Calcium Pump (SERCA)

1SU4

Toyoshima et al., 2000 (PDB 1SU4) — sarcoplasmic-reticulum Ca²⁺-ATPase, E1·2Ca²⁺.

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Reaction cycle

1E1·2Ca · Two Ca²⁺ bind from the cytoplasm; ATP binds the N-domain
2E1P · Asp351 phosphorylated; the calcium sites are occluded
3E2P · The A-domain rotates — Ca²⁺ is released into the SR lumen
4E2 → E1 · Dephosphorylation of Asp351 resets the pump

Moving parts

Transmembrane domain (10 helices)
P + A domains (actuator)
N-domain (ATP binding)
Ca²⁺transported

How it works

SERCA is the calcium pump that relaxes muscle — and the P-type ATPase whose structures first revealed how this whole family moves. It is a close cousin of the proton pump: the same actuator (A), nucleotide (N) and phosphorylation (P) domains run the same Post-Albers cycle. The N-domain phosphorylates Asp351 (E1→E1P), the A-domain then rotates to switch the membrane sites from inward- to outward-facing (E1P→E2P), and two Ca²⁺ ions are pumped from the cytoplasm into the sarcoplasmic-reticulum lumen each cycle before dephosphorylation resets the pump.