The assignment of carbon monoxide association rate constants to the alpha and beta subunits in native and mutant human deoxyhemoglobin tetramers

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

Tertiary and quaternary structural basis of oxygen affinity in human hemoglobin as revealed by multiscale simulations

Human hemoglobin (Hb) is a benchmark protein of structural biology that shaped our view of allosterism over 60 years ago, with the introduction of the MWC model based on Perutz structures of the oxy(R) and deoxy(T) states and the more recent Tertiary Two-State model that proposed the existence of individual subunit states -“r” and “t”-, whose structure is yet unknown. Cooperative oxygen binding is essential for Hb function, and despite decades of research there are still open questions related to how tertiary and quaternary changes regulate oxygen affinity. In the present work, we have determined the free energy profiles of oxygen migration and for HisE7 gate opening, with QM/MM calculations of the oxygen binding energy in order to address the influence of tertiary differences in the control of oxygen affinity. Our results show that in the α subunit the low to high affinity transition is achieved by a proximal effect that mostly affects oxygen dissociation and is the driving force of the allosteric transition, while in the β subunit the affinity change results from a complex interplay of proximal and distal effects, including an increase in the HE7 gate opening, that as shown by free energy profiles promotes oxygen uptake.

Differential control of heme reactivity in alpha and beta subunits of hemoglobin: a combined Raman spectroscopic and computational study

The use of hybrid hemoglobin (Hb), with mesoheme substituted for protoheme, allows separate monitoring of the α or β hemes along the allosteric pathway. Using resonance Raman (rR) spectroscopy in silica gel, which greatly slows protein motions, we have observed that the Fe-histidine stretching frequency, νFeHis, which is a monitor of heme reactivity, evolves between frequencies characteristic of the R and T states, for both α or β chains, prior to the quaternary R-T and T-R shifts. Computation of νFeHis, using QM/MM and the conformational search program PELE, produced remarkable agreement with experiment. Analysis of the PELE structures showed that the νFeHis shifts resulted from heme distortion and, in the α chain, Fe-His bond tilting. These results support the tertiary two-state model of ligand binding (Henry et al., Biophys. Chem. 2002, 98, 149). Experimentally, the νFeHis evolution is faster for β than for α chains, and pump-probe rR spectroscopy in solution reveals an inflection in the νFeHis time course at 3 μs for β but not for α hemes, an interval previously shown to be the first step in the R-T transition. In the α chain νFeHis dropped sharply at 20 μs, the final step in the R-T transition. The time courses are fully consistent with recent computational mapping of the R-T transition via conjugate peak refinement by Karplus and co-workers (Fischer et al., Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 5608). The effector molecule IHP was found to lower νFeHis selectively for α chains within the R state, and a binding site in the α1α2 cleft is suggested.

Autoxidation and oxygen binding properties of recombinant hemoglobins with substitutions at the αVal-62 or βVal-67 position of the distal heme pocket

The E11 valine in the distal heme pocket of either the α- or β-subunit of human adult hemoglobin (Hb A) was replaced by leucine, isoleucine, or phenylalanine. Recombinant proteins were expressed in Escherichia coli and purified for structural and functional studies. (1)H NMR spectra were obtained for the CO and deoxy forms of Hb A and the mutants. The mutations did not disturb the α1β2 interface in either form, whereas the H-bond between αHis-103 and βGln-131 in the α1β1 interfaces of the deoxy α-subunit mutants was weakened. Localized structural changes in the mutated heme pocket were detected for the CO form of recombinant Hb (rHb) (αV62F), rHb (βV67I), and rHb (βV67F) compared with Hb A. In the deoxy form the proximal histidyl residue in the β-subunit of rHb (βV67F) has been altered. Furthermore, the interactions between the porphyrin ring and heme pocket residues have been perturbed in rHb (αV62I), rHb (αV62F), and rHb (βV67F). Functionally, the oxygen binding affinity (P50), cooperativity (n50), and the alkaline Bohr Effect of the three α-subunit mutants and rHb (βV67L) are similar to those of Hb A. rHb (βV67I) and rHb (βV67F) exhibit low and high oxygen affinity, respectively. rHb (βV67F) has P50 values lower that those reported for rHb (αL29F), a B10 mutant studied previously in our laboratory (Wiltrout, M. E., Giovannelli, J. L., Simplaceanu, V., Lukin, J. A., Ho, N. T., and Ho, C. (2005) Biochemistry 44, 7207-7217). These E11 mutations do not slow down the autoxidation and azide-induced oxidation rates of the recombinant proteins. Results from this study provide new insights into the roles of E11 mutants in the structure-function relationship in hemoglobin.

