Ligand binding subsequent to CO photolysis of methionine-modified cytochrome c

In this report the kinetics of CO recombination to ferrocytochrome c in which Met80 has been oxidized to a sulfoxide are examined. Transient optical difference spectra suggest that the species formed immediately after photolysis contains a five-coordinate high spin heme. Single wavelength transient absorption data show triphasic kinetics with rate constants of (2.1+/-0.08)x10(4), (2.0+/-0.01)x10(3), and 57+/-0.01 s(-1). The data suggest a model for ligand recombination in which the methionine sulfoxide and CO compete for binding to the five-coordinate heme with rate constants of (2.1+/-0.08)x10(4) and (2.0+/-0.01)x10(3) s(-1), respectively. Carbon monoxide then binds to the population of cytochrome c containing the methionine sulfoxide with a rate constant of 57 s(-1). In addition, the slower than expected rate of methionine sulfoxide recombination (much smaller rate constant than expected for a ligand restricted to the distal heme pocket) is attributed to hydrogen bonding between the unbound methionine sulfoxide and Tyr(68).