PRODUCTIVE AND NON-PRODUCTIVE COMPLEXES IN CYTOCHROME P450-CONTAINING SYSTEM
The equilibrium dissociation constants KD, the complex association / dissociation rate constants (kon / koff) 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 (τlt) to the time required for a single hydroxylation cycle (τcat) - 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.