One‐Electron Photoreduction of Bacterial Cytochrome P‐450 by Ultraviolet Light

The intramolecular photoreduction of the bacterial cytochrome P‐450 demonstrated by the formation of the complex of reduced cytochrome m with camphor and CO has been studied by photolysis with a laser flash at 265 nm. This photoreduction of the heme follows the photoionization of a proximal aromatic amino acid residue and especially tryptophan in the nanosecond time scale as evidenced by the N 2 O effect on the reduction yield. However most of the hydrated electrons released in the solution do not react with the ferric porphyrin. In agreement with steady state studies, the quantum yield for formation of the complex of reduced cytochrome m with camphor and CO is proportional to the concentration of the high‐spin species. The low‐spin ferric porphyrin is thus not reducible by photoejected electrons. It is also shown that the growth kinetics of the transient absorbance at 446 nm characterizing the complex of reduced cytochrome m with camphor and CO correspond to a structural change affecting the protein and not to the formation of the complex of CO with the camphor‐bound reduced cytochrome m which occurs on a shorter time scale in the present experimental conditions.