CO Photodissociation Dynamics in Cytochrome P450BM3 Studied by Subpicosecond Visible and Mid-Infrared Spectroscopy

Abstract

Cytochrome P450BM3 is a bacterial enzyme with a heme cofactor that binds small diatomic ligands. Here we report the first study of carbon monoxide (CO) photodissociation and rebinding in ferrous P450BM3 on an ultrafast time scale. We monitored dissociation of carbon monoxide upon Soret band excitation using visible and infrared femtosecond spectroscopy between 100 fs and 4 ns. The dynamics of the ferric P450 was probed for reference in the visible spectral region. In the photodissociated ferrous P450-CO complex, the vibrational hot deligated ground state is populated in 0.2 ps and relaxes on a picosecond time scale. The onset of geminate recombination of CO with the heme is observed on a nanosecond time scale. In the mid-infrared spectral region, the bleached absorption due to the bound C=O stretch vibration is constant on the picosecond to 1 ns time scale, indicating that the photodissociation yield is 100% and that rebinding occurs after 1 ns. In the infrared absorption difference spectra, we additionally resolve two small bands of dissociated CO molecules at 2092 and 2114 cm(-1). This indicates that the escape of photolyzed CO to solvent and the geminate recombination are preceded by transient docking within the protein in a manner similar to that of globins. The bands partially decay with a time constant of 1 ps, possibly due to a relaxation of the protein around the CO docking site, allowing for greater orientational freedom of the CO molecules.