Preference for aromatic substitutions at tryptophan-120, which is highly conserved and a potential mediator of electron transfer in cytochrome P450 2C2

A proposal that tryptophan-120 of P450 2C2 might mediate electron transfer has been tested by substitution of a series of aliphatic and aromatic residues at this position. Activity in transfected COS1 cells expressing enzymes substituted with the aromatic residues, tyrosine, phenylalanine, and histidine, was 25% to 60% of wild type. Activities in enzymes containing serine, arginine, or aliphatic amino acid substitutions were less than 10%, except for alanine (25%). Alanine is the only naturally occurring substitution for tryptophan at this position in mammalian cytochromes P450 and is present in the electron pathway proposed on the basis of the crystal structure of cytochrome c and cytochrome c peroxidase. The preference for aromatic residues or the small aliphatic amino acid, alanine, at position 120 is consistent with a role for this tryptophan in electron transfer.