Substrate-induced spin-state transition in cytochrome P450LM2: a temperature-jump relaxation study.

The kinetics of the benzphetamine-P450LM2 binding reaction were studied by the T-jump relaxation technique, using the substrate-induced type I spectral change (which reflects transformation of the heme from the low- to the high-spin state) as the criterion for binding. The reciprocal relaxation time (kobs) exhibited a linear dependence on [E]eq+[S]eq. The kinetically determined dissociation equilibrium constant (68 +/- 10 microM) and that determined by direct titration of the spectral change (61 +/- 4 microM) were very similar. These results indicate that the substrate-induced spin-state transition follows a simple biomolecular binding mechanism; that is, the substrate-induced low- to high-spin transition reflects substrate binding.