SPIN-STATE CHANGES IN CYTOCHROME P-450cam ON BINDING OF SPECIFIC SUBSTRATES

TLDR: It is concluded that P-450(cam) as isolated is equal to or more than 95% in a low spin form probably having sulfur as one of the axial ligands.

Abstract: The electron paramagnetic resonance signals of the soluble P-450 cytochrome from Pseudomonas putida were observed at temperatures from 42 to 80°K As isolated, P-450 has a signal typical of a low spin ferric-heme compound with sulfur as one of the axial ligands (g = 245, 226, 1915) We also detected a minor signal typical of high spin ferric heme (g = 8, 4, 18) equivalent to less than 7% of the heme at temperatures below 20°K On titration with the substrate, (+)-camphor, the low spin signal decreased and the high spin signal increased, maximally representing about 60% of the heme For reasons not thus far understood, 40% of the heme is not converted to high spin by either (+) or (-)-camphor The high spin signal has a rhombic character which is stronger than any previously observed with a heme compound (E = 033 cm-1; D = 38 cm-1; E/D = 0087) We conclude that P-450cam as isolated is equal to or more than 95% in a low spin form probably having sulfur as one of the axial ligands The binding of substrate displaces this ligand sufficiently to allow for conversion from a low to a high spin form