Determination of the hemoglobin surface domains that react with cytochrome b5

Methemoglobin formation in mutant hemoglobin α chains: electron transfer parameters and rates

Hemoglobin-mediated transport of dioxygen (O₂) critically depends on the stability of the reduced (Fe²⁺) form of the heme cofactors. Some protein mutations stabilize the oxidized (Fe³⁺) state (methemoglobin, Hb M), causing methemoglobinemia, and can be lethal above 30%. The majority of the analyses of factors influencing Hb oxidation are retrospective and give insights only for inner-sphere mutations of heme (His58, His87). Herein, we report the first all-atom molecular dynamics simulations on both redox states and calculations of the Marcus electron transfer (ET) parameters for the α chain Hb oxidation and reduction rates for Hb M. The Hb wild-type (WT) and most of the studied α chain variants maintain globin structure except the Hb M Iwate (H87Y). The mutants forming Hb M tend to have lower redox potentials and thus stabilize the oxidized (Fe³⁺) state (in particular, the Hb Miyagi variant with K61E mutation). Solvent reorganization (λ_solv 73-96%) makes major contributions to reorganization free energy, whereas protein reorganization (λ_prot) accounts for 27-30% except for the Miyagi and J-Buda variants (λ_prot ∼4%). Analysis of heme-solvent H-bonding interactions among variants provide insights into the role of Lys61 residue in stabilizing the Fe²⁺ state. Semiclassical Marcus ET theory-based calculations predict experimental k_ET for the Cyt b5-Hb complex and provide insights into relative reduction rates for Hb M in Hb variants. Thus, our methodology provides a rationale for the effect of mutations on the structure, stability, and Hb oxidation reduction rates and has potential for identification of mutations that result in methemoglobinemia.

Evolving the [myoglobin, cytochrome b(5)] complex from dynamic toward simple docking: charging the electron transfer reactive patch

We describe photoinitiated electron transfer (ET) from a suite of Zn-substituted myoglobin (Mb) variants to cytochrome b(5) (b(5)). An electrostatic interface redesign strategy has led to the introduction of positive charges into the vicinity of the heme edge through D/E → K charge-reversal mutation combinations at "hot spot" residues (D44, D60, and E85), augmented by the elimination of negative charges from Mb or b(5) by neutralization of heme propionates. These variations create an unprecedentedly large range in the product of the ET partners' total charges (-5 < -q(Mb)q(b(5)) < 40). The binding affinity (K(a)) increases 1000-fold as -q(Mb)q(b(5)) increases through this range and exhibits a surprisingly simple, exponential dependence on -q(Mb)q(b(5)). This is explained in terms of electrostatic interactions between a "charged reactive patch" (crp) on each partner's surface, defined as a compact region around the heme edge that (i) contains the total protein charge of each variant and (ii) encompasses a major fraction of the "reactive region" (Rr) comprising surface atoms with large matrix elements for electron tunneling to the heme. As -q(Mb)q(b(5)) increases, the complex undergoes a transition from fast to slow-exchange dynamics on the triplet ET time scale, with a correlated progression in the rate constants for intracomplex (k(et)) and bimolecular (k(2)) ET. This progression is analyzed by integrating the crp and Rr descriptions of ET into the textbook steady-state treatment of reversible binding between partners that undergo intracomplex ET and found to encompass the full range of behaviors predicted by the model. The generality of this approach is demonstrated by its application to the extensive body of data for the ET complex between the photosynthetic reaction center and cytochrome c(2). Deviations from this model also are discussed.

Kinetic-dynamic model for conformational control of an electron transfer photocycle: mixed-metal hemoglobin hybrids

