MECHANISM FOR OXYGEN ACTIVATION BY CYTOCHROME P450cam

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Cytochrome P450cam is the terminal mono-oxygenase in the d-camphor hydroxylating system of Pseudomonas putida. Recently the x-ray crystal structure of this enzyme has been solved by Poulos and his associates enabling us to study its exact structure-function. relationship. The enzyme catalyzes the reaction indicated below.d-Camphor+NADH+H^++O_2*5-exo-hydroxycamphor+NAD^++H_2OOur previous study with conventional site-directed mutagenesis of the enzyme revealed that a hydroxy (OH) group of Thr252 at the oxygen pocket of the cytochrome plays an important role in the mono-oxygenase reaction : In the absence of the OH group, oxygen consumption and the hydroxylation of d-camphor was uncoupled (Proc.Natl.Acad.Sci.U.S.A.86,7823-7827). Based on this and other evidence, the role of Thr252 has been proposed by us to form a hydrogen bonding net work which is formed this threonine and carboxyl group of Asp251. Such a hydrogen-bonding net work is required for the activation of heme-bound dioxygen (O_ … More 2), i.e.reductive cleavage of the O-O bond in O_2, in the above reaction.to investigate further the role of the OH group of Thr252 in cytochrome P450cam, artificial amino acids with oxygen-, sulfur-, or nitrogen-containing groups were introduced to the 252 position (in place of OH group of Thr) by the aid of in vitro translation system. Measurrements of the enzyme activity of such mutants showed that only those containing oxygen atoms at that position (-OCH_3, COOCH_3) in place of the OH group retained a large portion of the mono-oxygenase activity (>75% formation of hydroxylated product per O_2 consumed). On the other hand, mutants containing a residue with sulfur or nitrogen (-SCH_3, -NH_3^+) had lost most of the activity (<10% formation of hydroxylated product per O_2 consumed). Thus oxygen atom at the residue of 252 position is important for the mono-oxygenase activity.On the basis of these results, we propose that the property of the oxygen atom in Thr252 as an acceptor of a hydrogen bond is essential for the formation of a hydrogen-bonding network for proton transfer necessary for the mono-oxygenase reaction described above. Less