Distal residue-CO interaction in carbonmonoxy myoglobins: a molecular dynamics study of three distal mutants.

Six 90-ps molecular dynamics trajectories, two for each of three distal mutants of sperm whale carbonmonoxy myoglobin, are reported; solvent waters within 16 A of the active site have been included. In both His64GIn trajectories, the distal side chain remains part of the heme pocket, forming a "closed" conformation similar to that of the wild type 64N delta H tautomer. Despite a connectivity more closely resembling the N epsilon H histidine tautomer, close interactions with the carbonyl ligand similar to those observed for the wild type 64N epsilon H tautomer are prevented in this mutant by repulsive interactions between the carbonyl O and the 64O epsilon. The aliphatic distal side chain of the His64Leu mutant shows little interaction with the carbonyl ligand in either His64Leu trajectory. Solvent water molecules move into and out of the active site in the His64Gly mutant trajectories; during all the other carbonmonoxy myoglobin trajectories, including the wild type distal tautomers considered in an earlier work, solvent molecules rarely encroach closer than 6 A of the active site. These results are consistent with a recent structural interpretation of the wild type infrared spectrum, and the current reinterpretation that the distal-ligand interaction in carbonmonoxy myoglobin is largely electrostatic, not steric, in nature.     

Influence of the heme pocket conformation on the structure and vibrations of the Fe-CO bond in myoglobin: a QM/MM density functional study.

The influence of the distal pocket conformation on the structure and vibrations of the heme-CO bond in carbonmonoxy myoglobin (MbCO) is investigated by means of hybrid QM/MM calculations based on density functional theory combined with a classical force field. It is shown that the heme-CO structure (QM treated) is quite rigid and not influenced by the distal pocket conformation (MM treated). This excludes any relation between FeCO distortions and the different CO absorptions observed in the infrared spectra of MbCO (A states). In contrast, both the CO stretch frequency and the strength of the CO...His64 interaction are very dependent on the orientation and tautomerization state of His64. Our calculations indicate that the CO...N(epsilon) type of approach does not contribute to the A states, whereas the CO...H-N(epsilon) interaction is the origin of the A(1) and A(3) states, the His64 residue being protonated at N(epsilon). The strength of the CO...His64 interaction is quantified, in comparison with the analogous O(2)...His64 interaction and with the observed changes in the CO stretch frequency. Additional aspects of the CO...His64 interaction and its biological implications are discussed.     

Structural dynamics of distal histidine replaced mutants of myoglobin accompanied with the photodissociation reaction of the ligand

Protein dynamics observed by the transient grating (TG) method are studied for some site-directed mutants at the distal histidine of myoglobin (H64L, H64Q, H64V). The time profiles of the TG signals are very sensitive to the amino acid residue of the 64 position. It was found that the sensitivity is mostly caused by the different rates of the ligand escape from the protein to solvent and the magnitude of the molecular volume change. Several molecular origins of the volume difference between MbCO and Mb, such as the electrostatic interaction in the distal pocket, movement of helices, and distal water, are proposed. Interestingly, the volume difference between the CO-trapped Mb inside the protein interior and Mb is similar to that of the partial molar volume of CO in organic solvent. The effect of mutation on the nature of the CO trapped site is discussed.     

Structural characterization of the proximal and distal histidine environment of cytoglobin and neuroglobin

Cytoglobin (Cgb) and neuroglobin (Ngb) are the first examples of hexacoordinated globins from humans and other vertebrates in which a histidine (His) residue at the sixth position of the heme iron is an endogenous ligand in both the ferric and ferrous forms. Static and time-resolved resonance Raman and FT-IR spectroscopic techniques were applied in examining the structures in the heme environment of these globins. Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy of transient five-coordinate heme species produced by the photolysis of carbon monoxide (CO) adducts of Cgb and Ngb showed Fe-His stretching (nu(Fe-His)) bands at 229 and 221 cm(-1), respectively. No time-dependent shift in the nu(Fe-His) band of Cgb and Ngb was detected in the 20-1000 ps time domain, in contrast to the case of myoglobin (Mb). These spectroscopic data, combined with previously reported crystallographic data, suggest that the structure of the heme pocket in Cgb and Ngb is altered upon CO binding in a manner different from that of Mb and that the scales of the structural alteration are different for Cgb and Ngb. The structural property of the heme distal side of the ligand-bound forms was investigated by observing the sets of (nu(Fe-CO), nu(C-O), delta(Fe-C-O)) and (nu(Fe-NO), nu(N-O), delta(Fe-N-O)) for the CO and nitric oxide (NO) complexes of Cgb and Ngb. A comparison of the spectra of some distal mutants of Cgb (H81A, H81V, R84A, R84K, and R84T) and Ngb (H64A, H64V, K67A, K67R, and K67T) showed that the CO adducts of Cgb and Ngb contained three conformers and that the distal His (His81 in Cgb and His64 in Ngb) mainly contributes to the interconversion of the conformers. These structural characteristics of Cgb and Ngb are discussed in relation to their ligand binding and physiological properties.     

