The Low‐Spin ⇌ High‐Spin Transition of Camphor‐Bound Cytochrome P‐450

Abstract

The spin state of camphor-bound cytochrome P-450 is shown to depend largely on medium and temperature in aqueous as well as in mixed organic buffer. At sub-zero temperatures a variation of pa_H, ionic strength or camphor concentration modifies the spin equilibrium from nearly pure high-spin form to nearly pure low-spin form. Since the apparent pK_a of transition is a linear function of log I, the spin state seems to be controlled by the electrostatic potential in the heme proximity. K⁺ is found to have a specific effect on the spin state. The change of enthalpy, ΔH values, of the spin transition depends on the same parameters as the equilibrium constant, in the organic cosolvent as well as in aqueous buffer. As the cosolvent effect is reflected by higher ΔH values, and KCI and pH tend to lower ΔH, the cosolvent effect can easily be compensated. Therefore kinetic studies of the spin conversion might well be undertaken at sub-zero temperature in this solvent.