Received July 10, 2020; Revised September 5, 2020; Accepted September 5, 2020
Cytochrome bd-I is a terminal oxidase of the Escherichia coli respiratory chain. This integral membrane protein contains three redox-active prosthetic groups (hemes b558, b595, and d) and couples the electron transfer from quinol to molecular oxygen to the generation of proton motive force, as one of its important physiological functions. The study was aimed at examining the effect of the membrane environment on the ligand-binding properties of cytochrome bd-I by absorption spectroscopy. The membrane environment was found to modulate the ligand-binding characteristics of the hemoprotein in both oxidized and reduced states. Absorption changes upon the addition of exogenous ligands, such as cyanide or carbon monoxide (CO), to the detergent-solubilized enzyme were much more significant and heterogeneous than those observed with the membrane-bound enzyme. In the native membranes, both cyanide and CO interacted mainly with heme d. An additional ligand-binding site (heme b558) appeared in the isolated enzyme, as was evidenced by more pronounced changes in the absorption in the Soret band. This additional reactivity could also be detected after treatment of E. coli membranes with a detergent. The observed effect did not result from the enzyme denaturation, since reconstitution of the isolated enzyme into azolectin liposomes restored the ligand-binding pattern close to that observed for the intact membranes.
KEY WORDS: cytochrome, terminal oxidase, respiratory chain, heme, membrane environment, ligand binding, Escherichia coli