It is becoming increasingly clear that the transfer of an electron across a protein-protein interface is coupled to the dynamics of conformational conversion between and within ensembles of interface conformations. Electron transfer (ET) reactions in conformationally mobile systems provide a "clock" against which the rapidity of a dynamic process may be measured, and we here report a simple kinetic (master equation) model that self-consistently incorporates conformational dynamics into an ET photocycle comprised of a photoinitiated "forward" step and thermal return to ground. This kinetic/dynamic (KD) model assumes an ET complex exists as multiple interconverting conformations which partition into an ET-optimized (reactive; R) population and a less-reactive population ( S). We take the members of each population to be equivalent by constraining them to have the same conformational energy, the same average rate constant for conversion to members of the other population, and the same rate constants for forward and back ET. The result is a mapping of a complicated energy surface onto the simple "gating", two-well surface, but with rate constants that are defined microscopically. This model successfully describes the changes in the ET photocycle within the "predocked" mixed-metal hemoglobin (Hb) hybrid, [alpha(Zn), beta(Fe3+N 3 (-))], as conformational kinetics are modulated by variations in viscosity (eta = 1-15 cP; 20 degrees C). The description reveals how the conformational "routes" by which a hybrid progresses through a photocycle differ in different dynamic regimes. Even at eta = 1 cP, the populations are not in fast exchange, and ET involves a complex interplay between conformational and ET processes; at intermediate viscosities the hybrid exhibits "differential dynamics" in which the forward and back ET processes involve different initial ensembles of configurational substates; by eta = 15 cP, the slow-exchange limit is approached. Even at low viscosity, the ET-coupled motions are fairly slow, with rate constants of <10 (3) s (-1). Current ideas about Hb function lead to the testable hypothesis that ET in the hybrid may be coupled to allosteric fluctuations of the two [alpha 1, beta 2] dimers of Hb.

Dynamic Docking of Cytochrome b5 with Myoglobin and α-Hemoglobin: Heme-Neutralization “Squares” and the Binding of Electron-Transfer-Reactive Configurations

Intracomplex electron transfer (ET) occurs most often in intrinsically transient, low affinity complexes. As a result, the means by which adequate specificity and reactivity are obtained to support effective ET is still poorly understood. We report here on two such ET complexes: cytochrome b5 (cyt b5) in reaction with its physiological partners, myoglobin (Mb) and hemoglobin (Hb). These complexes obey the Dynamic Docking (DD) paradigm: a large ensemble of weakly bound protein-protein configurations contribute to binding in the rapid-exchange limit, but only a few are ET-active. We report the ionic-strength dependence of the second-order rate constant, k2, for photoinitiated ET from within all four combinations of heme-neutralized Zn deuteroporphyrin-substituted Mb/alphaHb undergoing ET with cyt b5, the four "corners" of a "heme-neutralization square". These experiments provide insights into the relative importance of both global and local electrostatic contributions to the binding of reactive configurations, which are too few to be observed directly. To interpret the variations of k2 arising from heme neutralization, we have developed a procedure by which comparisons of the ET rate constants for a heme-neutralization square permit us to decompose the free energy of reactive binding into individual local electrostatic contributions associated with interactions between (i) the propionates of the two hemes and (ii) the heme of each protein with the polypeptide of its partner. Most notably, we find the contribution from the repulsion between propionates of partner hemes to the reactive binding free energy to be surprisingly small, DeltaG(Hb) approximately +1 kcal/mol at ambient temperature, 18 mM ionic strength, and we speculate about possible causes of this observation. To confirm the fundamental assumption of these studies, that the structure of a heme-neutralized protein is unaltered either by substitution of Zn or by heme neutralization, we have obtained the X-ray structure of ZnMb prepared with the porphyrin dimethyl ester and find it to be nearly isostructural with the native protein.

Expression ofAspergillus hemoglobin domain activities inAspergillus oryzae grown on solid substrates improves growth rate and enzyme production

DNA fragments coding for hemoglobin domains (HBD) were isolated from Aspergillus oryzae and Aspergillus niger. The HBD activities were expressed in A. oryzae by introduction of HBD gene fragments under the control of the promoter of the constitutively expressed gpdA gene. In the transformants, oxygen uptake was significantly higher, and during growth on solid substrates the developed biomass was at least 1.3 times higher than that of the untransformed wild-type strain. Growth rate of the HBD-activity-producing strains was also significantly higher compared to the wild type. During growth on solid cereal substrates, the amylase and protease activities in the extracts of the HBD-activity-producing strains were 30-150% higher and glucoamylase activities were at least 9 times higher compared to the wild-type strain. These results suggest that the Aspergillus HBD-encoding gene can be used in a self-cloning strategy to improve biomass yield and protein production of Aspergillus species.