A study of vibrational relaxation of B-state carbon monoxide in the heme pocket of photolyzed carboxymyoglobin.

The vibrational energy relaxation of dissociated carbon monoxide in the heme pocket of sperm whale myoglobin has been studied using equilibrium molecular dynamics simulation and normal mode analysis methods. Molecular dynamics trajectories of solvated myoglobin were run at 300 K for both the delta- and epsilon-tautomers of the distal histidine, His64. Vibrational population relaxation times were estimated using the Landau-Teller model. For carbon monoxide (CO) in the myoglobin epsilon-tautomer, for a frequency of omega0 = 2131 cm-1 corresponding to the B1 state, T1epsilon(B1) = 640 +/- 185 ps, and for a frequency of omega0 = 2119 cm-1 corresponding to the B2 state, T1epsilon(B2) = 590 +/- 175 ps. Although the CO relaxation rates in both the epsilon- and delta-tautomers are similar in magnitude, the simulations predict that the vibrational relaxation of the CO is faster in the delta-tautomer. For CO in the myoglobin delta-tautomer, it was found that the relaxation times were identical within error for the two CO substate frequencies, T1delta(B1) = 335 +/- 115 ps and T1delta(B2) = 330 +/- 145 ps. These simulation results are in reasonable agreement with experimental results of Anfinrud and coworkers (unpublished results). Normal mode calculations were used to identify the dominant coupling between the protein and CO molecules. The calculations suggest that the residues of the myoglobin pocket, acting as a first solvation shell to the CO molecule, contribute the primary "doorway" modes in the vibrational relaxation of the oscillator.     

Crystal structures of myoglobin-ligand complexes at near-atomic resolution.

We have used x-ray crystallography to determine the structures of sperm whale myoglobin (Mb) in four different ligation states (unligated, ferric aquomet, oxygenated, and carbonmonoxygenated) to a resolution of better than 1.2 A. Data collection and analysis were performed in as much the same way as possible to reduce model bias in differences between structures. The structural differences among the ligation states are much smaller than previously estimated, with differences of <0.25 A root-mean-square deviation among all atoms. One structural parameter previously thought to vary among the ligation states, the proximal histidine (His-93) azimuthal angle, is nearly identical in all the ferrous complexes, although the tilt of the proximal histidine is different in the unligated form. There are significant differences, however, in the heme geometry, in the position of the heme in the pocket, and in the distal histidine (His-64) conformations. In the CO complex the majority conformation of ligand is at an angle of 18 +/- 3 degrees with respect to the heme plane, with a geometry similar to that seen in encumbered model compounds; this angle is significantly smaller than reported previously by crystallographic studies on monoclinic Mb crystals, but still significantly larger than observed by photoselection. The distal histidine in unligated Mb and in the dioxygenated complex is best described as having two conformations. Two similar conformations are observed in MbCO, in addition to another conformation that has been seen previously in low-pH structures where His-64 is doubly protonated. We suggest that these conformations of the distal histidine correspond to the different conformational substates of MbCO and MbO(2) seen in vibrational spectra. Full-matrix refinement provides uncertainty estimates of important structural parameters. Anisotropic refinement yields information about correlated disorder of atoms; we find that the proximal (F) helix and heme move approximately as rigid bodies, but that the distal (E) helix does not.     

