EFFECTS OF MONOVALENT CATIONS ON CYTOCHROME-P-450 CAMPHOR - EVIDENCE FOR PREFERENTIAL BINDING OF POTASSIUM

Binding of monovalent cations of increasing ionic radius to ferric cytochrome P-450(cam) was measured. Potassium has the highest affinity for the cation binding site observed in the X-ray crystallographic structure with K-d cat = 12 mM, compared with the smaller cation lithium, (K-d cat = 37 mM) and the larger cation cesium (K-d cat = 20 mM). Coupling between cation binding and camphor binding is established by the observation of a linear relationship between the corresponding binding free energies. Potassium binding favours a conformational change of tyrosine 96 which increases the affinity of the protein for camphor and fully dehydrates the active site.