title: Sarcoplasmic reticulum calcium pump: a model for Ca2+ binding and Ca2+-coupled phosphorylation.
abstract: The conventional alternating access model for Ca2+ transport by the sarcoplasmic reticulum Ca2+ pump is modified, partly on the basis of the proposed MacLennan-Green domain structure for the Ca2+-pump protein. The present model divides the uptake state (E1) of the protein into three substates, differing in the condition of the Ca2+-binding domain. The domain is an open cavity in the first substate and can bind only a single Ca2+ ion. A fast "jaw-closing" (or "hinge-bending") step then partially closes the cavity to generate the second substate that has a second Ca2+-binding site. Occupation of this site is followed by another jaw-closing step that closes the binding cavity and occludes the bound ions. The subsequent translocation step (to form E2) remains unchanged from previous models. The modified model predicts a constant transport stoichiometry of two Ca2+ per pump reaction cycle. It suggests a plausible mechanism for coupling between Ca2+ binding and ATP utilization: the model predicts (in agreement with experiment) that Ca2+ binding should be a mandatory requirement for phosphorylation of the pump protein, though ATP binding per se does not require Ca2+. The model is consistent with high cooperativity in equilibrium binding of Ca2+, both in the absence and presence of ATP.
mesh_terms: D001665:Binding Sites; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D007700:Kinetics; D008954:Models, Biological; D010766:Phosphorylation; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: D000252:Calcium-Transporting ATPases; D002118:Calcium