Mercaptide-anion as a trans-ligand in oxycytochrome P450

Oxygenation of heme-mercaptide as well as spectroscopic characteristics of the dioxygen complex formed have been studied. Absorption and magnetic circular dichroism spectra of the O₂ complex support the retention of mercaptide in the heme fifth position. A release of $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ in the decomposition of the oxygenated complex and an independent formation of the latter from hemine-dimercaptide and $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ together with electron paramagnetic resonance and Mössbauer data support the O₂ presence in the heme coordination sphere. The similarity of optical and magnetic circular dichroism spectra and the closeness of the ratio for oxy-heme-mercaptide and oxycytochrome P450 unequivocally confirm the presence of an axial cystein mercaptide ligand in oxycytochrome P450.