Assignment of the quaternary structure of Fe/Zn hybrid hemoglobins: implications for allosteric theories

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.

Conformational dependence of hemoglobin reactivity under high viscosity conditions: the role of solvent slaved dynamics

The concept of protein dynamic states is introduced. This concept is based on (i) protein dynamics being organized hierarchically with respect to solvent slaving and (ii) which tier of dynamics is operative over the time window of a given measurement. The protein dynamic state concept is used to analyze the kinetic phases derived from the recombination of carbon monoxide to sol-gel-encapsulated human adult hemoglobin (HbA) and select recombinant mutants. The temperature-dependent measurements are made under very high viscosity conditions obtained by bathing the samples in an excess of glycerol. The results are consistent with a given tier of solvent slaved dynamics becoming operative at a time delay (with respect to the onset of the measurement) that is primarily solvent- and temperature-dependent. However, the functional consequences of the dynamics are protein- and conformation-specific. The kinetic traces from both equilibrium populations and trapped allosteric intermediates show a consistent progression that exposes the role of both conformation and hydration in the control of reactivity. Iron-zinc symmetric hybrid forms of HbA are used to show the dramatic difference between the kinetic patterns for T state alpha and beta subunits. The overall results support a model for allostery in HbA in which the ligand-binding-induced transition from the deoxy T state to the high -affinity R state proceeds through a progression of T state intermediates.

Liganded hemoglobin structural perturbations by the allosteric effector L35

Effector binding to liganded hemoglobin (Hb) provides a new understanding of structural determinants of Hb function. L35, a bezafibrate-related compound, is one of the more potent synthetic regulators of Hb oxygen (O(2)) affinity. In the presence of inositol hexaphosphate and bezafibrate (or derivatives), liganded Hb at low pH (pH approximately 6.5) exhibits extremely low O(2) affinity and very low cooperativity. In this study, the nature of L35 binding to COHbA at pH 6.35, an altered R-state, is presented. Solution-active site-specific spectroscopic probings by front-face fluorescence and circular dichroism reveal that L35 induces a global heterogeneous conformation in COHbA at pH 6.35 that includes: a T-like structural feature at the alpha1beta2 interface; an R-like structural feature within the heme environment; and an intermediate-like state at the central cavity. These long-range structural perturbations appear to stem from L35 binding to two classes of binding sites: the central cavity (primarily at the alphaalpha cleft) and the surface. These results indicate that L35 induces an allosteric transition species, characterized by domain-specific tertiary and quaternary-like conformation within a global R-quaternary structure.

Characterization of long-range electron transfer in mixed-metal [zinc,iron] hybrid hemoglobins

Measurements characterizing electron transfer from a photoexcited zinc protoporphyrin triplet (3ZnP) to a ferriheme electron acceptor within the [alpha 1,beta 2] electron-transfer complex of [FeIII,Zn] hybrid hemoglobins are reported. Analytical results demonstrate that the hybrids studied are pure, homogeneous proteins with 1:1 ZnP:FeP content. Within the T quaternary structure adopted by these hybrids, the optical spectrum of a FeIIIP is perturbed by the protein environment. Room temperature kinetic studies of the rate of 3ZnP decay as a function of the heme oxidation and ligation state demonstrate that quenching of 3ZnP by FeIII(H2O)P occurs by long-range intramolecular electron transfer with rate constant kt = 100 (+/- 10) s-1 and is not complicated by spin-quenching or energy-transfer processes; results are the same for alpha(Zn) and beta(Zn) hybrids. Replacement of H2O as a ligand to the ferriheme changes the 3ZnP----FeIIIP electron-transfer rate constant, kt, which demonstrates that electron transfer, not conformational conversion, is rate limiting. However, the trend is not readily explained by simple considerations of spin-state and bonding geometry: kt decreases in the order imidazole greater than H2O greater than F- approximately CN- approximately N3-. The reverse electron-transfer process FeIIP----ZnP+ has not been observed directly but has been shown to be much more rapid, with rate constant kb greater than 10(3) s-1, consistent with the possible importance of "hole" superexchange in electron tunneling within protein complexes.

Extended X-ray absorption fine structure studies of Zn2Fe2 hybrid hemoglobins: absence of heme bond length changes in half-ligated species

Metal hybrid hemoglobins, in which Zn(II) replaces Fe(II), have been structurally characterized by extended X-ray absorption structure (EXAFS) studies. Since Zn and Fe have very different K absorption edge energies, the structures of the ligated (Fe) and unligated (Zn) sites could be examined independently within a single molecule that mimics an intermediate ligation state. The observed EXAFS spectra and associated structural parameters are compared among the ligand free (alpha Zn)2(beta Zn)2, half-ligated (alpha FeCO)2(beta Zn)2 and (alpha Zn)2(beta FeCO)2, and fully ligated (alpha FeCO)2(beta FeCO)2 systems.