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.