Differential influence of dynamic processes on forward and reverse electron transfer across a protein-protein interface

We propose that the forward and reverse halves of a flash-induced protein-protein electron transfer (ET) photocycle should exhibit differential responses to dynamic interconversion of configurations when the most stable configuration is not the most reactive, because the reactants exist in different initial configurations: the flash-photoinitiated forward ET process begins with the protein partners in an equilibrium ensemble of configurations, many of which have little or no reactivity, whereas the reactant of the thermal back ET (the charge-separated intermediate) is formed in a nonequilibrium, "activated" protein configuration. We report evidence for this proposal in measurements on (i) mixed-metal hemoglobin hybrids, (ii) the complex between cytochrome c peroxidase and cytochrome c, and (iii and iv) the complexes of myoglobin and isolated hemoglobin alpha-chains with cytochrome b(5). For all three systems, forward and reverse ET does respond differently to modulation of dynamic processes; further, the response to changes in viscosity is different for each system.

Molecular Mechanism of AHSP-Mediated Stabilization of α-Hemoglobin

Hemoglobin A (HbA), the oxygen delivery system in humans, comprises two alpha and two beta subunits. Free alpha-hemoglobin (alphaHb) is unstable, and its precipitation contributes to the pathophysiology of beta thalassemia. In erythrocytes, the alpha-hemoglobin stabilizing protein (AHSP) binds alphaHb and inhibits its precipitation. The crystal structure of AHSP bound to Fe(II)-alphaHb reveals that AHSP specifically recognizes the G and H helices of alphaHb through a hydrophobic interface that largely recapitulates the alpha1-beta1 interface of hemoglobin. The AHSP-alphaHb interactions are extensive but suboptimal, explaining why beta-hemoglobin can competitively displace AHSP to form HbA. Remarkably, the Fe(II)-heme group in AHSP bound alphaHb is coordinated by the distal but not the proximal histidine. Importantly, binding to AHSP facilitates the conversion of oxy-alphaHb to a deoxygenated, oxidized [Fe(III)], nonreactive form in which all six coordinate positions are occupied. These observations reveal the molecular mechanisms by which AHSP stabilizes free alphaHb.

Conformational reorganisation in interfacial protein electron transfer

Protein-protein electron transfer (ET) plays an essential role in all redox chains. Earlier studies which used cross-linking and increased solution viscosity indicated that the rate of many ET reactions is limited (i.e., gated) by conformational reorientations at the surface interface. These results are later supported by structural studies using NMR and molecular modelling. New insights into conformational gating have also come from electrochemical experiments in which proteins are noncovalently adsorbed on the electrode surface. These systems have the advantage that it is relatively easy to vary systematically the driving force and electronic coupling. In this review we summarize the current knowledge obtained from these electrochemical experiments and compare it with some of the results obtained for protein-protein ET.

Dynamic docking and electron transfer between Zn-myoglobin and cytochrome b(5)

