[Productive and non-productive complexes in cytochrome P450-containing system]

The equilibrium dissociation constants K(D), the complex association / dissociation rate constants (k(on)/ k(off)) and the lifetimes of redox partners' complexes were measured for three cytochrome P450-containing monooxygenase systems (P450cam, P450 2B4 and P450scc). To estimate the productivity of complexes formed within the systems studied, the Q parameter--i.e. the ratio of protein-protein complex lifetime (T(LT)) to the time required for a single hydroxylation cycle (tau(cat))--was determined. It was shown that Q was changed (albeit insignificantly) upon transition from the oxidation to hydroxylation conditions in all the three P450 monooxygenase systems studied. It was shown that the binary complexes formed within the P450cam and the P450scc systems requiring an intermediate electron-transfer protein between the reductase and cytochrome P450 were non-productive while the binary complexes formed within the P450 2B4 system, not requiring such an intermediate electron-transfer protein, proved to be productive. Formation of ternary complexes within the three systems was demonstrated under hydroxylation conditions. Analysis of Q values led to the conclusion that the ternary complexes formed within the P450cam and the P450scc systems were virtually 100% productive. Within the P450 2B4 system, more than half (about 60%) ternary complexes were also found to be productive.