The influence of oxygen donor ligation on the spectroscopic properties of ferric cytochrome P-450: ester, ether and ketone co-ordination to the haem iron.

Abstract

Homogeneous low-spin complexes of cytochrome P-450-CAM with esters, ethers and ketones have been prepared and characterized by u.v.-visible absorption, circular dichroism (CD), magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopy. Direct haem iron ligation has been verified by competition for binding with well-known haem ligands. The u.v.-visible spectra of the oxygen-donor complexes exhibit near-u.v. (delta) transitions near 356 nm, Soret maxima at 417 +/- 1 nm, beta bands near 536 nm and alpha peaks near 569 nm, with alpha greater than beta in intensity. Negative delta and Soret CD troughs are seen. The MCD spectra have minima at approximately 356 nm, intense derivative-shaped Soret features centred at approximately 416 nm and four characteristic features beyond 450 nm. The EPR spectra of these complexes, while similar to that of the native enzyme, exhibit slight variances. Anomalous spectral and substrate binding properties have been reported in the study of cytochrome P-450 under conditions employing solvents and non-phosphate buffers containing oxygen functionalities, and have been attributed to 'solvent effects'. The present work, in combination with our previous report of alcohol, amide and carboxylate oxygen donor complexes of cytochrome P-450, is evidence that a wide variety of oxygen-donor species are capable of direct ligation to the haem iron of cytochrome P-450. This leads us to suggest oxygen-donor ligation to cytochrome P-450 as the origin of spectral and substrate binding anomalies previously attributed to solvent effects.