Elusive Intermediate State Key in the Conversion of ATP Hydrolysis into Useful Work Driving the Ca²⁺ Pump SERCA

A key event in the ATP-driven transport cycle of the calcium pump sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) occurs when autophosphorylation of the pump with two bound ions Ca²⁺ triggers a large conformational change that opens a gate on the luminal side of the membrane allowing the release of the ions. It is believed that this conformational transition proceeds through a two-step mechanism, with an initial rearrangement of the three cytoplasmic domains of the pump responsible for ATP binding and hydrolysis followed by the opening of the gate toward the luminal side in the transmembrane region. Here, molecular dynamics computation of the free energy landscapes associated with this transition show how, in response to phosphorylation, the cytoplasmic domains are partially reconfigured into an intermediate state on the path toward the E2 state with a closed luminal gate. It is suggested that the free energy associated with this conformational reorganization must subsequently be used to drive the opening of the gate on the luminal side.