The Journal of Biological Physics and Chemistry

2001 , September, Volume 1, Issue 1, p.12-18

Z. Kometiani and M. Leladze
I. Beritashvili Institute of Physiology, Georgian Academy of Sciences, 14 Gotua St., 380060 Tbilisi, Georgia

Na+,K+-activation of the Mg2+-dependent cycle of Na,K-ATPase

When the Mg2+/ATPfree ratio is high, in the molecular mechanism of the Na,K-ATPase system the Mg-dependent cycle is created, the most peculiar trait of which is dephosphorylation of the Mg-bound phosphoenzyme. Under these conditions, with the aid of a specially designed method (analysis of the geometrical forms of kinetic curves), the dependence of the number of essential Na+-, and K+-activators on the concentration of the Na+, K+ and Mg2+ ions, and the MgATP-complex, was investigated. At relatively low [Na+], an increase of [K+] elicits an increase of the number of essential Na+-activators – from 3 to 4. At [K+] = 0, the ouabain-sensitive ATPase system does not equal zero and K ions become the activators as modifiers with partial effect. The activation is a result of the binding of one or two K ions. In the regime of Mg-dependent dephosphorylation the apparent activation constant of Na ions does not depend on the MgATP and Mg2+ concentrations. On the basis of these experimental facts the principal scheme of the Mg-dependent Na,K-ATPase cycle is deduced, in which instead of the Na+:K+ transport stoichiometry being 3:2 we can have the ratios 3:0, 3:1 or 3:2. Therefore, certain cation-transporting sites become the non-transporting regulatory ones.

back