Thermodynamics of carbon monoxide photodissociation from the fully reduced cytochrome aa3 oxidase from Rb. sphaeroides

Photodissociation of the fully reduced carbonmonoxy bound cytochrome aa3 from Rb. sphaeroides results in ultrafast ligand transfer between heme a3 and CuB, which is followed by thermal dissociation from CuB on longer time scales. We have utilized photoacoustic calorimetry to obtain a detailed thermodynamic description of the mechanism of ligand photodissociation and transfer between heme a3 and CuB. Subsequent to ligand photodissociation an additional process, which has not been characterized previously, was observed with the lifetime of 485 ns at 18 °C and is coupled to a volume expansion of 3.3 ml mol−1. From the temperature dependence, an activation barrier of 4 kcal mol−1 was determined. We attribute the observed 500 ns process to changes in CuB ligation subsequent to ligand translocation. In a photoacoustic study on CO photodissociation from bovine heart aa3 oxidase, no volume changes were observed on the ns timescale, indicating that a different mechanism may control ligand dissociation and binding within the binuclear center of the bacterial and bovine enzymes.