Temperature jump relaxation kinetics of the P-450cam spin equilibrium

The ferric spin-state equilibrium and relaxation rate of cytochrome P-450 has been examined with temperature jump spectroscopy using a number of camphor analogues known to induce different mixed spin states in the substrate-bound complexes [Gould, P., Gelb, M., & Sligar, S. G. (1981) J. Biol. Chem. 256, 6686]. All temperature-induced spectral changes were monophasic, and the spin-state relaxation rate reached a limiting value at high substrate concentrations. The ferric spin equilibrium constant, Kspin, is defined in terms of the rate constants k1 and k-1 via Kspin = k1/k-1 = [P-450(HS)]/[P-450(LS)] where HS and LS represent high-spin (S = 5/2) and low-spin (S = 1/2) ferric iron, respectively, and the spectrally observed spin-state relaxation rate by kobsd = k1 + k-1. A strong correlation between the fraction of high-spin species and the rate constant, k-1, is observed. For a 3 degrees C temperature jump (from 10 to 13 degrees C), the 23% high-spin tetramethylcyclohexanone complex (Kd = 45 +/- 20 microM) is characterized by a ferric spin relaxation rate of kobsd = 1990 s-1, while the rates for the d-fenchone (41% high spin, Kd = 42 +/- 10 microM) and kobsd = 1990 s-1, while the rates for the d-fenchone (41% high spin, Kd = 42 +/- 10 microM) and camphoroquinone (75% high spin, Kd = 15 +/- 5 microM) complexes are 1430 and 346 s-1, respectively.