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The membrane motor $F_0$ contains a rotating ring of $c$ subunits beside stationary subunit $a$. Two offset water-accessible half-channels connect opposite membrane sides to the ring. A proton enters one half-channel and protonates a glutamic acid residue on a $c$ subunit; neutralization permits that site to enter the hydrophobic lipid bilayer. Rotation carries it around to the other half-channel, where it releases the proton. A conserved arginine in subunit $a$ helps separate the routes and promotes release. Downhill proton transfer biases rotation; this is not a freely open transmembrane pore.^[ref]

The central shaft transmits torque to $F_1$; a peripheral stalk prevents the catalytic head from simply co-rotating. Its three $\beta$ catalytic sites, within an ${\alpha }_3{\beta }_3$ head, cycle through different binding shapes. In the simplified binding-change picture, a loose site captures ADP and inorganic phosphate, a tight site favours bound ATP, and opening releases ATP. Rotation changes binding and release as well as chemistry.^[ref]

One complete, tightly coupled turn cycles each catalytic site once, producing three ATP molecules. If the ring has $n$ proton-binding subunits, the ideal motor budget is $n$ transferred protons per turn, or $n/3$ per ATP. Ring size varies between organisms; the worked example chooses $n=10$, as in the studied bacterial systems, rather than assigning ten to every ATP synthase.^[ref]

For this example the ring's repeat angle is $360^{\circ }/10=36^{\circ }$, whereas the three-site catalytic cycle repeats after $120^{\circ }$. Their ratio is $10/3$, not an integer. Counting complete turns avoids inventing a fixed integer proton count for every catalytic event. The schematic does not require both parts to advance simultaneously as rigid gears; elasticity allows temporary twisting while their cycles proceed.^[ref]

Researcher in structural biology. Guo and collaborators determined bacterial ATP synthase structures, connecting subunit organization with the coupling of a rotary mechanism to ATP production.^[ref]

Researcher in structural biology. Rubinstein coauthored studies of bacterial ATP synthase, connecting cryo-electron microscopy structures with the mechanics and regulation of a rotary energy-converting complex.^[ref]

A major source of ATP is chemiosmosis: an electron transport chain uses downhill electron transfer to establish a proton-motive force, and ATP synthase converts that electrochemical gradient into ATP formation. In aerobic oxidative phosphorylation, reduced carriers supply electrons, ultimately reducing oxygen to water; selected respiratory complexes couple the favorable oxidation-reduction reaction to uphill proton translocation. In a mitochondrion, this takes place across the inner membrane, creating a relatively positive, acidic exterior side and a relatively negative, alkaline matrix. Bacteria use their cell membrane. In photosynthetic systems, light-driven electron transfer can supply the gradient instead. Substrate-level phosphorylation is another ATP-producing route, so the description is not universal to every organism and condition.

The membrane $F_O$ region of ATP synthase contains a rotating ring of c-subunits next to subunit a. Two offset aqueous half-channels expose the ring's proton-binding sites to opposite membrane sides. Proton binding and release bias ring rotation down the electrochemical gradient. The central shaft transfers torque to the $F_1$ catalytic head, while a peripheral stator prevents the head from simply co-rotating. This is the ATP synthase rotary mechanism, rather than unrestricted proton leakage through a pore.

The three catalytic beta subunits cycle through different binding states as the shaft turns: ADP and inorganic phosphate bind, bound ATP is favored, then ATP is released. The binding-change mechanism uses mechanical work to alter binding affinities and product release as well as the chemical step. A full coupled revolution produces three ATP molecules; a ring with $n_c$ c-subunits transfers approximately $n_c$ protons per revolution in the tightly coupled model, or $n_c/3$ per ATP.^[ref][ref]

The overall cycle must have a favorable Gibbs free energy change. If the proton-motive force becomes too small, synthesis stops; under suitable conditions the same molecular motor can reverse, using ATP hydrolysis to pump protons uphill. A catalyst changes kinetics, not the thermodynamic condition for synthesis.

Toshiharu Suzuki coauthored structural research on bacterial ATP synthase. The structure relates the membrane sector involved in proton movement to the rotary and catalytic portions of the complex.

A static molecular structure constrains possible coupling mechanisms but does not directly record a live rotation trajectory. Combining structure with functional measurements connects molecular architecture with energy conversion.^[ref]

A biochemist.^[ref]