ATP駆動ポンプII：P型ポンプ

P-type pumps contain three cytoplasmic domains: the nucleotide-binding (N) domain where ATP binds, the phosphorylation (P) domain where an aspartic acid residue is reversibly phosphorylated, and the actuator (A) domain that allows association of the N and P domains. These domains work together to enable proper pump functioning and solute translocation across the membrane.

The SERCA pump binds two calcium ions from the cytosol at calcium-binding sites within its membrane-spanning domain. ATP hydrolysis provides energy, triggering a conformational change that opens a passageway to the sarcoplasmic reticulum lumen and releases the calcium ions. Hydrogen ions then bind to the empty sites, and the pump returns to its initial conformation, completing the cycle.

Calcium pumps maintain cytoplasmic calcium concentration roughly 10,000 times lower than extracellular concentration, which is essential for cell signaling and muscle function. Failure to maintain this concentration gradient is one of the causes of muscle cramps. Calcium pumps account for about 80% of sarcoplasmic reticulum membrane protein in skeletal muscles, highlighting their critical role.

During the catalytic cycle, inorganic phosphate attaches to the P-domain after ATP hydrolysis, forming a covalent phosphoenzyme intermediate. This phosphorylation at a highly conserved aspartic acid residue leads to conformational changes in the pump's transmembrane segments, enabling the pump to open its passageway and translocate solutes across the membrane.

Calcium pumps are found in multiple cellular locations. SERCA pumps are located on the sarcoplasmic and endoplasmic reticulum membranes in skeletal and heart muscles. PMCA pumps are expressed on the plasma membrane in various tissues including the brain. SPCA pumps are found on Golgi membranes. All these pumps are P-type ATPases that regulate intracellular calcium concentration.

The SERCA pump hydrolyzes one ATP molecule to transport two calcium ions into the sarcoplasmic reticulum against their concentration gradient. This stoichiometry of 2:1 (calcium to ATP) makes the pump highly efficient at maintaining the steep calcium concentration gradient necessary for primary active transport and cellular homeostasis.

The three cytoplasmic domains—nucleotide-binding, phosphorylation, and actuator domains—are connected to the membrane-spanning helices by short amino acid segments. This structural arrangement allows the domains to coordinate ATP hydrolysis and covalent phosphoenzyme intermediate formation, which are crucial parts of the catalytic cycle for solute translocation.