Modulation of redox potential in electron transfer proteins: Effects of complex formation on the active site microenvironment of cytochrome b(5)

The reduction potential of cytochrome b(5) is modulated via the formation of a complex with polylysine at the electrode surface (Rivera et al., Biochemistry, 1998, 37, 1485). This modulation is thought to originate from the neutralization of a solvent exposed heme propionate and from dehydration of the complex interface. Although direct evidence demonstrating that neutralization of the charge on the heme propionate contributes to the modulation of the redox potential of cytochrome b(5) has been obtained, evidence demonstrating that water exclusion from the complex interface plays a similar role has not been conclusive. Herein we report the preparation of the V45I/V61I double mutant of rat liver outer mitochondrial membrane (OM) cytochrome b(5). This mutant has been engineered with the aim of restricting water accessibility to the exposed heme edge of cytochrome b(5). The X-ray crystal structure of the V45I/V61I mutant revealed that the side chain of Ile at positions 45 and 61 restricts water accessibility to the interior of the heme cavity and protects a large section of the heme edge from the aqueous environment. Electrochemical studies performed with the V45I/V61I mutant of cytochrome b(5), and with a derivative in which the heme propionates have been converted into the corresponding dimethyl ester groups, clearly demonstrate that dehydration of the heme edge contributes to the modulation of the reduction potential of cytochrome b(5). In fact, these studies showed that exclusion of water from the complex interface exerts an effect (similar to 40 mV shift) that is comparable, if not larger, than the one originating from neutralization of the charge on the solvent exposed heme propionate (similar to 30 mV shift).