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 Cu_B, which is followed by thermal dissociation from Cu_B 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 Cu_B. 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 Cu_B 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.