The importance of the spin equilibrium in cytochrome P-450 for the reduction rate of the heme iron.

Based on the existence of a spin equilibrium of P-450 LM a thermodynamic model including the first two elementary steps has been developed. This model has been compared with a modified model of P-450 CAM first presented by SLIGAR [1]. For the quantitative description of the model of P-450 LM seven necessary constants have been calculated. In the model a 5-coordinated ferric heme complex as intermediate and precursor of the reduced state is included. The model is based on experimental data of the spin equilibrium and the binding of benzphetamine to P-450 LM2, on the redox potential of P-450 LM and further on the assumption of a structure of the precursor well adapted to reduction. The dissociation constant (KD) of the 6th heme iron ligand is calculated to be 0.03 meaning that only 3% of the high spin ferric state are 5-coordinated. From the thermodynamic parameters of the model the sequential order of the reaction process can be derived and conclusions can be drawn on possibly reaction controlling steps. Experimentally it is shown that the substrate induced spin shift is correlated with the reduction rate. This has been shown to be valid in dependence on the degree of saturation with only one substrate as well as with different substrates indicating that independent of the individual substrate only the shift of the spin state controls the reduction. From the experiments as well as from the model it can be concluded that P-450 works in the first reaction step in two states (different in structural and functional properties) being in equilibrium with each other. Different affinities of these two states towards the substrate shift the equilibrium. This shift increases the concentration of the reducible intermediate and by this the reduction as second elementary step is regulated.