OXYGEN REACTIONS WITH BACTERIAL OXIDASES AND GLOBINS - BINDING, REDUCTION AND REGULATION

Oxygen is favoured as terminal electron acceptor in aerobic and facult ative microorganisms because of its appropriate physical state, satisf actory solubility and its desirable combinations of kinetic and thermo dynamic properties. Oxygen is generally reduced by four electrons to y ield oxygen, but there are important biological consequences of, and r oles for, the partial reduction to superoxide and peroxide. Complex an d multiple regulatory networks ensure (i) the utilization of oxygen in preference to other oxidants, (ii) the synthesis of oxygen-consuming enzymes with appropriate properties (particularly affinity for the lig and), and (iii) appropriate cellular protection in the event of oxidat ive stress. This contribution reviews the terminal respiratory oxidase s of selected Gram-negative bacteria and microbial haemoglobin-like pr oteins. Recent studies of the cytochrome bd-type oxidases of Escherich ia coli and Azotobacter vinelandii suggest that, despite probable simi larity at the amino acid level, the reactivities of these oxidases wit h oxygen are strikingly different. The respiratory protection afforded to nitrogenase in the obligately aerobic diazotroph A. vinelandii by the cytochrome bd complex appears to be accompanied by, and may be the result of, a low affinity for oxygen and a high V-max. The poorly cha racterized cytochrome o-containing oxidase in this bacterium is not re quired for respiratory protection. In E. coli, the cytochrome bd-type oxidase has a remarkably high affinity for oxygen, consistent with the view that this is an oxygen-scavenging oxidase utilized under microae robic conditions. The demonstration of substrate (i.e. oxygen) inhibit ion in this complex suggests a mechanism whereby wasteful electron flu x through a non-proton-pumping oxidase is avoided at higher dissolved oxygen tensions. The demonstration of two ligand-binding sites (haems d and b(595)) in oxidases of this type suggests plausible mechanisms f or this phenomenon. In E. coli, assembly of the cytochrome bd-type oxi dase (and of periplasmic cytochromes b and c) requires the presence of an ABC transporter, which may serve to export haem or some ''assembly factor' to the periplasm. There is at least one additional oxygen-con suming protein in E. coli - the flavohaemoglobin encoded by the hmp ge ne. Globin-like proteins are also widely distributed in other bacteria , fungi and protozoa, but most have unknown functions. The function of HMP and the related chimaeric flavohaemoglobins in other bacteria and yeast is unknown; one of several possibilities for HMP is that its re latively low affinity for oxygen during turnover with NADH as substrat e could enable it to function as a sensor of falling (or rising) cytop lasmic oxygen concentrations.