Ionization dependence of camphor binding and spin conversion of the complex between cytochrome P-450 and camphor. Kinetic and static studies at sub-zero temperatures.

The kinetic rate constants of formation and dissociation of the cytochrome-P-450 - camphor complex (Fe3+-RH) have been obtained by low-temperature (+ 5 degrees C to -20 degrees C) stopped-flow experiments. Simiarly the high-spin/low-spin equilibrium of this complex has been studied as a function of temperature and protonic activity. Both the camphor-binding mechanism and the high-spin/low-spin thermodynamic parameters of Fe3+-RH depend on the protonic activity of the medium in the physiological pH range. The binding rate constants are shown to depend on the ionization of a residue of the protein, probably a histidine. Linear enthalpy-entropy compensation is observed for the camphor binding as well as for the spin-state transition. A camphor-binding-induced change of the electrostatic potential is discussed.