DYNAMICS OF CARBON-MONOXIDE BINDING WITH CYTOCHROMES P-450

The dynamics of CO rebinding with cytochromes P-450(cam), P-450(scc), and P-450(LM2) after laser flash photolysis have been investigated fro m 293 to 77 K, and the distribution functions of the rate parameters P (k) and of the activation enthalpy P(H) were determined using the maxi mum entropy method. In a fluid solvent, geminate rebinding is nonexpon ential, presumably because of a spectral shift induced by protein rela xation on the same time scale. Substrate binding increases the yield o f the bimolecular process and decreases the bimolecular rate by I or 2 orders of magnitude. The-amplitude of these effects seems to correlat e with substrate specificity, In a rigid environment at low temperatur e, cytochromes P-450 exhibit a bimodal distribution of activation enth alpy; P(H) consists of two distinct bands which are in a thermal equil ibrium even at 77 K. The results lead to a scheme in which a common st ructural perturbation splits the conformational substates of cytochrom es P-450 into pairs of ''doublet'' substates with different dynamic pr operties. The hierarchy of conformational substates of cytochromes P-4 50 thus contrasts with that of oxygen-binding hemoproteins such as myo globin.