Redox-Bohr effect

The most important and universal type of chemical equilibria to which a heme protein is subjected in natural environments is pH equilibria, involving the protein aminoacid residues and the ionizable groups of the porphyrin, the propionate side chains. Beside the effects on the overall protein structure, which are outside the scope of this review, the major consequences of proton equilibria may be the changes of the heme reduction potential (electron affinity), called the redox-Bohr effect (see Sect. 3.2), by analogy to the pH effect on the oxygen affinity of hemoglobin (Bohr effect). 

The following consideration of cytochrome c oxidase reviews (1) composition, (2) structure, (3) overall reaction, (4) the electron transfer steps of cytochrome c oxidase, (5) status of proton translocation and proposed aqueous D- and K-channels for proton ingress, (6) the redox Bohr effect and its correlation with electrochemical transduction in elastic-contractile model proteins, and (7) possible molecular sources of protons for translocation with an abundant and uniquely positioned functional side chain that exhibits interesting parallels to the states of QH2 with, however, single rather than double proton changes and coordination to a metal ion required for electron transfer capability. 

The Redox Bohr Effect of Complex IV Represents the Hydrophobic Consilient Mechanism s AGap in Action... 

Coherence of Phenomena for Electro-chemical Transduction Demonstrated in Designed Elastic-contractile Model Protein and the Redox Bohr Effect in Complex IV... 

S.3.3 Equivalence of the Reduction/Oxidation (Redox) Bohr Effect in Complex IV and the Original Deoxygenation/Oxygenation Bohr Effect in Hemoglobin... 

N. Capitanio, TV. Vygodina, G. Capitanio, A.K. Konstantinov, P. Nichols, and S. Papa, Redox-Linked Proteolytic Reactions in Soluble Cytochrome-c Oxidase from Beef Heart Mitochondria Redox Bohr Effects. Biochim. Biophys. Acta, 1318, 255-265, 1997. 

The consequence of eqn (3.2) is an increase in EE upon accumulation of positive charge, as expressed by the redox-Bohr effect (see above) a more protonated species is a stronger oxidant. Similarly, the redox potential difference between two adjacent redox couples Ox " /Red" and Ox" /Red " is mainly a result of the different charges. 

Protons that could logically be involved in a membrane Bohr effect are those present on imidazole rings coordinated to Fe or Cu in redox proteins. Removal of an electron from the metal ion could be accompanied by displacement of electrons within the imidazole, within a peptide group that is hydrogen-bonded to an imidazole, or within some other acidic group. A hypothetical example is illustrated in Eq. 18-12 in which a carboxyl group loses a proton when "handed" a second. If the transiently enolized peptide linkage formed in... 

The number of protons involved in the redox-Bohr cooperativity was titrated, showing that this effect involves two protons [60]. Furthermore, kinetic NMR studies have shown that the intramolecular electron exchange (heme to heme) is extremely fast [61], that these two redox-Bohr protons have diffusion controlled exchange rates and that they titrate with the same pKa [54]. 

The functional consequence of the haem orientation appears to be dependent on the particular protein. The influence of the haem orientation on the O2 affinity is negligible in sperm whale Mb, but is relatively significant in insect Chiwnomus thummi thummi) Hbs. Similarly, the Bohr effect of the insect Hb, the autoxidation rate of oxy-Mb and tetrameric human adult oxy-Hb, the redox potential of ferricyto-chrome bs, and the thermal spin equilibrium for both met-azido insect Hb and ferricytochrome have been reported to be influenced by the haem orientation. Thus, an understanding of the molecular mechanism underlying the modulation of functional properties for Z -type haemoproteins through the interaction of the prosthetic haem periphery with the protein matrix demands detailed scrutiny of the influence of the haem orientation on the molecular and electronic structures of the haem-active site. 

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