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Raman dispersion spectroscopy probes heme distortions in deoxyHb-trout IV involved in its T-state Bohr effect
Journal article   Open access   Peer reviewed

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

Reinhard Schweitzer-Stenner, Michael Bosenbeck and Wolfgang Dreybrodt
Biophysical journal, v 64(4), pp 1194-1209
01 Apr 1993
DOI: https://doi.org/10.1016/S0006-3495(93)81485-1
PMID: 19431886
Featured in Collection :   UN Sustainable Development Goals @ Drexel
url
https://doi.org/10.1016/s0006-3495(93)81485-1View
Published, Version of Record (VoR)Open Access (Publisher-Specific),  Open

Abstract

Proteins
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 δ Q Γ of the heme group that can be classified in terms of the symmetry races Γ = 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 p K values of the involved titrable side chains. We have applied this method to the ν 4 (1,355 cm -1 ) and ν 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 p K values identical to those of the Bohr groups, which were earlier derived from the corresponding O 2 -binding isotherms. These are p K α1 = p K α2 = 8.5 for the α and p K β1 = 7.5, p K β2 = 7.4 for the β chains. We also obtain the specific distortion parameters for each protonation state. As shown in earlier studies, the ν 4 mode mainly probes distortions from interactions between the proximal histidine and atoms of the heme core (i.e., the nitrogens and the C α 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 δ Q Γ derived from the Raman dispersion data. Thus, we found that the pH dependence of the heme distortions δ Q A1g (totally symmetric) and δ Q 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 ν 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 δ Q Γ affecting the hemes in the α and β chains, respectively. From this, we find that in the α subunit structural changes induced on protonation of the corresponding Bohr groups are mainly transferred via the Fe—N ε bond and give rise to changes in the azimuthal angle. In the β 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 β). This rearranges the net of H bonds between His HC3 β, Ser (F9 β), and Glu (F7 β).

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