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Roles of cytochromes c and b5 in mitochondria and microsomes: Classical and murburn perspectives

While cytochrome c (Cyt. c) is a soluble protein involved in mitochondrial electron transfer (ET) reactions between Complex III and Complex IV, cytochrome b5 (Cyt. b5) is a microsomal membrane protein acting as a redox aide for diverse cytochrome P450s and their unique reductase. We found little conservation in the sequence and surface amino acid residues of Cyt. c and b5 proteins among evolutionarily diverse species. Hence, the logic that these proteins mediate ET through affinity binding via specific surface residues is weak. Also, analysis of putative protein-protein interactions in the crystal structures of these proteins and their redox partners did not point to any specific interaction logic. The literature on kinetic and thermodynamic constants of mutants (with altered surface residues) did not provide strong evidence to support the binding-based ET paradigm. Topographically divergent Cyt. b5 from one species has been shown to enhance the activity of CYP450 from another species, implying the involvement of non-specific interactions. These observations downplay the classical protein-protein biding based long range ET mechanism. Further, we provide evidence to show that murburn concept presents better chemico-physical logic for ETs mediated by Cyt. c and b5. To explain for the promiscuity of interactions, we conclude that the two proteins act as non-specific/generic redox capacitors, mediating a one-electron redox equilibrium involving diffusible reactive oxygen species (DROS) and ions.