The crustacean gill (Na +, K +)-ATPase: Allosteric modulation of high- and low-affinity ATP-binding sites by sodium and potassium

Abstract

The blue crab, Callinectes danae, tolerates exposure to a wide salinity range employing mechanisms of compensatory ion uptake when in dilute media. Although the gill (Na +, K +)-ATPase is vital to hyperosmoregulatory ability, the interactions occurring at the sites of ATP binding on the molecule itself are unknown. Here, we investigate the modulation by Na + and K + of homotropic interactions between the ATP-binding sites, and of phosphoenzyme formation of the (Na +, K +)-ATPase from the posterior gills of this euryhaline crab. The contribution of the high- and low-affinity ATP-binding sites to maximum velocity was similar for both Na + and K +. However, in contrast to Na +, a threshold K + concentration triggers the appearance of the high-affinity binding sites, displacing the saturation curve to lower ATP concentrations. Further, a low-affinity site for phosphorylation is present on the enzyme. These findings reveal notable differences in the catalytic mechanism of the crustacean (Na +, K +)-ATPase compared to the vertebrate enzyme.