Structural dynamics of the Ca²⁺-ATPase studied by time-resolved infrared spectroscopy

Abstract

This review discusses the contribution of time-resolved infrared spectroscopy to the understanding of the Ca²⁺ pump in the sarcoplasmic reticulum membrane of skeletal muscle cells (SERCA1a). The focus is on interactions of the substrate ATP with the ATPase and on the bond parameters of the phosphoenzyme phosphate group. Functional groups throughout the ATP molecule are important for stabilising the closed conformation of the ATP–ATPase complex and for fast phosphorylation of the ATPase. Dissociation of the reaction product ADP after phosphorylation leads to a more open average conformation of the enzyme and does not trigger the transition from the first phosphoenzyme Ca₂E1P to the second E2P. The P–O bond between phosphate and aspartyl moieties is weaker in Ca₂E1P and E2P than in acetyl phosphate in aqueous solution, which explains the high reactivity of the phosphoenzymes. This ground state property of the phosphoenzymes prepares for a phosphate transfer reaction with dissociative character.