Measurements of CO geminate recombination in cytochromes P450 and P420

The kinetics of CO geminate recombination in cytochrome P450cam are studied at room temperature subsequent to laser photolysis. The geminate rebinding kinetics of P450 are strongly affected by the presence of the camphor substrate. We observe a approximately 2% geminate yield for substrate-bound P450 and a 90% geminate yield when the substrate is absent. The drastic difference in the geminate kinetics suggests that the presence of camphor significantly alters the CO rebinding and escape rates by modifying the heme pocket environment. Two geminate phases and two bimolecular rebinding phases in the substrate free protein were observed, which could arise from slowly interconverting protein conformations. When the temperature or the viscosity of the solution is changed, the fast geminate rate remains the same, whereas the slow geminate rate and the two bimolecular rates change significantly. The geminate rebinding yield of substrate-free P420 is smaller than that of substrate free P450, but its geminate rebinding rate is faster. This demonstrates that in the absence of substrate, CO escapes from the pocket of P420 much more rapidly than from P450 and suggests that the distal pocket environment is altered in the P420 form.