Blocking the gate to ligand entry in human hemoglobin

His(E7) to Trp replacements in HbA lead to markedly biphasic bimolecular CO rebinding after laser photolysis. For isolated mutant subunits, the fraction of fast phase increases with increasing [CO], suggesting a competition between binding to an open conformation with an empty E7 channel and relaxation to blocked or closed, slowly reacting states. The rate of conformational relaxation of the open state is ∼18,000 s(-1) in α subunits and ∼10-fold faster in β subunits, ∼175,000 s(-1). Crystal structures were determined for tetrameric α(WT)β(Trp-63) HbCO, α(Trp-58)β(WT) deoxyHb, and Trp-64 deoxy- and CO-Mb as controls. In Trp-63(E7) βCO, the indole side chain is located in the solvent interface, blocking entry into the E7 channel. Similar blocked Trp-64(E7) conformations are observed in the mutant Mb crystal structures. In Trp-58(E7) deoxy-α subunits, the indole side chain fills both the channel and the distal pocket, forming a completely closed state. The bimolecular rate constant for CO binding, k'(CO), to the open conformations of both mutant Hb subunits is ∼80-90 μm(-1) s(-1), whereas k'(CO) for the completely closed states is 1000-fold slower, ∼0.08 μm(-1) s(-1). A transient intermediate with k'(CO) ≈ 0.7 μm(-1) s(-1) is observed after photolysis of Trp-63(E7) βCO subunits and indicates that the indole ring blocks the entrance to the E7 channel, as observed in the crystal structures of Trp(E7) deoxyMb and βCO subunits. Thus, either blocking or completely filling the E7 channel dramatically slows bimolecular binding, providing strong evidence that the E7 channel is the major pathway (≥90%) for ligand entry in human hemoglobin.

Distal histidine stabilizes bound O2 and acts as a gate for ligand entry in both subunits of adult human hemoglobin

The role of the distal histidine in regulating ligand binding to adult human hemoglobin (HbA) was re-examined systematically by preparing His(E7) to Gly, Ala, Leu, Gln, Phe, and Trp mutants of both Hb subunits. Rate constants for O(2), CO, and NO binding were measured using rapid mixing and laser photolysis experiments designed to minimize autoxidation of the unstable apolar E7 mutants. Replacing His(E7) with Gly, Ala, Leu, or Phe causes 20-500-fold increases in the rates of O(2) dissociation from either Hb subunit, demonstrating unambiguously that the native His(E7) imidazole side chain forms a strong hydrogen bond with bound O(2) in both the alpha and beta chains (DeltaG(His(E7)H-bond) approximately -8 kJ/mol). As the size of the E7 amino acid is increased from Gly to Phe, decreases in k(O2)', k(NO)', and calculated bimolecular rates of CO entry (k(entry)') are observed. Replacing His(E7) with Trp causes further decreases in k(O2)', k(NO)', and k(entry)' to 1-2 microM(-1) s(-1) in beta subunits, whereas ligand rebinding to alphaTrp(E7) subunits after photolysis is markedly biphasic, with fast k(O2)', k(CO)', and k(NO)' values approximately 150 microM(-1) s(-1) and slow rate constants approximately 0.1 to 1 microM(-1) s(-1). Rapid bimolecular rebinding to an open alpha subunit conformation occurs immediately after photolysis of the alphaTrp(E7) mutant at high ligand concentrations. However, at equilibrium the closed alphaTrp(E7) side chain inhibits the rate of ligand binding >200-fold. These data suggest strongly that the E7 side chain functions as a gate for ligand entry in both HbA subunits.

