Is the Sodium Pump a Functional Dimer?

Abstract

It is presently unclear whether the membrane-embedded sodium pump works as an (αß) monomer (4) according to the single site model (15,32) or as an (αß)2 diprotomer of interacting α subunits (21–23,33). The single site model includes as an essential step the conversion of the low affinity ATP binding site (E2ATP site) to the high affinity ATP binding site (E1ATP site). This model implies that high and low affinity ATP binding sites cannot exist simultaneously, since they are two different forms of the same ATP binding site that can exist either in the E1ATP or the E2ATP conformational state. There are a number of data which are difficult to reconcile with the idea that the membrane-embedded sodium pump works according to the single site model as an (αß) monomer: (a) a negative cooperativity in ATP hydrolysis is seen (3,24) which may indicate an interaction of ATP binding sites; this is also seen under conditions when the catalytically active (αß) protomer of the sodium pump is existent as shown by active-enzyme analytical centrifugation of detergent solubilized Na+/K+-ATPase (37). (b) radiation inactivation reveals a bigger target size for the overall reaction than for partial reactions of the pump (7, 16, 17); (c) the enzyme shows the phenomenon of superphosphorylation at high concentrations of ATP (20); (d) simultaneous binding of TNP-ADP and phosphate to the FITC-modified enzyme have been recorded (27) (FITC binds to the high affinity ATP binding site in the E1ATP state); (e) Förster energy transfer measurements between the FITC-site and an ATP-protectable erythrosin isothiocyanate-site reveals that both sites are 5.5 nm apart (2); (f) Consistent with the idea that the onset of turnover of the sodium pump might cause (αß)protomers to form (αß)2 diprotomers is the observation that various concentrations of ATP, protein and phosphatidylserine affect the (αß) protomer ⇌ (αß)2 diprotomer equilibrium (14,10).KeywordsConformational StateTryptic DigestionSodium PumpNegative CooperativityChromium ComplexThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.