Adenosine 5′-triphosphate (ATP) synthesis by oxidative phosphorylation or photophosphorylation

phosphorylation or photophosphorylation is a multi-step, membrane-located process that provides the bulk of cellular energy in eukaryotes and many prokaryotes. Most of the ATP synthesis in these cells is catalyzed by the enzyme, F1Fo-ATP synthase, also called F1Fo-ATPase (F-ATPase), which in its simplest, bacterial, form is composed of eight subunits (α3:β3:γ:δ:ε:a:b2:c9–12). The archaeal A1Ao-ATP synthase (AATPase) has ten subunits (A3:B3:C:D:E:F:G:H:I:Kx), the actual subunit stoichiometry being unknown. The term ATPase reflects the fact that the Fand A-enzymes are reversible and can act as proton (or Na+)-pumping complexes. The Fand the A-ATPases transform energy from a gradient of ions across the membrane to synthesize ATP (Mitchell, 1961; Dimroth, 1997; Müller et al., 1999). Conversely, the free energy of ATP hydrolysis can be coupled to proton (or Na+) translocation and generate an ion-motive force (IMF), as in the genetically related vacuolar-type, H+-translocating ATPases (V-ATPases). The V-ATPases, consisting of at least twelve distinct subunits (A3:B3:C:D:E:F:Gy:Hz:a:d:e:c6), generate IMFs that are used for ligand trafficking, signaling, nutrient uptake and diverse activities in endomembranes and plasma membranes of animal cells (Wieczorek et al., 1999). A-, Vand F-ATPases consist of a mosaic of globular structural units, including domain and secondary structures, which also serve as functional units. Morphologically each of these enzymes has three components: a membrane-bound sector, Ao/Fo/Vo, which contains the ion channel, a central connecting stalk, and an approximately spherical assembly, A1/F1/V1, which contains the catalytic sites (Schäfer et al., 1999; Leslie and Walker, 2000; Forgac, 2000). Side-view projections of the F1Fo(Wilkens and Capaldi, 1998) and V1Vo-ATPases (Boekema et al., 1997) show a second stalk (stator) as a fourth distinct feature extending from the Fo or Vo portion. In the case of the Escherichia coli F1 moiety the central stalk is composed of γec and εec, which are the equivalent of δm in mitochondrial F1Fo, and the stator is formed by the δec and b subunits (Pedersen et al., 2000). The bacterial δ subunit (δec) bears homology to one of the mitochondrial Fo subunits called OSCP (Table 1). The mitochondrial F1 ε subunit (εm) has no counterpart in the bacterial F1Fo enzyme. The central element of the F1 complex, subunit γ, has been shown to move relative to the α3β3 complex during ATP hydrolysis (Capaldi et al., 1996; Junge et al., 1997; Masaike et al., 2000). This rearrangement is proposed to drive the motion of a ring of c9–14 subunits (Fillingame, 1996; Stock et al., 1999; Seelert et al., 2000; Stahlberg et al., 2001) in the Fo domain (Sambongi et al., 1999; Pänke et al., 2000; Tsunoda et al., 2597 The Journal of Experimental Biology 204, 2597–2605 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3339