Membrane subunit b is a component of the Fo complex of FoF1-ATP synthase. The F-type ATP synthases (FoF1-ATPase) consist of two structural domains: the F1 (assembly factor one) complex containing the soluble catalytic core, and the Fo (oligomycin sensitive factor) complex containing the membrane proton channel, linked together by a central stalk and a peripheral stalk. F1 is composed of alpha (or A), beta (B), gamma (C), delta (D) and epsilon (E) subunits with a stoichiometry of 3:3:1:1:1, while Fo consists of the three subunits a, b, and c (1:2:10-14). An oligomeric ring of 10-14 c subunits (c-ring) make up the Fo rotor. The flux of protons through the ATPase channel (Fo) drives the rotation of the c-ring, which in turn is coupled to the rotation of the F1 complex gamma subunit rotor due to the permanent binding between the gamma and epsilon subunits of F1 and the c-ring of Fo. The F-ATP synthases are primarily found in the inner membranes of eukaryotic mitochondria, in the thylakoid membranes of chloroplasts or in the plasma membranes of bacteria. The F-ATP synthases are the primary producers of ATP, using the proton gradient generated by oxidative phosphorylation (mitochondria) or photosynthesis (chloroplasts). Alternatively, under conditions of low driving force, ATP synthases function as ATPases, thus generating a transmembrane proton or Na(+) gradient at the expense of energy derived from ATP hydrolysis. This group also includes F-ATP synthase that has also been found in the archaea Candidatus Methanoperedens.
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