CO-MP4, a polyethylene glycol-conjugated haemoglobin derivative and carbon monoxide carrier that reduces myocardial infarct size in rats

BACKGROUND AND PURPOSE

MP4 (Hemospan) is a Hb-based oxygen therapeutic agent, based on polyethylene-glycol (PEG) conjugation to Hb, undergoing clinical trials as an oxygen carrier. This study describes the functional interaction between MP4 and carbon monoxide (CO), as a CO delivery agent, and the effects of CO-MP4 on myocardial infarct size following ischaemia and reperfusion in rats.

EXPERIMENTAL APPROACH

Kinetic measurements of CO-MP4 binding were used to evaluate the effects of PEG modification on Hb subunit structure/function and to calculate CO-MP4 equilibrium constants. CO transport by CO-MP4 was shown by ligand (O2/CO) partitioning between MP4 and red blood cell (RBC)-Hb. Pharmacological effects of CO-MP4 were studied on myocardial infarction in rats.

KEY RESULTS

CO binding kinetics show primary structural/functional effects on beta chains in MP4, with alpha chains maintaining the ability to undergo tertiary conformational transition. CO confers long-term, room-temperature stability and is able to rapidly re-equilibrate between MP4 and RBCs. In a rat model of myocardial infarct, in contrast to oxy-MP4, CO-MP4 reduced infarct size when administered prior to the induction of ischaemia.

CONCLUSIONS AND IMPLICATIONS

MP4 PEGylation chemistry modifies the individual function of Hb subunits, but results in an overall CO equilibrium constant similar to that for unmodified Hb. CO-MP4 is able to deliver CO to the circulation and reduces ischaemia/reperfusion injury in rats, providing the first evidence for this drug as a CO therapeutic agent.

Effects of amino acid substitutions at beta 131 on the structure and properties of hemoglobin: evidence for communication between alpha 1 beta 1- and alpha 1 beta 2-subunit interfaces

Substitutions of Asn, Glu, and Leu for Gln at the beta131 position of the hemoglobin molecule result in recombinant hemoglobins (rHbs) with moderately lowered oxygen affinity and high cooperativity compared to human normal adult hemoglobin (Hb A). The mutation site affects the hydrogen bonds present at the alpha(1)beta(1)-subunit interface between alpha103His and beta131Gln as well as that between alpha122His and beta35Tyr. NMR spectroscopy shows that the hydrogen bonds are indeed perturbed; in the case of rHb (beta131Gln --> Asn) and rHb (beta131Gln --> Leu), the perturbations are propagated to the other alpha(1)beta(1)-interface H-bond involving alpha122His and beta35Tyr. Proton exchange measurements also detect faster exchange rates for both alpha(1)beta(1)-interface histidine side chains of the mutant rHbs in 0.1 M sodium phosphate buffer at pH 7.0 than for those of Hb A under the same conditions. In addition, the same measurements in 0.1 M Tris buffer at pH 7.0 show a much slower exchange rate for mutant rHbs and Hb A. One of the mutants, rHb (beta131Gln --> Asn), shows the conformational exchange of its interface histidines, and exchange rate measurements have been attempted. We have also conducted studies on the reactivity of the SH group of beta93Cys (a residue located in the region of the alpha(1)beta(2)-subunit interface) toward p-mercuribenzoate, and our results show that low-oxygen-affinity rHbs have a more reactive beta93Cys than Hb A in the CO form. Our results indicate that there is communication between the alpha(1)beta(1)- and alpha(1)beta(2)-subunit interfaces, and a possible communication pathway for the cooperative oxygenation of Hb A that allows the alpha(1)beta(1)-subunit interface to modulate the functional properties in conjunction with the alpha(1)beta(2) interface is proposed.

