Measurements of Na(+)-occluded intermediates during the catalytic cycle of the Na(+)/K(+)-ATPase provide novel insights into the mechanism of Na(+) transport

The Na(+)/K(+)-ATPase is an integral plasma membrane glycoprotein of all animal cells that couples the exchange of intracellular Na(+) for extracellular K(+) to the hydrolysis of ATP. The asymmetric distribution of Na(+) and K(+) is essential for cellular life and constitutes the physical basis of a series of fundamental biological phenomena. The pumping mechanism is explained by the Albers–Post model. It involves the presence of gates alternatively exposing Na(+)/K(+)-ATPase transport sites to the intracellular and extracellular sides and includes occluded states in which both gates are simultaneously closed. Unlike for K(+), information is lacking about Na(+)-occluded intermediates, as occluded Na(+) was only detected in states incapable of performing a catalytic cycle, including two Na(+)-containing crystallographic structures. The current knowledge is that intracellular Na(+) must bind to the transport sites and become occluded upon phosphorylation by ATP to be transported to the extracellular medium. Here, taking advantage of epigallocatechin-3-gallate to instantaneously stabilize native Na(+)-occluded intermediates, we isolated species with tightly bound Na(+) in an enzyme able to perform a catalytic cycle, consistent with a genuine occluded state. We found that Na(+) becomes spontaneously occluded in the E1 dephosphorylated form of the Na(+)/K(+)-ATPase, exhibiting positive interactions between binding sites. In fact, the addition of ATP does not produce an increase in Na(+) occlusion as it would have been expected; on the contrary, occluded Na(+) transiently decreases, whereas ATP lasts. These results reveal new properties of E1 intermediates of the Albers–Post model for explaining the Na(+) transport pathway.