PRODUCTIVE AND NON-PRODUCTIVE COMPLEXES IN CYTOCHROME P450-CONTAINING SYSTEM
The equilibrium dissociation constants K<sub>D</sub>, the complex association / dissociation rate constants <i>(k</i><sub><i>on</i></sub><i> / k</i><sub><i>off</i></sub><i>) </i>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 <i>Q </i>parameter - i.e. the ratio of protein-protein complex lifetime <i>(τ</i><sub><i>lt</i></sub><i>) </i>to the time required for a single hydroxylation cycle <i>(τ</i><sub><i>cat</i></sub><i>) - </i>was determined. It was shown that <i>Q </i>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 <i>Q </i>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.