Distal ligand reactivity and quaternary structure studies of proximally detached hemoglobins

The linkage between the proximal histidines and the proximal polypeptide in normal adult human hemoglobin (Hb A) has been proposed to play a major role in transmitting allosteric effects between oxygen binding sites [Perutz, M. F. (1970) Nature 228, 726-734]. Here we present circular dichroism (CD), (1)H NMR, analytical ultracentrifugation, and stopped-flow kinetic data to better define the quaternary structure of hemoglobins in which the linkage between the proximal histidines and the polypeptide backbone has been broken [Barrick et al. Nat. Struct. Biol. 4, 78-83 (1997)] and to characterize the distal ligand binding properties of these proximally detached Hbs. CD spectroscopy indicates that rHb (alphaH87G) and rHb (alphaH87G/betaH92G) retain at least partial T-quaternary structure with distal ligand bound, whereas rHb (betaH92G) does not, consistent with (1)H NMR spectra. Analytical ultracentrifugation reveals significant tetramer dissociation in rHb (betaH92G) to be the likely cause of loss of T-state markers. These quaternary structure studies indicate that in distally liganded Hb, the T-state is compatible with proximal linkages in the beta- but not the alpha-chains. (1)H NMR titrations of rHb (alphaH87G) with n-butyl isocyanide demonstrate the alpha-chains to be of high affinity as compared with the beta-chains. Comparing ligand association and dissociation rates between the rHb (alphaH87G) variant with the T- and R-states of wild-type Hb A indicates that at the alpha-chains, carbon monoxide affinity is modulated entirely by the proximal linkage, rather than from distal interactions. Some residual allosteric interactions may remain operative at the beta-chains of rHb (alphaH87G).

Novel recombinant hemoglobin, rHb (beta N108Q), with low oxygen affinity, high cooperativity, and stability against autoxidation

Using our Escherichia coli expression system, we have constructed rHb (beta N108Q), a new recombinant hemoglobin (rHb), with the amino acid substitution located in the alpha(1)beta(1) subunit interface and in the central cavity of the Hb molecule. rHb (beta N108Q) exhibits low oxygen affinity, high cooperativity, enhanced Bohr effect, and slower rate of autoxidation of the heme iron atoms from the Fe(2+) to the Fe(3+) state than other low-oxygen-affinity rHbs developed in our laboratory, e.g., rHb (alpha V96W) and rHb (alpha V96W, beta N108K). It has been reported by Olson and co-workers [Carver et al. (1992) J. Biol. Chem. 267, 14443-14450; Brantley et al. (1993) J. Biol. Chem. 268, 6995-7010] that the substitution of phenylalanine for leucine at position 29 of myoglobin can inhibit autoxidation in myoglobin and at position 29 of the alpha-chain of hemoglobin can lower NO reaction in both the deoxy and the oxy forms of human normal adult hemoglobin. Hence, we have further introduced this mutation, alpha L29F, into beta N108Q. rHb (alpha L29F, beta N108Q) is stabilized against auto- and NO-induced oxidation as compared to rHb (beta N108Q), but exhibits lower oxygen affinity at pH below 7.4 and good cooperativity as compared to Hb A. Proton nuclear magnetic resonance (NMR) studies show that rHb (beta N108Q) has similar tertiary structure around the heme pockets and quaternary structure in the alpha(1)beta(1) and alpha(1)beta(2) subunit interfaces as compared to those of Hb A. The tertiary structure of rHb (alpha L29F, beta N108Q) as measured by (1)H NMR, especially the alpha-chain heme pocket region (both proximal and distal histidyl residues), is different from that of CO- and deoxy-Hb A, due to the amino acid substitution at alpha L29F. (1)H NMR studies also demonstrate that rHb (beta N108Q) can switch from the R quaternary structure to the T quaternary structure without changing ligation state upon adding an allosteric effector, inositol hexaphosphate, and reducing the temperature. On the basis of its low oxygen affinity, high cooperativity, and stability against autoxidation, rHb (beta N108Q) is considered a potential candidate for the Hb-based oxygen carrier in a blood substitute system.

