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.

A kinetic comparison between E2P and the E2P-like state induced by a beryllium fluoride complex in the Na,K-ATPase. Interactions with Rb.

Metal-fluoride complexes have been used to induce E2P-like states with the aim of studying the events that occur during E2P hydrolysis in P-type ATPases. In the present work, we compared the E2P-like state induced by a beryllium fluoride complex (BeFx) with the actual E2P state formed through backdoor phosphorylation of the Na,K-ATPase. Formation of E2P and E2P-like states were investigated employing the styryl dye RH421. We found that BeFx is the only fluorinated phosphate analog that, like Pi, increases the RH421 fluorescence. The observed rate constant, kobs, for the formation of E2P decreases with [Pi] whereas that of E2BeFx increases with [BeFx]. This might wrongly be taken as evidence of a mechanism where the binding of BeFx induces a conformational transition. Here, we rather propose that, like for Pi, binding of BeFx follows a conformational-selection mechanism, i.e. it binds to the E2 conformer forming a complex that is much more stable than E2P, as seen from its impaired capacity to return to E1 upon addition of Na+. Although E2P and E2BeFx are able to form states with 2 occluded Rb+, both enzyme complexes differ in that the affinity for the binding and occlusion of the second Rb+ is much lower in E2BeFx than in E2P. The higher rates of Rb+ occlusion and deocclusion observed for E2BeFx, as compared to those observed for other E2P-like transition and product states suggest a more open access to the cation transport sites, supporting the idea that E2BeFx mimics the E2P ground state.

The E2P-like state induced by magnesium fluoride complexes in the Na,K-ATPase. Kinetics of formation and interaction with Rb(+).

The first X-ray crystal structures of the Na,K-ATPase were obtained in the presence of magnesium and fluoride as E2(K2)Mg-MgF4, an E2∙Pi-like state capable to occlude K(+) (or Rb(+)). This work presents a functional characterization of the crystallized form of the enzyme and proposes a model to explain the interaction between magnesium, fluoride and Rb(+) with the Na,K-ATPase. We studied the effect of magnesium and magnesium fluoride complexes on the E1-E2 conformational transition and the kinetics of Rb(+) exchange between the medium and the E2(Rb2)Mg-MgF4 state. Our results show that both in the absence and in the presence of Rb(+), simultaneous addition of magnesium and fluoride stabilizes the Na,K-ATPase in an E2 conformation, presumably the E2Mg-MgF4 complex, that is unable to shift to E1 upon addition of Na(+). The time course of conformational change suggests the action of fluoride and magnesium at different steps of the E2Mg-MgF4 formation. Increasing concentrations of fluoride revert along a sigmoid curve the drop in the level of occluded Rb(+) caused by Mg(2+). Na(+)-induced release of Rb(+) from E2(Rb2)Mg-MgF4 occurs at the same rate as from E2(Rb2) but is insensitive to ADP. The rate of Rb(+) occlusion into the E2Mg-MgF4 state is 5-8 times lower than that described for the E2Mg-vanadate complex. Since the E2Mg-MgF4 and E2Mg-vanadate complexes represent different intermediates in the E2-P→E2 dephosphorylation sequence, the variation in occlusion rate could provide a tool to discriminate between these intermediates.