We present a broad study of the effect of neutralizing the two negative charges of the Mb propionates on the interaction and electron transfer (ET) between horse Mb and bovine cyt b(5), through use of Zn-substituted Mb (ZnMb, 1) to study the photoinitiated reaction, ((3)ZnP)Mb + Fe(3+)cyt b(5) --> (ZnP)(+)Mb + Fe(2+)cyt b(5). The charge neutralization has been carried out both by replacing the Mb heme with zinc-deuteroporphyrin dimethylester (ZnMb(dme), 2), which replaces the charges by small neutral hydrophobic patches, and also by replacement with the newly prepared zinc-deuteroporphyrin diamide (ZnMb(diamide), 3), which converts the charged groups to neutral, hydrophilic ones. The effect of propionate neutralization on the conformation of the zinc-porphyrin in the Mb heme pocket has been studied by multinuclear NMR with an (15)N labeled zinc porphyrin derivative (ZnMb((15)N-diamide), 4). The rates of photoinitiated ET between the Mb's (1-3) and cyt b(5) have been measured over a range of pH values and ionic strengths. Isothermal titration calorimetry (ITC) and NMR methods have been used to independently investigate the effect of charge neutralization on Mb/b(5) binding. The neutralization of the two heme propionates of ZnMb by formation of the heme diester or, for the first time, the diamide increases the second-order rate constant of the ET reaction between ZnMb and cyt b(5) by as much as several 100-fold, depending on pH and ionic strength, while causing negligible changes in binding affinity. Brownian dynamic (BD) simulations and ET pathway calculations provide insight into the protein docking and ET process. The results support a new "dynamic docking" paradigm for protein-protein reactions in which numerous weakly bound conformations of the docked complex contribute to the binding of cyt b(5) to Mb and Hb, but only a very small subset of these are ET active, and this subset does not include the conformations most favorable for binding; the Mb surface is a large "target" with a small "bullseye" for the cyt b(5) "arrow". This paradigm differs sharply from the more familiar, "simple" docking within a single, or narrow range of conformations, where binding strength and ET reactivity increase in parallel. Likewise, it is distinct from, although complementary to, the well-known picture of conformational control of ET through "gating", or a related picture of "conformational coupling". The new model describes situations in which tight binding does not correlate with efficient ET reactivity, and explains how it is possible to modulate reactivity without changing affinity. Such "decoupling" of reactivity from binding clearly is of physiological relevance for the reduction of met-Mb in muscle and of met-Hb in a red cell, where tight binding of cyt b(5) to the high concentration of ferrous-Mb/Hb would prevent the cytochrome from finding and reducing the oxidized proteins; it likely is of physiological relevance in other situations, as well.

Molten-globule and other conformational forms of zinc cytochrome C. Effect of partial and complete unfolding of the protein on its electron-transfer reactivity

To test the effect of protein conformation on reactivity, we use laser flash photolysis to compare the electron-transfer properties of the triplet state of zinc-substituted cytochrome c, designated (3)Zncyt, in the folded forms at low (F(low)) and high (F(high)) ionic strength, molten-globule (MG) form, and the forms unfolded by acid (U(acid)) and urea (U(urea)) toward the following four oxidative quenchers: Fe(CN)(6)(3-), Co(acac)(3), Co(phen)(3)(3+), and iron(III) cytochrome c. We characterize the conformational forms of Zncyt on the basis of the far-UV circular dichroism, Soret absorption, and rate constant for natural decay of the triplet state. This rate constant in the absence of quencher increases in the order F(high) < F(low) < MG < U(acid) < U(urea) because the exposure of porphyrin to solvent increases as Zncyt unfolds. Bimolecular rate constants for the reaction of (3)Zncyt with the four quenchers show significant effects on reactivity of electrostatic interactions and porphyrin exposure to solvent. This rate constant at the ionic strength of 20 mM increases upon unfolding by urea and acid, respectively, as follows: 1340-fold and 466-fold when the quencher is Co(phen)(3)(3+) and 168-fold and 36-fold when the quencher is cyt(III). To compare reactivity of (3)Zncyt in the F(low), F(high), MG, U(acid), and U(urea) forms without complicating effects of electrostatic interactions, we used the electroneutral quencher Co(acac)(3). Indeed, reactivity of folded (3)Zncyt with Co(acac)(3) was independent of ionic strength. Reactivity of (3)Zncyt with Co(acac)(3) upon partial and complete unfolding increases 10-fold, 54-fold, and 64-fold in the molten-globule, urea-unfolded, and acid-unfolded forms.

Electron transfer between cytochrome b(5) and some oxidised haemoglobins: the role of ionic strength

We have compared experimental measurements and Brownian dynamic calculations for the reduction of oxidised adult human haemoglobin by reduced bovine cytochrome b(5) over a range of ionic strengths. Our calculations suggest that the presence of molecular electrostatic fields have a significant role to play in the formation of the electron transfer complexes. These results predict that electron transfer occurs within an ensemble of similarly weakly docked complexes, the formation of which is strongly ionic strength dependent. Application of electron tunneling analysis to the complexes allows us to predict the rates of electron transfer within each ensemble of complexes as a function of ionic strength. The outcome of this theoretical study is compared with experimental rate measurements. A comparison of the results obtained from adult and embryonic haemoglobins, at a fixed ionic strength, indicates a significant difference in the characteristics of complex formation. These data emphasise the role played by electrostatic interactions in this important physiological reaction.