The proton price of ATP is not a constant

In Short. How many protons buy one ATP is fixed by rotor geometry. Chlamydomonas carries a 13-subunit c-ring, not the familiar plant c14, and that changes the energy cost of fixing carbon.

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ATP synthase is a rotary motor. Protons flowing down an electrochemical gradient turn a membrane ring of identical c-subunits. Each full turn of that ring drives the catalytic head through three ATP-making steps. So the number of teeth on the rotor is not a decorative detail. It sets how many protons must pass for each ATP formed. Change the tooth count and you change a fundamental price of metabolism.

That price is written H⁺/ATP. Because three ATP leave per revolution, H⁺/ATP equals the c-subunit count divided by three. Across life the ring is not one size. Bovine mitochondria run a tight c8 ring (predicted H⁺/ATP of about 2.7). Some cyanobacteria push toward c15 (about 5). Within chloroplasts, though, the high-resolution reference has long been spinach, with a fourteen-membered ring and a predicted H⁺/ATP of 4.67.

A Sheffield-led cryo-EM study now reports the chloroplast ATP synthase of the green alga Chlamydomonas reinhardtii at 2.2 Å. The algal rotor is a thirteen-membered c-ring. Thirteen protons per revolution yield a predicted H⁺/ATP of 4.33, lower than spinach. The authors call this the first clear departure from c14 in a chloroplast motor. A prior algal map had modelled fourteen subunits into weaker Fo density. Higher resolution settles the count at thirteen.

Why does one fewer subunit matter? Green algae run a carbon-concentrating mechanism that packs CO₂ around Rubisco and suppresses photorespiration, but at an extra ATP cost. Linear electron transfer from water to NADP⁺ deposits a fixed proton budget of six protons per NADPH. How much ATP that budget buys depends on the synthase ratio. On the paper’s arithmetic, a c14 enzyme recovers about 1.28 ATP per NADPH from those six protons. A c13 enzyme recovers about 1.39. Carbon fixation wants 1.5. Neither geometry closes the gap alone. The smaller ring halves the shortfall relative to the vascular-plant reference, from roughly 0.22 to 0.11 ATP per NADPH, and so eases (without removing) the alga’s reliance on alternative electron routes that make extra ATP without net NADPH. The hardware change is a mechanistic answer to an ATP-expensive lifestyle in water, not a metaphor about efficiency.

The honest limit is sharper than the headline. A predicted H⁺/ATP from ring geometry is not a measured in vivo flux. The structure fixes how many proton-binding sites travel with one turn of the rotor. It does not, by itself, prove how many protons the living thylakoid spends per ATP under every light and metabolic regime. Stoichiometry sets the exchange rate the motor can offer. Physiology still has to show what rate the cell actually pays.

Life’s energy exchange rate is hardware. Count the c-subunits, and you count the proton price of ATP. Chlamydomonas counted thirteen.

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Sources

1. Lorencik et al., “A 13-subunit c-ring in the Chlamydomonas chloroplast ATP synthase lowers the H⁺/ATP cost of carbon fixation,” bioRxiv (2026-09-06), DOI 10.64898/2026.09.03.749062, version v1. EMDB entries cited include EMD-59504.
2. Spinach chloroplast c14 reference discussed in the preprint (and cited PDB 6FKF / prior c14 literature). Ratios 4.67 and 4.33 are the preprint’s geometry-predicted values (c/3), not independent in vivo measurements.