Tautomerism,acid-base equilibria,and H-bonding of the six histidines in subtilisin BPN' by NMR

We have determined by (15)N, (1)H, and (13)C NMR, the chemical behavior of the six histidines in subtilisin BPN' and their PMSF and peptide boronic acid complexes in aqueous solution as a function of pH in the range of from 5 to 11, and have assigned every (15)N, (1)H, C(epsilon 1), and C(delta2) resonance of all His side chains in resting enzyme. Four of the six histidine residues (17, 39, 67, and 226) are neutrally charged and do not titrate. One histidine (238), located on the protein surface, titrates with pK(a) = 7.30 +/- 0.03 at 25 degrees C, having rapid proton exchange, but restricted mobility. The active site histidine (64) in mutant N155A titrates with a pK(a) value of 7.9 +/- 0.3 and sluggish proton exchange behavior, as shown by two-site exchange computer lineshape simulation. His 64 in resting enzyme contains an extremely high C(epsilon 1)-H proton chemical shift of 9.30 parts per million (ppm) owing to a conserved C(epsilon 1)-H(.)O=C H-bond from the active site imidazole to a backbone carbonyl group, which is found in all known serine proteases representing all four superfamilies. Only His 226, and His 64 at high pH, exist as the rare N(delta1)-H tautomer, exhibiting (13)C(delta1) chemical shifts approximately 9 ppm higher than those for N(epsilon 2)-H tautomers. His 64 in the PMSF complex, unlike that in the resting enzyme, is highly mobile in its low pH form, as shown by (15)N-(1)H NOE effects, and titrates with rapid proton exchange kinetics linked to a pK(a) value of 7.47 +/- 0.02.     

Resonance raman investigations of site-directed mutants of myoglobin: effects of distal histidine replacement

The resonance Raman spectra of met-, deoxy-, and (carbonmonoxy)myoglobin (MbCO) are studied as a function of amino acid replacement at the distal histidine-E7 position. The synthetic wild type is found to be spectroscopically identical with the native material. The methionine and glycine replacements do not affect the met or deoxy spectra but do lead to distinct changes in the nu Fe-CO region of the MbCO spectrum. The native MbCO displays a pH-dependent population redistribution of the nu Fe-CO modes, while the analogous population in the mutant systems is found to be pH independent. This indicates that histidine-E7 is the titratable group in native MbCO. Moreover, the pH dependence of the population dynamics is found to be inconsistent with a simple two-state Henderson-Hasselbalch analysis. Instead, we suggest a four-state model involving the coupling of histidine protonation and conformational change. Within this model, the pK of the distal histidine is found to be 6.0 in the "open" configuration and 3.8 in the "closed" conformation. This corresponds to a 3 kcal/mol destabilization of the positively charged distal histidine within the hydrophobic pocket and suggests how protonation can lead to a larger population of the "open" conformation. At pH 7, the pocket is found to be "open" approximately 3% of the time. Further work, involving both IR and Raman measurements, allows the electron-nuclear coupling strengths of the various nu Fe-CO and nu C-O Raman modes to be determined. The slowly rebinding conformational state, corresponding to nu Fe-CO = 518 cm-1 (nu C-O = 1932 cm-1), displays unusually weak coupling of the Fe-CO mode to the Soret transition. Studies of the nu Fe-CO region as a function of temperature reveal that the equilibria between the conformational states are quenched in both the native and glycine mutant below the freezing point of the solvent. Unusual line narrowing of the nu Fe-CO modes at the phase transition is also observed in all samples studied. This line narrowing stands in marked contrast to the other heme Raman modes and suggests that Fe-CO librational motion and/or distal pocket vibrational (or conformational) excitations are involved in the line broadening at room temperature.     

Alternative carbon monoxide binding modes for horseradish peroxidase studied by resonance Raman spectroscopy

Resonance Raman (RR) spectroscopy and infrared spectroscopy have been used to characterize the three vibrational modes, CO and FeC stretching and FeCO bending, for carbon monoxide bound to reduced horseradish peroxidase, with the aid of 13CO and C18O isotope shifts. At high pH, one species, I, is observed, with nu FeC = 490 cm-1 and nu CO = 1932 cm-1. The absence of a band attributable to delta FeCO suggests a linear FeCO unit normal to the heme plane. The data were consistent with I having a strongly H-bonded proximal histidine, as shown by a comparison with imidazole and imidazolate adducts of FeIIPPDME(CO) (PPDME = protoporphyrin IX dimethyl ester), with nu FeC = 497 and 492 cm-1 and nu CO = 1960 and 1942 cm-1. At low pH an additional species, II, is observed, with nu FeC = 537 cm-1, nu CO = 1904 cm-1, and delta FeCO = 587 cm-1; it is attributed to FeCO that is H bonded to a protonated distal histidine, the H bond strongly lowering nu CO and raising nu FeC. The appearance of delta FeCO in the RR spectrum suggests that the FeCO unit in II is tilted with respect to the heme plane. At low pH, the population of I and II depends on the CO concentration. I dominates at low CO/protein levels but is replaced by II as the amount of CO is increased. This behavior is suggested to arise from secondary binding of CO, which induces a conformation change involving the distal residues of the heme pocket.     

