H⁺-ATPase Proton Pump

5KSD

Focht et al., 2017 (PDB 5KSD); Post-Albers / Albers-Post cycle.

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Reaction cycle

1E1 · Ground state — ATP binds the N-domain
2E1P · Asp329 phosphorylated; a proton is taken up from the cytoplasm
3E2P · The A-domain rotates — the proton is released to the cell exterior
4E2 → E1 · Dephosphorylation of Asp329 resets the pump

Moving parts

Transmembrane domain (10 helices)
P + A domains (actuator)
N-domain (ATP binding)
H⁺transported

How it works

The plant plasma-membrane H⁺-ATPase (AHA2) is a P-type ATPase that pumps protons out of the cell. Its cytoplasmic motor cycles through the Post-Albers scheme: the N-domain binds ATP and phosphorylates Asp329 (E1→E1P); the actuator (A) domain then rotates, switching the transmembrane helices from inward- to outward-facing (E1P→E2P) and releasing the proton to the cell exterior; the A-domain finally dephosphorylates Asp329, resetting the pump (E2P→E2→E1). Here those domain motions are reconstructed as a loop, with a proton crossing the bilayer each cycle.