Ligand binding to the haem-copper binuclear catalytic site of cytochrome bo, a respiratory quinol oxidase from Escherichia coli.

The Escherichia coli quinol oxidase, cytochrome bo, is closely related to the cytochrome-c oxidase, cytochrome aa3 and reacts with ligands to the high-spin ferric haem or the high-spin ferric-cupric binuclear catalytic site in similar ways. Cyanide reacts with the isolated enzyme to give a low-spin complex, manifested by a red shift in the Soret band, the loss of an absorption band at 630 nm and the appearance of a low-spin ferric haem EPR resonance at g = 3.3. Sulphide also elicits a low-spin complex, whereas azide gives a mixture of low-spin and high-spin species. Formate and fluoride (and azide) give a blue shift in the Soret band and the development of a modified absorption band in the 600-650 nm range. These latter species are attributed to an integral spin compound involving the binuclear centre.