Rearrangement of the distal pocket accompanying E7 His----Gln substitution in elephant carbonmonoxy- and oxymyoglobin: 1H NMR identification of a new aromatic residue in the heme pocket

Two-dimensional 1H NMR methods have been used to assign side-chain resonances for the residues in the distal heme pocket of elephant carbonmonoxymyoglobin (MbCO) and oxymyoglobin (MbO2). It is shown that, while the other residues in the heme pocket are minimally perturbed, the Phe CD4 residue in elephant MbCO and MbO2 resonates considerably upfield compared to the corresponding residue in sperm whale MbCO. The new NOE connectivities to Val E11 and heme-induced ring current calculations indicate that Phe CD4 has been inserted into the distal heme pocket by reorienting the aromatic side chain and moving the CD corner closer to the heme. The C zeta H proton of the Phe CD4 was found to move toward the iron of the heme by approximately 4 A relative to the position of sperm whale MbCO, requiring minimally a 3-A movement of the CD helical backbone. The significantly altered distal conformation in elephant myoglobin, rather than the single distal E7 substitution, forms a plausible basis for its altered functional properties of lower autoxidation rate, higher redox potential, and increased affinity for CO ligand. These results demonstrate that one-to-one interpretation of amino acid residue substitution (E7 His----Gln) is oversimplified and that conformational changes of substituted proteins which are not readily predicted have to be considered for interpretation of their functional properties.     

Resonance Raman evidence that distal histidine protonation removes the steric hindrance to upright binding of carbon monoxide by myoglobin

The resonance Raman band assigned to Fe--CO stretching in the sperm whale myoglobin CO adduct shifts from 507 cm-1 at neutral pH to 488 cm-1 at low pH, in concert with a shift of the C-O stretching infrared band from 1947 to 1967 cm-1 (Fuchsman & Appleby, 1979), while the 575-cm-1 Fe-C-O bending RR band loses intensity. The pKa that characterizes these changes is approximately 4.4. The vibrational frequencies at low pH are well modeled by the protein-free CO, imidazole adduct of protoheme in a nonpolar solvent while those at high pH are modeled by the adduct of a heme with a covalent strap (Yu et al., 1983) which inhibits upright CO binding. It is inferred that the Fe-C-O unit changes from a tilted to an upright geometry when the distal histidine is protonated, because its side chain swings out of the heme pocket due to electrostatic repulsion with a nearby arginine residue. A different protonation step (pKa = 5.7), which has been shown to modulate the CO rebinding kinetics (Doster et al., 1982) as well as the optical spectrum (Fuchsman & Appleby, 1979), is suggested to involve a global structure change associated with protonation of histidine residues distant from the heme.     

Discrimination between oxygen and carbon monoxide and inhibition of autooxidation by myoglobin. Site-directed mutagenesis of the distal histidine

Sperm whale myoglobin mutants were constructed using site-directed mutagenesis to replace the highly conserved distal histidine residue (His(E7)-64). His-64 was substituted with Gly, Val, Phe, Cys, Met, Lys, Arg, Asp, Thr, and Tyr, and all 10 mutant proteins expressed to approximately 10% of the total soluble cell protein in Escherichia coli as heme containing myoglobin. With the exception of His-64----Tyr, which did not form a stable oxygen (O2) complex, all mutant proteins could be reduced and bound O2 and carbon monoxide (CO) reversibly. However, removal of the distal histidine increased the rate of autooxidation 40-350-fold. The His-64----Gly, Val, Phe, Met, and Arg mutants all showed markedly increased O2 dissociation rate constants which were approximately 50-1500-fold higher than those for wild-type myoglobin and increased O2 association rate constants which were approximately 5-15-fold higher than those for the native protein. All mutants studied (except His-64----Tyr) showed approximately 10-fold increased CO association rates and relatively unchanged CO dissociation rates. These altered O2 and CO association and dissociation rate constants resulted in 3-14-fold increased CO affinities, 10-200-fold decreased O2 affinities, and 50-380-fold greater M (KCO/KO2) values for the mutants compared to the wild-type protein. Thus, the distal histidine of myoglobin discriminates between CO and O2 binding by both sterically hindering bound CO and stabilizing bound O2 through hydrogen bonding. The increased autooxidation rates observed for the mutants appear to be due to a decrease in oxygen affinity and an increase in solvent anion accessibility to the distal pocket.     

