ATP synthase: a tentative structural model

Abstract

Adenosine triphosphate (ATP) synthase produces ATP from ADP and inorganic phosphate at the expense of proton‐ or sodium‐motive force across the respective coupling membrane in Archaea, Bacteria and Eucarya. Cation flow through the intrinsic membrane portion of this enzyme (F_O, subunits ab ₂ c _9–12) and substrate turnover in the headpiece (F₁, subunits α ₃ β ₃ γ δ ϵ) are mechanically coupled by the rotation of subunit γ in the center of the catalytic hexagon of subunits (α β)₃ in F₁. ATP synthase is the smallest rotatory engine in nature. With respect to the headpiece alone, it probably operates with three steps. Partial structures of six out of its at least eight different subunits have been published and a 3‐dimensional structure is available for the assembly (α β)₃ γ. In this article, we review the available structural data and build a tentative topological model of the holoenzyme. The rotor portion is proposed to consist of a wheel of at least nine copies of subunits c, ϵ and a portion of γ as a spoke, and another portion of γ as a crankshaft. The stator is made up from a, the transmembrane portion of b ₂, δ and the catalytic hexagon of (α β)₃. As an educated guess, the model may be of heuristic value for ongoing studies on this fascinating electrochemical‐to‐mechanical‐to‐chemical transducer.