Review
The cytochrome bd respiratory oxygen reductases

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Abstract

Cytochrome bd is a respiratory quinol:O2 oxidoreductase found in many prokaryotes, including a number of pathogens. The main bioenergetic function of the enzyme is the production of a proton motive force by the vectorial charge transfer of protons. The sequences of cytochromes bd are not homologous to those of the other respiratory oxygen reductases, i.e., the heme–copper oxygen reductases or alternative oxidases (AOX). Generally, cytochromes bd are noteworthy for their high affinity for O2 and resistance to inhibition by cyanide. In E. coli, for example, cytochrome bd (specifically, cytochrome bd-I) is expressed under O2-limited conditions. Among the members of the bd-family are the so-called cyanide-insensitive quinol oxidases (CIO) which often have a low content of the eponymous heme d but, instead, have heme b in place of heme d in at least a majority of the enzyme population. However, at this point, no sequence motif has been identified to distinguish cytochrome bd (with a stoichiometric complement of heme d) from an enzyme designated as CIO. Members of the bd-family can be subdivided into those which contain either a long or a short hydrophilic connection between transmembrane helices 6 and 7 in subunit I, designated as the Q-loop. However, it is not clear whether there is a functional consequence of this difference. This review summarizes current knowledge on the physiological functions, genetics, structural and catalytic properties of cytochromes bd. Included in this review are descriptions of the intermediates of the catalytic cycle, the proposed site for the reduction of O2, evidence for a proton channel connecting this active site to the bacterial cytoplasm, and the molecular mechanism by which a membrane potential is generated.

Highlights

► Physiological functions and genetics of cytochrome bd terminal oxidases reviewed. ► Structural and catalytic properties of cytochromes bd discussed. ► Phylogenetic analysis of cytochromes bd and their homologues presented.

Abbreviations

AOX
alternative oxidase
CIO
cyanide-insensitive quinol oxidase
Em
apparent midpoint redox potential
IC50
the half maximal inhibitory concentration
PMF
proton motive force
TMPD
N,N,N′,N′-tetramethyl-p-phenylendiamine
Q
quinone
QH2
quinol
UQH2
ubiquinol
MQH2
menaquinol
PQH2
plastoquinol
A1
one electron-reduced O2-bound species
A3
fully reduced O2-bound species
R1
one electron-reduced species
R3
fully reduced species
O
fully oxidized species
F
oxoferryl species
P
peroxide-bound species
ΔμH+
transmembrane difference in the electrochemical H+ potentials
τ
time constant reciprocal of rate constant (t1/e)

Keywords

Metabolism
Molecular bioenergetics
Oxidoreduction
Bacterial physiology
Microbe
Disease

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