Heme-apoprotein interaction in the modified oxyhemoglobin-bis(N-maleimidomethyl)ether and in oxyhemoglobin at high Cl-concentration detected by resonance Raman scattering

Raman dispersion spectroscopy probes heme distortions in deoxyHb-trout IV involved in its T-state Bohr effect

The depolarization ratios of heme protein Raman lines arising from vibrations of the heme group exhibit significant dependence on the excitation wavelength. From the analysis of this depolarization ratio dispersion, one obtains information about symmetry-lowering distortions deltaQ(Gamma) of the heme group that can be classified in terms of the symmetry races Gamma = A(1g), B(1g), B(2g), and A(2g) in D(4h) symmetry. The heme-protein interaction can be changed by the protonation of distinct amino acid side chains (i.e., for instance the Bohr groups in hemoglobin derivates), which gives rise to specific static heme distortions for each protonation state. From the Raman dispersion data, it is possible to obtain parameters by fitting to a theoretical expression of the Raman tensor, which provide information on these static distortions and also about the pK values of the involved titrable side chains. We have applied this method to the nu(4) (1,355 cm(-1)) and nu(10) (1,620 cm(-1)) lines of deoxygenated hemoglobin of the fourth component of trout and have measured their depolarization ratio dispersion as a function of pH between 6 and 9. From the pH dependence of the thus derived parameters, we obtain pK values identical to those of the Bohr groups, which were earlier derived from the corresponding O(2)-binding isotherms. These are pK(alpha1) = pK(alpha2) = 8.5 for the alpha and pK(beta1) = 7.5, pK(beta2) = 7.4 for the beta chains. We also obtain the specific distortion parameters for each protonation state. As shown in earlier studies, the nu(4) mode mainly probes distortions from interactions between the proximal histidine and atoms of the heme core (i.e., the nitrogens and the C(alpha) atoms of the pyrroles). Group theoretical argumentation allows us to relate specific changes of the imidazole geometry as determined by its tilt and azimuthal angle and the iron-out-of-plane displacement to distinct variations of the normal distortions deltaQ(Gamma) derived from the Raman dispersion data. Thus, we found that the pH dependence of the heme distortions deltaQ(A1g) (totally symmetric) and deltaQ(B1g) (asymmetric) is caused by variations of the azimuthal rather than the tilt angle of the Fe-His (F8) bond. In contrast to this, the nu(10) line mainly monitors changes resulting from the interaction between peripheral substituents of the porphyrin macrocycle (vinyl). From the pH dependence of the parameters, it is possible to separately identify distortions deltaQ(Gamma) affecting the hemes in the alpha and beta chains, respectively. From this, we find that in the alpha subunit structural changes induced on protonation of the corresponding Bohr groups are mainly transferred via the Fe-N(epsilon) bond and give rise to changes in the azimuthal angle. In the beta subunit, however, in addition, structural changes of the heme pocket arise, which most probably result from protonation of the imidazole of the COOH-terminal His (HC3 beta). This rearranges the net of H bonds between His HC3 beta, Ser (F9 beta), and Glu (F7 beta).

Allosteric linkage-induced distortions of the prosthetic group in haem proteins as derived by the theoretical interpretation of the depolarization ratio in resonance Raman scattering

The relationship between functional properties of haem proteins, particularly ligand binding and Bohr effect, and associated variations of the tertiary and quaternary structures is one of the main objectives of haem protein research. In this context one aims to get detailed knowledge of the coupling mechanisms which are involved in the transduction of structural changes from the protein to the functional haem group along distinct pathways.

The influence of structural variations in the F- and FG-helix of the beta-subunit modified oxyHb-NES on the heme structure detected by resonance Raman spectroscopy

The dispersion of the depolarization ratio of two prominent Raman lines (1,375 cm-1 and 1,638 cm-1) of oxyhemoglobin-N-ethyl succinimide have been examined for pH values between pH = 6.0 and 8.5. Both exhibit a significant pH dependence. Calculation of the Raman tensor in terms of a fifth-order time dependent theory provides information about the pH-dependence of parameters reflecting symmetry classified distortions of the prosthetic heme group. To correlate these distortions with the functional properties of the molecule the following protocol was used: 1) An allosteric model suggested by Herzfeld and Stanley (1974) has been applied to O2-binding curves measured at different pH values between 6.5 and 9.0. From this calculation one obtains both, the energy differences between different molecular conformations and the equilibrium constants of oxygen and proton binding. 2) A titration model was formulated relating each conformation of a molecule to a distinct set of distortion parameters of the heme group. 3) The distortion parameters resulting from the analysis of our Raman data were assigned as an effective value due to incoherent superposition of the distortion parameters related to the different titration states. The application of this procedure yields an excellent reproduction of the pH-dependent effective distortion parameters of both Raman lines investigated. It is shown that the protonation of two tertiary effector groups located in the beta-subunits affect the symmetry of the heme in a contrary manner: the protonation of a His-residue (pK = 8.2, probably His(FG4) beta) causes a symmetric position of the proximal imidazole thus lowering the perturbations of the heme core. Further it influences the interaction between amino acid residues of the heme cavity and pyrrole side chains (probably Val(FG5) beta-vinyl (pyrrole 3) thus causing a decrease of the distortions related to the peripheral part of the heme. In contrast, the protonation of Lys (EF6) beta causes a tilt position of the proximal imidazole and an increase of asymmetric perturbations of the heme core, whereas the interaction between the pyrrole side chains and the heme cavity is weakened. Our results are consistent with stereochemical predictions of Moffat (1971) concerning the existence of a H-bond between His(FG4) beta and Cys(F9) beta.