Inversion of axial coordination in myoglobin to create a "proximal" ligand binding pocket

A ligand binding pocket has been created on the proximal side of the heme in porcine myoglobin by site-directed mutagenesis. Our starting point was the H64V/V68H double mutant which has been shown to have bis-histidine (His68 and His93) heme coordination [Dou, Y., Admiraal, S. J., Ikeda-Saito, M., Krzywda, S., Wilkinson, A. J., Li, T., Olson, J. S., Prince, R. C., Pickering, I. J., George, G. N. (1995) J. Biol. Chem. 270, 15993-16001]. The replacement of the proximal His93 ligand by noncoordinating Ala (H64V/V68H/H93A) or Gly (H64V/V68H/H93G) residues resulted unexpectedly in a six-coordinate low-spin species in both ferric and ferrous states. To test the hypothesis that the sixth coordinating ligand in the triple mutants was the imidazole of His97, this residue was mutated to Phe, in the quadruple mutants, H64V/V68H/H93A/H97F and H64V/V68H/H93G/H97F. The ferric quadruple mutants show a clear water/hydroxide alkaline transition and high cyanide and CO affinities, characteristics similar to those of wild-type myoglobin. The nu(Fe-CO) and nu(C-O) stretching frequencies in the ferrous-CO state of the quadruple mutants indicate that the "proximal" ligand binding heme pocket is less polar than the distal pocket in the wild-type protein. Thus, we conclude that the proximal heme pocket in the quadruple mutants has a similar affinity for exogenous ligands to the distal pocket of wild-type myoglobin but that the two pockets have different polarities. The quadruple mutants open up new approaches for developing heme chemistry on the myoglobin scaffold.     

Dual nature of the distal histidine residue in the autoxidation reaction of myoglobin and hemoglobin comparison of the H64 mutants.

The oxygenated form of myoglobin or hemoglobin is oxidized easily to the ferric met-form with generation of the superoxide anion. To make clear the possible role(s) of the distal histidine (H64) residue in the reaction, we have carried out detailed pH-dependence studies of the autoxidation rate, using some typical H64 mutants of sperm whale myoglobin, over the wide range of pH 5-12 in 0.1 M buffer at 25 degrees C. Each mutation caused a dramatic increase in the autoxidation rate with the trend H64V >/= H64G >/= H64L > H64Q > H64 (wild-type) at pH 7.0, whereas each mutant protein showed a characteristic pH-profile which is essentially different from that of the wild-type or native sperm whale MbO2. In particular, all the mutants have lost the acid-catalyzed process that can play a dominant role in the autoxidation reaction of most mammalian myoglobins or hemoglobins. Kinetic analyses of various types of pH-profiles lead us to conclude that the distal histidine residue can play a dual role in the nucleophilic displacement of O2- from MbO2 or HbO2 in protic, aqueous solution. One is in a proton-relay mechanism via its imidazole ring, and the other is in the maximum protection of the FeO2 center against a water molecule or an hydroxyl ion that can enter the heme pocket from the surrounding solvent.     