Understanding mechanisms in a cooperative protein: the CO ligation intermediates of hemoglobin

Hemoglobin is a regulatory component of the oxygen transport to the tissues, and for decades has been a prototype to develop new strategies for the study of the structure/function relationships in proteins. One of the most difficult, and so far, unattained objectives of hemoglobin research has been the study of the hemoglobin molecules in a state of partial ligation with oxygen, or intermediates, as a means of testing theories of cooperativity. A cryogenic technique has been developed for the isolation, identification and quantification of the reaction intermediates of hemoglobin and CO, which in many aspects is a close approximation to the physiological ligand. The technical features that are crucial for the evaluation of the significance of the experimental data obtained using this technique and various approaches to the analysis of the data are reported. The discussion points out the importance of accessing direct information on the nature and concentrations of the intermediates in solution to clarify mechanisms of cooperativity as opposed to the less informative studies of the bulk properties of the solution.

Rate constants for O2 and CO binding to the alpha and beta subunits within the R and T states of human hemoglobin

Despite a large amount of work over the past 30 years, there is still no universal agreement on the differential reactivities of the individual alpha and beta subunits in human hemoglobin. To address this question systematically, we prepared a series of hybrid hemoglobins in which heme was replaced by chromium(III), manganese(III), nickel(II), and magnesium(II) protoporphyrin IXs in either the alpha or beta subunits to produce alpha2(M)beta2(Fe)1 and alpha2(Fe)beta2(M) tetramers. None of the abnormal metal complexes react with dioxygen or carbon monoxide. The O2 affinities of the resultant hemoglobins vary from 3 microM-1 (Cr(III)/Fe(II) hybrids) to 0.003 microM-1 (Mg(II)/Fe(II) hybrids), covering the full range expected for the various high (R) and low (T) affinity quaternary conformations, respectively, of human hemoglobin A0. The alpha and beta subunits in hemoglobin have similar O2 affinities in both quaternary states, despite the fact that the R to T transition causes significantly different structural changes in the alpha and beta heme pockets. This functional equivalence almost certainly evolved to maintain high n values for efficient O2 transport.

Modulation of the association reaction between hemoglobin and carbon monoxide by proton and chloride

A cryogenic technique for the isolation of the ligation intermediates in the association reaction between hemoglobin and carbon monoxide at 20 degrees C [Perrella, M., Davids, N., and Rossi-Bernardi, L. (1992) J. Biol. Chem. 267, 8744-8751] was used to study the effects of proton and chloride concentrations on the rates of the stepwise reactions. The reaction rate was observed to increase continuously in the course of the ligation process, yet the acceleration of the reaction after the binding of two ligand molecules, observed previously in 100 mM KCl, pH 7, was not observed at other pH values. At pH 6.3, such an acceleration occurred after the binding of three ligands, and at pH 8.5, a large acceleration was observed after the binding of the first ligand molecule. Greater CO binding to the beta chains was observed under all conditions, as in the previous study. The functional heterogeneity of the chains in the first ligation step increased with pH. The chloride concentration did not influence the distribution of the ligand between the alpha and beta chains at pH 6.3 and 8.5. At pH 7, less binding to the alpha chains was observed at 7 mM chloride with respect to 100 mM. The nature of the biliganded component isolated at pH 7 in 100 mM KCl and unresolved by the cryogenic technique was studied using a combination of cryogenic and noncryogenic isoelectric focusing. This component was a mixture of intermediates (alpha beta) (alpha CO beta CO), about 65%, and (alpha beta CO) (alpha CO beta), about 35%. The experimental data were compared with the distributions of intermediates calculated according to the Monod kinetic model assuming rapid and concerted transitions between two quaternary structures at each ligation step. The model provided a qualitative fit of the observed distributions of intermediates at acidic and neutral pH. A large discrepancy between the experimental observations and the predictions of the model was found at alkaline pH. The mechanism of the association reaction is discussed in the light of the available information on the tertiary/quaternary structures of the intermediates, as obtained from the studies of the deoxy/cyanomet model of ligation.