Detection of the heme perturbations caused by the quaternary R----T transition in oxyhemoglobin trout IV by resonance Raman scattering

The depolarization ratio dispersion and the respective excitation profiles of two structural sensitive Raman lines of oxyhemoglobin-trout IV (1,375 and 1,638 cm-1) have been measured at pH-values between 6.5 and 8.5. They were analyzed by employing a fifth order time dependent perturbation theory to calculate the polarizability tensor. This provides information about the pH-dependence of parameters reflecting symmetry classified distortions of the prosthetic heme groups. In order to correlate these distortions with functional properties of the molecule the following protocol has been employed: (a) a titration model was formulated relating each conformation of the molecule to a distinct set of distortion parameters the incoherent superposition of which provides the respective distortion parameter obtained from our Raman data. (b) The thermodynamic constants determining the equilibrium between these molecular conformations (i.e., the quaternary T and R-states, the low affinity t and the high affinity r-states of the distinct subunits, the pK-values of the Root- and Bohr groups) were obtained from a set of O2-binding curves that were analyzed in terms of an allosteric model suggested by Herzfeld and Stanley 1974. J. Mol. Biol. 82:231. The application of this procedure yields excellent reproduction of the pH-dependent effective distortion parameters of both Raman lines investigated. Thus established correlation between hemoglobin function (O2-binding) and structure (asymmetric perturbation of the hemegroup) provides some interesting insights into the molecular basis of the allosteric Root effect.

An extended Monod-Wyman-Changeaux-model expressed in terms of the Herzfeld-Stanley formalism applied to oxygen and carbonmonoxide binding curves of hemoglobin trout IV

An extended Monod, Wyman, Changeaux (MWC)-model, the mathematical basis of which had been formulated by Herzfeld and Stanley (J. Mol. Blol. 82:231. 1974.) was used to fit oxygen and CO-binding curves of hemoglobin trout IV measured at different pH-values between pH = 8.0 and 6.0. From this calculation one obtains that even the fully liganded molecule exhibits a R----T quaternary transition upon approaching the acid pH-region. In the case of O2-binding, the cooperativity becomes negative below pH = 6.5. This can be related to the difference between the equilibrium constants of proton binding to the alpha- and beta-subunits. Furthermore, it can be shown that the interaction between the quaternary T----R- and the tertiary t----r-transitions is different for the alpha- and beta-subunits.

pH-dependent absorption in the B and Q bands of oxyhemoglobin and chemically modified oxyhemoglobin (BME) at low Cl- concentrations

We have measured the optical absorbance in the maxima of the Q and B bands for oxyhemoglobin and oxyhemoglobin (BME) in dependence on the pH value of the solution in the region between pH 4.4 and pH 10. From the absorbance data optical titration curves are derived for both bands. These yield for oxyhemoglobin pK values 4.3, 5.3, 6.8, 7.8, and 9.0, whereas for oxyhemoglobin (BME) only one pK value at 4.3 is observed. These data are in good agreement to those derived recently from resonance Raman spectroscopy. The changes of the oscillator strengths in the Q bands are interpreted in terms of Gouterman's four-orbital model to arise from A1g-distortions of the heme group, resulting from changes of the heme-apoprotein interactions due to protonation processes of amino acid-side groups in the beta-chains. The difference between the sets of pK values in oxyhemoglobin and oxyhemoglobin BME is explained from the fact that the bifunctional reagent BME blocks important pathways of heme-apoprotein interactions. The fact that in any case increase of the Q band absorbance is accompanied by a corresponding increase in the B band absorbance leads us to the conclusion that the electronic structure of the B bands has to be described in terms of a six-orbital model, taking into account configurational interaction with the L and N bands.

Correspondence of the pK values of oxyHb-titration states detected by resonance Raman scattering to kinetic data of ligand dissociation and association

The dispersion of the depolarization ratio of oxidation and spinmarker lines of oxyhemoglobin at low C1- concentration (less than 0.08 M) have been examined for different pH values in the acid and alkaline region. Interpreting the depolarization ratio dispersion curves by fifth order Loudon theory of the polarizibility tensor, we obtain tensor parameters depending linearly on symmetry classified distortions of the functional hemegroup. The pH dependence of these parameters are explained by assuming the influence of three titrable groups with pK = 7.8, 6.6, and 5.8 on the heme. Using these pK values, we are able to interpret the pH dependence of CO(O2)-dissociation and CO-association of the fourth hemoglobin subunit. We conclude from our measurements that the change of the Tyr HC2 beta-configuration induces heme-apoprotein interaction via the Tyr HC2 beta-Val FG5 beta H-bond, which are transduced to the heme via central and peripheral coupling.