P-type ATPase - Mechanism

Mechanism

All P-type ATPases use the energy derived from ATP to drive vectorial transport. They form a high-energy aspartyl-phosphoranhydride intermediate in the reaction cycle, and they interconvert between at least two different conformations, denoted by E1 and E2. The E1-E2 notation stems from the initial studies on this family of enzymes made on the Na+,K+-ATPase, where the sodium form and the potassium form are referred to as E1 and E2, respectively, in the "Post-Albers scheme".

The E1-E2 schema has been proven to work, but there exist more than two major conformational states. However, the E1-E2 notation highlights the selectivity of the enzyme. In E1, the pump has high affinity for the exported substrate and low affinity for the imported substrate. In E2, it has low affinity of the exported substrate and high affinity for the imported substrate.

Four major enzyme states form the cornerstones in the reaction cycle. Several additional reaction intermediates occur interposed. These are termed E1~P, E2P, E2-P*, and E1/E2, described below.

In the case of SERCA1a, energy from ATP is used to transport 2 Ca2+-ions from the cytoplasmic side to the lumen of the sarcoplasmatic reticulum, and to countertransport 1-3 protons into the cytoplasm.

Starting in the E1/E2 state, the reaction cycle begins as the enzyme releases 1-3 protons from the cation-ligating residues, in exchange for cytoplasmic Ca2+-ions. This leads to assembly of the phosphorylation site between the ATP-bound N domain and the P domain, while the A domain directs the occlusion of the bound Ca2+. In this occluded state, the Ca2+ ions are buried in a proteinaceous environment with no access to either side of the membrane.

The Ca₂E1~P state becomes formed through a kinase reaction, where the P domain becomes phosphorylated, producing ADP. The cleavage of the β,-phosphordiester bond releases the gamma-phosphate from ADP and unleashes the N domain from the P domain.

This then allows the A domain to rotate towards the phosphorylation site, making a firm association with both the P and the N domain. This movement of the A domain exerts a downward push on M3-M4 and a drag on M1-M2, forcing the pump to open at the luminal side and forming the E2P state. During this transition, the transmembrane Ca2+-binding residues are forces apart, destroying the high-affinity binding site. This is in agreement with the general model form substrate translocation (cf. 1.2), showing that energy in primary transport is not used to bind the substrate but to release it again from the buried counter ions. At the same time the N domain becomes exposed to the cytosol, ready for ATP exchange at the nucleotide-binding site.

As the Ca2+ dissociate to the luminal side, the cation binding sites are neutralised by proton binding, and this make a closure of the transmembrane segments favourable. This closure is coupled to a downwards rotation of the A domain and a movement of the P domain, which then leads to the E2-P* occluded state. Meanwhile, the N domain exchanges ADP for ATP.

The P domain is dephosphorylated by the A domain, and the cycle completes when the phosphate is released from the enzyme, stimulated by the newly bound ATP, while a cytoplasmic pathway opens to exchange the protons for two new Ca2+-ions.