Conformational fluctuations in deoxy hemoglobin revealed as a major contributor to anionic modulation of function through studies of the oxygenation and oxidation of hemoglobins A0 and Deer Lodge beta2(NA2)His --> Arg

Organisms rely on regulation at the molecular level, such as the allosteric regulation of hemoglobin (Hb) function by anions, to meet challenges presented by changing environmental and physiological conditions. A comparison of the effects of anions on oxygenation, oxidation, and sulfhydryl reactivity of Hb leads us to suggest that a large and significant part of the shift in oxygen affinity brought about by anion binding occurs as a result of increased conformational rigidity of the T state of deoxy Hb. As conformational rigidity increases, it becomes increasingly difficult for subunits in the deoxygenated T-state tetramer to assume higher oxygen affinity forms (T', T", T"'...) with less steric hindrance. The oxygen affinity reflects the average of the rapidly equilibrating conformations within the T state and is correspondingly decreased when anion levels are increased. The initial stage of the oxidation of Hb is relatively insensitive to steric alterations and thus reflects, primarily, the electronic aspects of the quaternary (T, T', T", T"'...) <--> equilibrium. We show that the reactivity of the sterically obscured sulfhydryl of beta93 Cys in deoxy Hb is much greater in chloride-free buffers than in buffers with added chloride. Anion-induced decreases in the extent and frequency of conformational fluctuations of subunits within the T-quaternary state thus reduce sulfhydryl reactivity as well as oxygen affinity. This parallel in anionic control of function allowed us to test, and disprove, the possibility that uncompensated positive charges in the central cavity of Hb Deer Lodge increase the frequency and extent of conformational fluctuations in its deoxy structure. This Hb variant exhibits increased anion sensitivity, increased oxygen affinity, and increased ease of oxidation, but without increased reactivity of its sulfhydryl groups, indicating that active-site alterations in deoxy Hb Deer Lodge are primarily electronic and not associated with increased conformational fluctuations within its T state. The restoration of normal properties in Hb Deer Lodge by addition of anions reinforces our conclusion that anionic control can be exerted through both steric and electronic alterations. The anionic control of fluctuations within the T state of Hb illustrates an important principle of macromolecular structure-function relationships: that functional regulation can be achieved by alterations in conformational rigidity.

Characterization of recombinant soybean leghemoglobin a and apolar distal histidine mutants

The cDNA for soybean leghemoglobin a (Lba) was cloned from a root nodule cDNA library and expressed in Escherichia coli. The crystal structure of the ferric acetate complex of recombinant wild-type Lba was determined at a resolution of 2.2 A. Rate constants for O2, CO and NO binding to recombinant Lba are identical with those of native soybean Lba. Rate constants for hemin dissociation and auto-oxidation of wild-type Lba were compared with those of sperm whale myoglobin. At 37 degrees C and pH 7, soybean Lba is much less stable than sperm whale myoglobin due both to a fourfold higher rate of auto-oxidation and to a approximately 600-fold lower affinity for hemin. The role of His61(E7) in regulating oxygen binding was examined by site-directed mutagenesis. Replacement of His(E7) with Ala, Val or Leu causes little change in the equilibrium constant for O2 binding to soybean Lba, whereas the same mutations in sperm whale myoglobin cause 50 to 100-fold decreases in K(O2). These results show that, at neutral pH, hydrogen bonding with His(E7) is much less important in regulating O2 binding to the soybean protein. The His(E7) to Phe mutation does cause a significant decrease in K(O2) for Lba, apparently due to steric hindrance of the bound ligand. The rate constants for O2 dissociation from wild-type and native Lba decrease significantly with decreasing pH. In contrast, the O2 dissociation rate constants for mutants with apolar E7 residues are independent of pH, suggesting that hydrogen bonding to the distal histidine residue in the native protein is enhanced under acid conditions. All of these results support the hypothesis that the high affinity of Lba for oxygen and other ligands is determined primarily by enhanced accessibility and reactivity of the heme group.

