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 cyt ochrome P-450(cam) was measured. Potassium has the highest affinity fo r the cation binding site observed in the X-ray crystallographic struc ture 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). Coupl ing between cation binding and camphor binding is established by the o bservation of a linear relationship between the corresponding binding free energies. Potassium binding favours a conformational change of ty rosine 96 which increases the affinity of the protein for camphor and fully dehydrates the active site.