Ionization Dependence of Camphor Binding and Spin Conversion of the Complex between Cytochrome P‐450 and Camphor

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Abstract
Kinetic rate constants of formation and dissociation of the [Pseudomonas putida] cytochrome-P-450 .cntdot. camphor complex (Fe3+ .cntdot. RH) were obtained by low-temperature (5.degree. C to -20.degree. C) stopped-flow experiments. The high-spin/low-spin equilibrium of this complex was studied as a function of temperature and protonic activity. The camphor-binding mechanism and high-spin/low-spin thermodynamic parameters of Fe3+ .cntdot. RH depend on the protonic activity of the medium in the physiological pH range. Binding rate constants depend on the ionization of a protein residue, probably a histidine. Linear enthalpy-entropy compensation is observed for camphor binding and for spin-state transition. A camphor-binding-induced change of the electrostatic potential is discussed.