironments in alpha- and beta-parvalbumins as determined by protonation and laser photochemically induced dynamic nuclear polarization effects

The microenvironments of the histidines in three isoforms of Ca(II)-bound parvalbumin (carp, pI = 4.25; pike, pI = 5.00; rat, pI = 5.50) have been examined with 1H NMR techniques to probe their protonation characteristics and photochemically induced dynamic nuclear polarizability (photo-CIDNP). The histidine at position 26 (or 25), present in all three of these proteins, shows absolutely no photo-CIDNP enhancement of its C2H or C5H resonances. Nor does this nonpolarizable histidine possess a normal pKa: values range only from 4.20 for carp to 4.32 for pike to 4.44 for rat. The C2H and C5H resonances of the histidine in this carp isoform split into doublets as the pH is lowered. The magnitude of this splitting depends on the magnetic field strength, temperature, and pH; however, the line intensities within each doublet are temperature-independent. Although the crystal structure of carp parvalbumin indicates that His-26 is exposed to solvent [Kretsinger, R. H., & Nockolds, C. E. (1973) J. Biol. Chem. 248, 3313-3326], we conclude that in solution this residue, in its unprotonated state, is part of the hydrophobic core of the protein. In contrast, His-48 in rat parvalbumin and His-106 in pike III parvalbumin show dramatic photo-CIDNP enhancements of their C2H, C5H, and beta-CH2 1H NMR resonances. Combined with its nearly normal pKa, 6.14, and exchange-broadened C2H resonance, the photo-CIDNP enhancement results for His-48 indicate that its microenvironment differs little from random-coil exposure, consistent with its presumed position on the solvent surface of helix C.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrogen-1 nuclear magnetic resonance investigation of high-potential iron-sulfur proteins from Ectothiorhodospira halophila and Ectothiorhodospira vacuolata: a comparative study of hyperfine-shifted resonances

Proton NMR spectra of the oxidized and reduced forms of high-potential iron-sulfur proteins (HiPIPs) were recorded at 200 MHz. The proteins studied were the HiPIPs I and II from Ectothiorhodospira halophila and Ectothiorhodospira vacuolata. Hyperfine-shifted peaks in spectra of the oxidized proteins were assigned to some of the protons of the cysteinyl ligands and aromatic residues at the active site on the basis of their chemical shifts, longitudinal relaxation times, and temperature-dependent behavior. The cysteinyl C beta-H protons were found to resonate downfield (about 100 ppm) and the C alpha-H protons upfield (about-25 ppm). This hyperfine shift pattern is consistent with the observed isotropic shift being contact in origin; it probably results from a pi-spin-transfer mechanism. The large magnitudes of the chemical shifts of peaks assigned to aromatic residues suggest that these residues interact with the iron-sulfur cluster via pi-pi overlap. Some of the hyperfine-shifted peaks observed in water were found to disappear in 2H2O solution. Such resonances probably arise from exchange-labile hydrogens of amino acid residues directly hydrogen bonded to the iron-sulfur cluster. In the case of HiPIPs I and II from E. vacuolata, whose spectra are similar except for the number of such peaks, the relative number of hydrogen bonds inferred to be present in the oxidized and reduced proteins qualitatively explains the difference between their midpoint redox potentials. On the other hand, for E. halophila HiPIPs I and II, consideration of the inferred number of hydrogen bonds alone fails to predict the sign of the difference between their midpoint redox potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Assignment of heme and distal amino acid resonances in the 1H-NMR spectra of the carbon monoxide and oxygen complexes of sperm whale myoglobin

Assignments of resonances of the heme and distal amino acid protons in spectra of the CO and O2 complexes of sperm whale myoglobin are reported. These resonances provide information on the conformation of the heme pocket. For oxymyoglobin, the assignments of the heme meso protons disagree with those proposed previously on the basis of partial deuteration experiments. Rapid ring flips about the C beta-C gamma bond are detected for Phe-CD1. Recent claims for two conformational substates of valine-E11 in carbonmonoxymyoglobin (Bradbury, J.H. and Carver, J.A. (1984) Biochemistry 23, 4905-4913) are shown to be in error. The pK of His-97 (FG3) in carbonmonoxymyoglobin has been determined (pK = 5.9). This residue appears to influence many spectroscopic properties of myoglobin. The distal His-E7 in carbonmonoxymyoglobin has pK less than 5.0. Differences in the heme pocket conformation in the CO complexes of myoglobin and leghemoglobin are discussed. These differences may be influential in O2 and CO association reactions.

Assignment of 1H NMR resonances of histidine and other aromatic residues in met-, cyano-, oxy-, and (carbon monoxy)myoglobins

The resolved 1H NMR resonances of the aromatic region in the 270-MHz NMR spectrum of sperm whale, horse, and pig metmyoglobin (metMb) have been assigned, including the observable H-2 and H-4 histidine resonances, the tryptophan H-2 resonances, and upfield-shifted resonances from one tyrosine residue. The use of different Mb species, carboxymethylation, and matching of pK values allows the assignment of the H-4 resonances, which agree in only three cases out of seven with scalar-correlated two-dimensional NMR spectroscopy assignments by others. The conversion to hydroxymyoglobin at high pH involves rearrangements throughout the molecule and is observed by many assigned residues. In sperm whale ferric cyanomyoglobin, nine H-2 and eight H-4 histidine resonances have been assigned, including the His-97 H-2 resonance and tyrosine resonances from residues 103 and 146. The hyperfine-shifted resonances from heme and near-heme protons observe a shift with a pK = 5.3 +/- 0.3 (probably due to deprotonation of His-97, pK = 5.6) and another shift at pK = 10.8 +/- 0.3. The spectrum of high-spin ferrous sperm whale deoxymyoglobin is very similar to that of metMb, which allows the assignment of seven surface histidine H-2 and H-4 resonances and also resonances from the two tryptophan residues and one tyrosine. In diamagnetic sperm whale (carbon monoxy)myoglobin (COMb), 10 His H-2 and 11 His H-4 resonances are observed, and 8 H-2 and 9 H-4 resonances are assigned, including His-64 H-4, the distal histidine. This important resonance is not observed in sperm whale oxymyoglobin, which in general shows very similar titration curves to COMb. Histidine-36 shows unusual titration behavior in the paramagnetic derivatives but normal behavior in the diamagnetic derivatives, which is discussed in the accompanying paper [Bradbury, J. H., & Carver, J. A. (1984) Biochemistry (following paper in this issue)].