Spin transition of camphor-bound cytochrome P-450. 2. Kinetics following rapid changes of the local paH at sub-zero temperatures.

The kinetics of the spin transition in the heme iron of the cytochrome P-450 substrate complex have been observed by stopped-flow measurements between 4 degrees C and -27 degrees C. Large displacements in the spin equilibrium are induced by small changes in the concentration of hydrogen or potassium ions. The kinetic and thermodynamic data indicate that the spin transition is rate-limited by conformational changes of the protein. The spin transition appears to be governed by the local paH, modulated in turn by external factors: a satisfying kinetic analysis is attained only by accounting for the difference between local paH (paH, in) and bulk paH (paH, out), as described in the preceding paper. Indeed, the low-spin to high-spin rate constant obeys a local paH titration curve with a pK = 5.4 +/- 0.1 at -17 degrees C.