Mechanism of allosteric effects of ATP on the kinetics of P-type ATPases

Abstract

The roles of allosteric effects of ATP and protein oligomerisation in the mechanisms of P-type ATPases belong to the most controversial and least well understood topics in the field. Recent crystal structural and kinetic data, however, now allow certain hypotheses to be definitely excluded and consistent hypotheses to be developed. The aim of this review is to critically discuss recent results and, in the light of them, to present a set of conclusions which could form the basis of future research. The major conclusions are: (1) at saturating ATP concentrations P-type ATPases function as monomeric enzymes, (2) the catalytic units of P-type ATPases only possess a single ATP binding site, (3) at non-saturating ATP concentrations P-type ATPases exist as diprotomeric (or higher oligomeric) complexes, (4) protein-protein interactions within a diprotomeric complex enhances the enzymes' ATP binding affinity, (5) ATP binding to both protomers within a diprotomeric complex causes it to dissociate into two separate monomers. The physiological role of protein-protein interactions within a diprotomer may be to enhance ATP binding affinity so as to scavenge ATP and maximize the ion pumping rate under hypoxic or anoxic conditions. For the first time a structural basis for the well-known ATP allosteric acceleration of the E2 --> E1 transition is presented. This is considered to be due to a minimization of steric hindrance between neighbouring protomers because of the ability of ATP to induce a compact conformation of the enzymes' cytoplasmic domains.

Comparative Properties of Caveolar and Noncaveolar Preparations of Kidney Na+/K+-ATPase

“…31−33 Hence, we suggest the need for more rigorous investigations, e.g., those using transient kinetics, on the potential differences between the turnover cycles of the caveolar and noncaveolar preparations, and the possible physiological implications of such differences. 33,34 …”

Hypothesized diprotomeric enzyme complex supported by stochastic modelling of palytoxin-induced Na/K pump channels

“…This justifies the linear dependence of the activation rate of the induced current on [PTX] o [4]. Therefore, only one of the channels must be induced, even when two PTX molecules are bound to the diprotomer [15]. …”

“…Therefore, to reproduce and investigate the induced current dynamics in single channels, we propose a model that describes the PTX effect on the pump acting as a diprotomer (figure 4). In the model, considering only Na + as the physiologic ligand, the pump substates and reactions were based on reported descriptions [15]. The following substates were assumed: (i) E1:E1—both enzymes with the intracellular gates opened; (ii) E2:E2—both enzymes with the extracellular gate opened; (iii) $\mathrm{N}{{\mathrm{normala}}^{+}}_3\mathrm{E}1:\mathrm{N}{{\mathrm{normala}}^{+}}_3\mathrm{E}1$—three Na + present in the intracellular face of each enzyme; (iv) ${(\mathrm{normalN}{\mathrm{normala}}^{+})}_2\mathrm{E}2:false(\mathrm{normalN}{\mathrm{normala}}^{+}{)}_2\mathrm{E}2$—two Na + occlusions in each enzyme.…”

“…There is evidence that the Na/K pump should function as an oligomeric complex [15–17]. In the diprotomer complex ( αβ ) 2 , a cooperative positive feedback between the proteins increases the affinity for the binding of the first ATP molecule.…”

“…Moreover, the second ATP molecule binding to the diprotomer exhibits a lower affinity (approx. 40 times less intense), causing the dissociation of the diprotomer into individual monomers and increasing the phosphorylation rate of the Na + /K + -ATPase [15]. …”