The molecular size required varies according to the reaction step round the sodium pump cycle

Abstract

Progress along the path of the sodium pump cycle requires a stepwise recruitment of additional subunits for maximal activity. These results show that whereas a particle the size of the c$ protomer presents Na+,K+-ATPase activity at 10 PM ATP, an additional subunit, perhaps a second a-chain, is required to obtain the much greater Na+,K+-ATPase activity resulting from the occupation of low-affinity ATP sites at physiological ATP concentrations.A non-phosphorylating ATP analogue, however, will modestly stimulate the Na+,K+-ATPase activity acting at an alternative low-affinity site or step on the @ protomer.

Radiation inactivation analysis of sarcoplasmic reticulum Ca‐ATPase in membrane‐bound form and in detergent‐solubilized monomeric states

“…In contrast to the analogous experiments carried out with the Na,K-ATPase [26], there was no difference between the decay curves obtained for the two ATP concentrations with CaATPase. Therefore for Ca-ATPase, the enhance- In (o), incubation with CrATP and %a" was performed before freezing and irradiation, to measure the effect of irradiation on maintenance of the occluded state.…”

Binding of 2′(3′)-O-(2,4,6-Trinitrophenyl)ADP to Soluble αβ Protomers of Na,K-ATPase Modified with Fluorescein Isothiocyanate

“…The considerable low affinity stimulation of the overall cycle (K 0.5(low) approximately 150 -300 M) seems to result from accelerating a rate-limiting step in the E 2 form of the dephosphoenzyme (4,5). ADP (6), nonphosphorylating ATP analogues (7), and acyl coenzymes A (8) can replace ATP in this low affinity effect. For these reasons, this is regarded as a regulatory nucleotide effect, in contrast with the "catalytic" (high affinity) ATP action that results in enzyme phosphorylation in the presence of Na ϩ ions.…”

Photoinactivation of Fluorescein Isothiocyanate-modified Na,K-ATPase by 2′(3′)-O-(2,4,6-Trinitrophenyl)8-azidoadenosine 5′-Diphosphate

“…Micromolar concentrations of ATP are sufficient to achieve maximal steady-state phosphorylation of the enzyme (K 0.5 Ͻ 1 M) and yet, in the presence of K ϩ , higher ATP concentrations further stimulate ATP hydrolysis more than 20-fold (K 0.5 Ϸ 100 -500 M). This stimulation seems to arise, at least in part, from an acceleration of K ϩ release to the cytosol (5-7) and can also be elicited by nonhydrolyzing ATP analogues (8). Low affinity ATP effects are also observed with several partial reactions of the E2 state of the enzyme, including the K ϩ -activated phosphatase activity (4,9), phosphorylation by P i (10), release of bound ouabain (11), and K ϩ -K ϩ exchange (12); in the latter case, ADP is also effective (13).…”