Kinetic, structural, and spectroscopic identification of geminate states of myoglobin: a ligand binding site on the reaction pathway

Elementary steps or geminate states in the reaction of gaseous ligands with transport proteins delineate the trajectory of the ligand and its rebinding to the heme. By use of kinetic studies of the 765-nm optical "conformation" band, three geminate states were identified for temperatures less than approximately 100 K. MbCO, which is accumulated by photolysis between 1.2 and approximately 10 K, was characterized by our previous optical and X-ray absorption studies [Chance, B., Fischetti, R., & Powers, L. (1983) Biochemistry 22, 3820-3829]. Between 10 and approximately 100 K, geminate states that are also identified that have recombination rates of approximately 10(3) s-1 and approximately 10(-5) s-1 (40 K). Thus, it is possible to maintain a steady-state nearly homogeneous population of the slowest recombining geminate state, Mb, by regulated continuous illumination (optical pumping). Both X-ray absorption and resonance Raman studies under similar conditions of optical pumping show that the heme structure around the iron in Mb is similar to that of MbCO. In both geminate states, the iron-proximal histidine distance remains unchanged (+/- 0.02 A) from that of MbCO while the iron to pyrrole nitrogen average distance has not fully relaxed to that of the deoxy state. In MbCO the CO remains close to iron but not bound, and the Fe...CO angle, which is bent in MbCO (127 +/- 4 degrees C), is decreased by approximately 15 degrees [Powers, L., Sessler, J. L., Woolery, G. L., & Chance, B. (1984) Biochemistry 23, 5519-5523]. The CO molecule in Mb, however, has moved approximately 0.7 A further from iron. Computer graphics modeling of the crystal structure of MbCO places the CO in a crevice in the heme pocket that is just large enough for the CO molecule end-on. Above approximately 100 K resonance Raman studies show that this structure relaxes to the deoxy state.     

N5-CAIR mutase: role of a CO2 binding site and substrate movement in catalysis

N5-Carboxyaminoimidazole ribonucleotide mutase (N5-CAIR mutase or PurE) from Escherichia coli catalyzes the reversible interconversion of N5-CAIR to carboxyaminoimidazole ribonucleotide (CAIR) with direct CO2 transfer. Site-directed mutagenesis, a pH-rate profile, DFT calculations, and X-ray crystallography together provide new insight into the mechanism of this unusual transformation. These studies suggest that a conserved, protonated histidine (His45) plays an essential role in catalysis. The importance of proton transfers is supported by DFT calculations on CAIR and N5-CAIR analogues in which the ribose 5'-phosphate is replaced with a methyl group. The calculations suggest that the nonaromatic tautomer of CAIR (isoCAIR) is only 3.1 kcal/mol higher in energy than its aromatic counterpart, implicating this species as a potential intermediate in the PurE-catalyzed reaction. A structure of wild-type PurE cocrystallized with 4-nitroaminoimidazole ribonucleotide (NO2-AIR, a CAIR analogue) and structures of H45N and H45Q PurEs soaked with CAIR have been determined and provide the first insight into the binding of an intact PurE substrate. A comparison of 19 available structures of PurE and PurE mutants in apo and nucleotide-bound forms reveals a common, buried carboxylate or CO2 binding site for CAIR and N5-CAIR in a hydrophobic pocket in which the carboxylate or CO2 interacts with backbone amides. This work has led to a mechanistic proposal in which the carboxylate orients the substrate for proton transfer from His45 to N5-CAIR to form an enzyme-bound aminoimidazole ribonucleotide (AIR) and CO2 intermediate. Subsequent movement of the aminoimidazole moiety of AIR reorients it for addition of CO2 at C4 to generate isoCAIR. His45 is now in a position to remove a C4 proton to produce CAIR.     

Structural and chemical requirements for histidine phosphorylation by the chemotaxis kinase CheA

The CheA histidine kinase initiates the signal transduction pathway of bacterial chemotaxis by autophosphorylating a conserved histidine on its phosphotransferase domain (P1). Site-directed mutations of neighboring conserved P1 residues (Glu-67, Lys-48, and His-64) show that a hydrogen-bonding network controls the reactivity of the phospho-accepting His (His-45) in Thermotoga maritima CheA. In particular, the conservative mutation E67Q dramatically reduces phosphotransfer to P1 without significantly affecting the affinity of P1 for the CheA ATP-binding domain. High resolution crystallographic studies revealed that although all mutants disrupt the hydrogen-bonding network to varying degrees, none affect the conformation of His-45. 15N-NMR chemical shift studies instead showed that Glu-67 functions to stabilize the unfavored N(delta1)H tautomer of His-45, thereby rendering the N(epsilon2) imidazole unprotonated and well positioned for accepting the ATP phosphoryl group.     

