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

Abstract

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)×10 4, (2.0±0.01)×10 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)×10 4 and (2.0±0.01)×10 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.