Functional heterogeneity of the alpha and beta subunits in the association reaction between hemoglobin and carbon monoxide

A technique is described for the rapid inactivation and removal of excess ferricyanide used for the non-cryogenic oxidation of the unliganded subunits of the intermediates in the association reaction between hemoglobin and carbon monoxide. Under these conditions the asymmetric oxidized intermediates, which dissociate into non-identical dimers, disproportionate into their parent tetramers and four species, Hb+, HbCO, alpha 2+ beta 2CO, alpha 2CO beta 2+, are isolated by non-cryogenic isoelectric focusing. The relative concentrations of species alpha 2CO beta 2+ and alpha 2+ beta 2CO measure the overall distribution of the ligand between the alpha and beta subunits in the association reaction. At 20 degrees C in 0.1 M KCl, pH 7, preferential CO binding to the beta subunits was observed, in agreement with observations made by the cryogenic technique for the isolation of the intermediates [M. Perrella, N. Davids and L. Rossi-Bernardi, J. Biol. Chem. 267 (1992) 8744].

Isolation and stability of partially oxidized intermediates of carp hemoglobin: kinetics of CO binding to the mono- and triferric species

The monoliganded and triliganded forms of the asymmetric valency hybrids of carp hemoglobin were isolated using high-performance liquid chromatography. These partially oxidized hybrids were shown to be sufficiently stable to permit the measurement of the kinetics of CO binding. The effects of protons and inositol hexaphosphate on the rates of these reactions were examined. The kinetics of CO recombination with these partially oxidized derivatives were compared to the kinetics of CO binding to the fully ferrous molecule. To a first approximation, the kinetic behavior of the monoferric derivative was consistent with a small shift in the T<==>R equilibrium in favor of the R state. The presence of three ferric ligands resulted in a still greater shift in the conformational equilibrium in favor of the R state. The kinetic behavior of the triferric molecule was similar, but not identical, to that of a fully ferrous molecule which is triliganded with CO. The properties of both asymmetric valency hybrids were responsive to the nature of the ligand; i.e., the rate of CO binding was increased more by the presence of cyanide on the ferric hemes than by water. Not all of the data could be accommodated within the two-state model. For example, there was evidence of an altered T state in the case of the tricyanomet derivative at low pH in the presence of inositol hexaphosphate.

Genetic engineering of myoglobin as a simple prototype for hemoglobin-based blood substitutes

Site-directed mutagenesis has been used to examine the structural and functional roles of distal pocket residues in regulating O2 affinity, CO binding, rates of association and dissociation, autooxidation, and hemin loss in mammalian myoglobins and human hemoglobin. In myoglobin, His-E7 inhibits CO binding by requiring displacement of distal pocket water. In the case of O2 binding, this displacement is compensated by a strong hydrogen bond between the bound ligand and the imidazole side chain. The isopropyl side chain Val-E11 also sterically restricts CO binding. The rates of ligand binding are regulated by distal pocket water displacement, steric restrictions near the iron atom, and an outer more global protein barrier. Autooxidation occurs by two mechanisms, direct dissociation of HO2 and bimolecular reaction of external O2 with unliganded heme. Both processes are inhibited markedly by hydrogen bonding interactions with His-E7. Double mutants have been constructed to decrease oxygen affinity, but still prevent oxidation. The apoprotein of His-E7-->Tyr myoglobin has been used to extract hemin from other myoglobins and hemoglobin, causing a brown to green color change. This assay has been used to show that polar interactions between residues CD3, E7, E10, F7, and the porphyrin propionates inhibit hemin dissociation markedly.

