The Quaternary Structure of F1-ATPase is Unique
The membrane-bound ATP synthase of animals, plants, and microorganisms is comprised of two functional units, FO and F1. The water-soluble F1 comprises five different subunits in the stoichiometry α3β3δεγ. The three catalytic β subunits alternate with the three α subunits around the centrally located γ subunit. The FO part sits in the membrane, where it mediates coupling of ATP synthesis to the flow of protons across the membrane.
The γ subunit acts as a crankshaft that is driven by the proton current. The β subunit of FO is bound to the external surface of a single α subunit. It holds the α3/β3 hexamer in place as the γ subunit rotates within the central cavity.
In the absence of FO, the F1 part is an ATPase, known as F1-ATPase.