The Quaternary Structure of F1-ATPase is Unique

The membrane-bound ATP synthase of animals, plants, and microorganisms is comprised of two functional units, F_O 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 F_O 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 F_O 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 F_O, the F₁ part is an ATPase, known as F₁-ATPase.

Opening Image: F₁-ATPase. The helical domain of the γ subunit is shown in red. It runs through the center of the "mushroom cap" formed by alternating α and β subunits. The δ and ε subunits are missing in this X-ray structure.

Toggle rotation and note that the tip of the γ subunit projects into a 15 Å-deep dimple in the top of the molecule.

Isolate the C-terminal segment of the γ subunit. It is a 92 Å-long α helix.

Toggle between the trace model and spacefilled model.

Measure the distance between the ends of the segment.

The lower half forms a bent coiled coil with the N-terminal segment of the γ subunit. The two helices create a stiff crankshaft. [Note: About half of the γ subunit is missing in this X-ray structure.]

The catalytic β subunit exists in two conformations, open and closed. Two β subunits are always in the closed state. At any step in the catalytic cycle, one fully-formed ATP is tightly bound to a closed β subunit, and ADP and inorganic phosphate are tightly bound to the second closed β subunit. The third β subunit exists in the open conformation and is empty.

The open β subunit makes extensive contacts with the crankshaft.

Toggle MgATP-β complex. AMP-PNP replaces ATP in the X-ray structure.

Toggle MgADP-β complex. P_i is missing in the crystal structure.

All three α subunits as well as the three β subunits are in contact with the γ subunit.

α/β cross-section of F₁-ATPase.

Toggle slab view. Note that the binding sites for AMP-PNP and ADP are located below the surface of the protein.