Application of molecular dynamics and free energy perturbation methods to metalloporphyrin-ligand systems II: CO and dioxygen binding to myoglobin

The protein contribution to the relative binding affinity of the ligands CO and O2 toward myoglobin (Mb) has been simulated using free energy perturbation calculations. The tautomers of the His E7 residue are different for the oxymyoglobin (MbO2) and carboxymyoglobin (MbCO) systems. This was modeled by performing two-step calculations that mutate the ligand and mutate the His E7 tautomers in separate steps. Differences in hydrogen bonding to the O2 and CO ligands were incorporated into the model. The O2 complex was calculated to be 2-3 kcal/mol more stable than the corresponding CO complex when compared to the same difference in an isolated heme control. This value agrees well with the experimental value of 2.0 kcal/mol. In qualitative agreement with experiments, the Fe-C-O bond is found to be bent (theta = 159.8 degrees) with a small tilt (theta = 6.2 degrees). The contributions made by each of the 29 residues--within the 9.0-A radius of the iron atom--to the free energy difference are separated into van der Waals and electrostatic contributions; the latter contributions are dominant. Aside from the proximal histidine and the heme group, the residues having the largest difference in free energy in mutating MbO2-->MbCO are His E7, Phe CD1, Phe CD4, Val E11, and Thr E10.

Serine92 (F7) contributes to the control of heme reactivity and stability in myoglobin

The effects of mutation of the conserved serine92 residue to alanine, valine, and leucine in pig myoglobin have been determined. In myoglobin crystal structures, the hydroxyl group of serine92 is within hydrogen-bonding distance of the N delta-H of histidine93, whose N epsilon coordinates the iron atom of the heme prosthetic group. The association equilibrium constants of the ferrous forms of the mutant myoglobins for O2, CO, and methyl and ethyl isocyanide are increased 1.3-13-fold relative to the wild-type protein. The rates of azide association with the mutant ferric proteins at neutral pH are decreased by factors of 2-5 consistent with an increased affinity for the iron-bound water molecule which must be displaced. The dissociation rates for azide appear to be decreased 4-10-fold, suggesting that the affinity of the mutant proteins for this ligand is also higher. Thus, the overall affinities are increased regardless of the chemical nature of the liganded species, indicating that the reactivity of the heme iron itself has been raised. Time courses for association of methyl and ethyl isocyanide at high concentrations show fast and slow phases in which the absorbance at 445 nm drops and then rises, respectively. Comparison of these traces with spectra following the reaction of isocyanide ligands with chelated proton heme in soap micelles indicates that the slow phase is associated with the breaking of the iron-proximal histidine bond and the binding of a second isocyanide species in the proximal heme pocket.(ABSTRACT TRUNCATED AT 250 WORDS)

Recombinant human hemoglobin: modification of the polarity of the beta-heme pocket by a valine67(E11)-->threonine mutation

Using the mutagenesis and a gene expression system previously described [Fronticelli et al. (1991) J. Protein Chem. 10, 495-501], we have replaced Val67E11 in the distal heme pocket of the beta-chains of hemoglobin with Thr. The valine to threonine substitution is isosteric and only modifies the polarity of the beta-heme environment. The absorption and CD spectra of the resultant mutant hemoglobin were essentially the same as that of wild-type protein, indicating that the mutation did not cause any large conformational changes and that a water molecule was not coordinated to the ferrous iron atom. Equilibrium measurements of oxygen binding to the mutant indicate a 2-fold decrease in overall affinity relative to native or wild-type human hemoglobin. Thermodynamic analyses of O2 binding curves, based either on the sequential Adair model or on the MWC two-state model, indicated that the overall decrease of O2 affinity in the system was due to a lower association equilibrium constant for the intermediates of oxygenation, particularly those involved at the third ligation step. The functional characteristics of the mutant hemoglobin in either the T- or R-state were not modified greatly by the mutation; however, the Bohr effect and sensitivity to C1- were increased, suggesting a role of the intermediates of oxygenation in the modulation of these parameters.(ABSTRACT TRUNCATED AT 250 WORDS)