Changes in the apomyoglobin folding pathway caused by mutation of the distal histidine residue

Factors governing the folding pathways and the stability of apomyoglobin have been examined by replacing the distal histidine at position 64 with phenylalanine (H64F). Acid and urea-induced unfolding experiments using CD and fluorescence techniques reveal that the mutant H64F apoprotein is significantly more stable than wild-type apoMb. Kinetic refolding studies of this variant also show a significant difference from wild-type apoMb. The amplitude of the burst phase ellipticity in stopped-flow CD measurements is increased over that of wild-type, an indication that the secondary structure content of the earliest kinetic intermediate is greater in the mutant than in the wild-type protein. In addition, the overall rate of folding is markedly increased. Hydrogen exchange pulse labeling was used to establish the structure of the initial intermediate formed during the burst phase of the H64F mutant. NMR analysis of the samples obtained at different refolding times indicates that the burst phase intermediate contains a stabilized E helix as well as the A, G, and H helices previously found in the wild-type kinetic intermediate. Replacement of the polar distal histidine residue with a nonpolar residue of similar size and shape appears to stabilize the E helix in the early stages of folding due to improved hydrophobic packing. The presence of a hydrophilic histidine at position 64 thus exacts a price in the stability and folding efficiency of the apoprotein, but this residue is nevertheless highly conserved among myoglobins due to its importance in function.     

Theoretical study of the discrimination between O(2) and CO by myoglobin

Combined quantum chemical and molecular mechanics geometry optimisations have been performed on myoglobin without or with O(2) or CO bound to the haem group. The results show that the distal histidine residue is protonated on the N(epsilon 2) atom and forms a hydrogen bond to the haem ligand both in the O(2) and the CO complexes. We have also re-refined the crystal structure of CO[bond]myoglobin by a combined quantum chemical and crystallographic refinement. Thereby, we probably obtain the most accurate available structure of the active site of this complex, showing a Fe[bond]C[bond]O angle of 171 degrees, and Fe[bond]C and C[bond]O bond lengths of 170-171 and 116-117 pm. The resulting structures have been used to calculate the strength of the hydrogen bond between the distal histidine residue and O(2) or CO in the protein. This amounts to 31-33 kJ/mol for O(2) and 2-3 kJ/mol for CO. The difference in hydrogen-bond strength is 21-22 kJ/mol when corrected for entropy effects. This is slightly larger than the observed discrimination between O(2) or CO by myoglobin, 17 kJ/mol. We have also estimated the strain of the active site inside the protein. It is 2-4 kJ/mol larger for the O(2) complex than for the CO complex, independent of which crystal structure the calculations are based on. Together, these results clearly show that myoglobin discriminates between O(2) and CO mainly by electrostatic interactions, rather than by steric strain.     

Tautomerism of histidine 64 associated with proton transfer in catalysis of carbonic anhydrase

The imidazole (15)N signals of histidine 64 (His(64)), involved in the catalytic function of human carbonic anhydrase II (hCAII), were assigned unambiguously. This was accomplished by incorporating the labeled histidine as probes for solution NMR analysis, with (15)N at ring-N(delta1) and N(epsilon2), (13)Cat ring-Cepsilon1, (13)C and (15)N at all carbon and nitrogen, or (15)N at the amide nitrogen and the labeled glycine with (13)C at the carbonyl carbon. Using the pH dependence of ring-(15)N signals and a comparison between experimental and simulated curves, we determined that the tautomeric equilibrium constant (K(T)) of His(64) is 1.0, which differs from that of other histidine residues. This unique value characterizes the imidazole nitrogen atoms of His(64) as both a general acid (a) and base (b): its epsilon2-nitrogen as (a) releases one proton into the bulk, whereas its delta1-nitrogen as (b) extracts another proton from a water molecule within the water bridge coupling to the zinc-bound water inside the cave. This accelerates the generation of zinc-bound hydroxide to react with the carbon dioxide. Releasing the productive bicarbonate ion from the inside separates the water bridge pathway, in which the next water molecules move into beside zinc ion. A new water molecule is supplied from the bulk to near the delta1-nitrogen of His(64). These reconstitute the water bridge. Based on these features, we suggest here a catalytic mechanism for hCAII: the tautomerization of His(64) can mediate the transfers of both protons and water molecules at a neutral pH with high efficiency, requiring no time- or energy-consuming processes.