Significance of Protein-Substrate Hydrogen Bonding for the Selectivity of P450-Catalysed Oxidations.

P450cin and P450cam are bacterial cytochromes P450 that specifically hydroxylate bicyclic monoterpenes. Protein-substrate H bonding has been previously proposed as crucial in the selectivity of P450cin oxidations, but not as essential for P450cam . To examine the difference in importance of H bonds in these two model P450s, the P450-catalysed oxidation products from thiocamphor were compared. Surprisingly, both P450s oxidised thiocamphor predominantly to the corresponding S-oxides, in contrast to previous reports, and this is the first report of P450-catalysed sulfine generation from a thioketone. Additionally, the result emphasised the importance of the protein-substrate H bond to selectivity in both P450cin and P450cam . The H bonding in P450cam was re-examined using camphane, another substrate for which the protein-substrate H bond is absent. The results indicated that both H bonding and hydrophobic interactions between substrate and protein play a role in selectivity. Interestingly, the protein-substrate H bond was not a factor in substrate affinity for the enzyme.