Ion Pumping by Calcium ATPase of Sarcoplasmic Reticulum

Abstract

Ca²⁺-ATPase of skeletal muscle sarcoplasmic reticulum (SERCA1a) is an integral membrane protein of 110K and the best characterised member of the P-type (or E1/E2-type) ion translocating ATPases. It was first identified by Ebashi in the “relaxing factor” of muscle contraction and gave rise to the calcium theory that Ca²⁺ is a fundamental and ubiquitous factor in the regulation of intracellular processes.1 There are several types of Ca²⁺-ATPases in different tissues; all transfer Ca²⁺ from the cytoplasm to the opposite side of the membrane and countertransport H⁺. Stoichiometry of Ca²⁺: ATP may be variable but it is well established that SERCA1a can transfer two Ca²⁺ per ATP hydrolysed.2 In the sarcoplasmic reticulum (SR) membrane, Ca²⁺-ATPase pumps Ca²⁺, released into muscle cells during muscle contraction, back into SR, thereby relaxes muscle cells. This pump runs as long as ATP and Ca²⁺ are present, and establishes more than 10⁴-fold concentration gradient across membranes. According to the classical E1/E2 theory, transmembrane Ca²⁺-binding sites have high affinity and face the cytoplasm in E1; in E2, the binding sites have low affinity and face the lumen of SR (extracellular side).2,3 Actual transfer of bound Ca²⁺ is thought to take place between two phosphorylated intermediates, E1P and E2P, in exchange of H⁺. Because 2 Ca²⁺ are transferred in the forward direction and 2 to 3 protons in the opposite direction, active transport of Ca²⁺ is an electrogenic process. Although no H⁺ gradient is built up across the SR membrane because it is leaky to H⁺, this Ca²⁺/H⁺ exchange may cause pathological pH effects with plasma membrane Ca²⁺-ATPase.4