Proton ATPases in bacteria: Comparison to Escherichia coli F¹F⁰ as the prototype

The F₁F₀ ATP synthase complex of Escherichia coli functions reversibly in coupling proton translocation to ATP synthesis or hydrolysis. The structural organization and subunit composition corresponds to that seen in many other bacteria, i.e. a membrane extrinsic F₁ sector with five subunits in an α₃β₃γδε stoichiometry, and a membrane-traversing F₀ sector with three subunits in an a₁b₂c₁₂ stoichiometry. The structure of much of the F₁ sector is known from a X-ray diffraction model. During function, the γ subunit is known to rotate within a hexameric ring of alternating α and β subunits to promote sequential substrate binding and product release from catalytic sites on the three β subunits. Proton transport through F₀ must be coupled to this rotation. Subunit c folds in the membrane as a hairpin to two a helices to generate the proton-binding site in F₀. Its structure was determined by NMR, and the structure of the c oligomer was deduced by cross-linking experiments and molecular mechanics calculations. The implications of the oligomeric structure of subunit c will be considered and related to the H₊/ATP pumping ratio, P/O ratios and the cation-binding site in other types of F₀. The possible limits of the structure in changing the ion-binding specificity, stoichiometry and routes of proton entrance/exit to the binding site will be considered.