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 αβ protomer presents Na⁺,K⁺-ATPase activity at 10 μM ATP, an additional subunit, perhaps a second α-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 αβ protomer.