Kinetics of carbon monoxide binding and electron transfer by cytochrome c polymers

Abstract

Studies on horse heart cytochrome c polymers were carried out by stopped-flow and photolysis techniques, to investigate the properties of the CO complex and the kinetics of electron transfer, mainly of the dimeric and tetrameric forms. CO binding, which does not occur with native monomers, proceeds at both pH7.0 and pH9.6, and appears to follow complex kinetics: an initial phase is observed, which is CO-concentration-dependent, followed by a very slow monomolecular phase (k~2x10(-3)s(-1) at pH7) before establishment of equilibrium. Photodissociation of the CO complex has a very low quantum yield, probably less than 0.1. Static titration data of the dimer gave an ;n' value of 0.4. These data strongly suggest heterogeneity of the population of binding sites, and have been interpreted in terms of the existence of different structures, probably owing to the non-unique type of binding of monomers during polymerization. Polymers of cytochrome c carboxymethylated on the methionine residue normally ligated to iron show simple CO recombination kinetics after photolytic removal (k(on)=1.5x10(6)m(-1).s(-1) at pH6). We therefore suggest that, for native cytochrome c, polymerization has an effect on the lability of the haem crevice, rendering the iron available for binding ligands, without, however, forming the structure of a truly open crevice. Electron transfer is, on the other hand, a simple process, and no gross differences are observed between monomer and polymers. A simple model, taking into account all these data, is suggested.