ABSTRACT
For an enzyme system which has been so extensively studied as P450, it is perhaps surprising that there are still many questions about its catalytic cycle and oxygen activation mechanism remaining to be answered. Indeed, even the currently-accepted reaction stages are subject to speculation and controversy: some of which can be focused on the oxene intermediate. This supposedly active oxygenating species remains to be observed in a P450 system and yet would be expected to contain a strong covalent bond, i.e. Fe=O, which should entail stability and an energetically unfavourable cleavage with respect to, say, a single Fe-O linkage that has been postulated to be present at an earlier stage of the catalytic cycle. A second difficulty with the currently in vogue oxenoid mechanism arises from the fact that the iron-oxene intermediate can be expected to be electrophilic in nature whereas, in fact, some P450-mediated reactions would appear to require a nucleophilic oxygenating species. However, there are some advantages to the oxene postulate, as it readily explains single oxygen insertion, particularly for the formation of epoxides and N-oxides, which are otherwise difficult to explain via a peroxide or superoxide intermediate. In contrast, there is considerable evidence for both superoxide and peroxide in P450-mediated oxygenations, including the observation of an O−O stretching vibration by Raman spectroscopy under catalytic conditions (Egawa et al., 1991). On balance, it seems possible that, depending on the reaction conditions, substrate, and type of P450 involved, either mechanism (i.e. oxenoid or peroxide) could occur. It is, therefore, reasonable to discuss both of these mechanisms of oxygen activation and insertion into substrates such that the reader can make up his or her own mind regarding whether either, or both, are possible.
