Cation binding to SERCA
Sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) is a transmembrane pump that plays an important role in transporting calcium into the sarcoplasmic reticulum (SR). While calcium (Ca2+) binds SERCA with micro-molar affinity, magnesium (Mg2+) and potassium (K+) also compete with calcium (Ca2+) binding. However, the molecular bases for these competing ions influence on SERCA function and the selectivity of the pump for Ca2+ are not well-established. We therefore used in silico methods to resolve molecular determinants of cation binding in the canonical site I and II Ca2+ binding sites: 1) triplicate molecular dynamics (MD) simulations of Mg2+, Ca2+and K+-bound SERCA. 2) mean spherical approximation (MSA) theory to determine the affinity and selectivity of cation binding to the MD-resolved structures and 3) state models of SERCA turnover informed from MSA-derived affinity data. Our key findings are that a) coordination at sites I and II are optimized for Ca2+and to a lesser extent for Mg2+ and K+, as determined by MD-derived cation-amino acid oxygen and bound water configurations, b) the impaired coordination and high desolvation cost for Mg2+ precludes favorable Mg2+ binding relative to Ca2+, while K+ has limited capacity to bind site I, c) Mg2+ most likely acts as inhibitor and K+ as intermediate in SERCAs reaction cycle, based on a best-fit state model of SERCA turnover. These findings provide a quantitative basis for SERCA function that leverages molecular-scale thermodynamic data and rationalize enzyme activity across broad ranges of K+, Ca2+ and Mg2+ concentrations.