Spin transition of camphor-bound cytochrome P-450. 1. local paH and electrostatic interactions.

Variations of the spin state in camphor-bound cytochrome P-450 are interpreted in the light of the polyelectrolyte theory and its implications on the microenvironment of the heme. The ratio of high-spin to low-spin iron can serve as a tool to determine the local paH in the microenvironment of a group (pK0, app approximately 5.4) which governs the spin state. The local paH depends on the electrostatic potential created by negatively charged groups (pKa = 5.6), modulated in turn by paH and by the screening effect of ionic strength. A model is given for the proton-coupled spin state change.