Na+,k+-pump: kinetics and electrogenicity of transport cycle partial reaction

A. M. Okan

Na+,K+-pump, one of the most important transport systems of cell was considered. А model of Na+, K+-ATPase functioning as a channel with gates, that are opening and closing during phosphorylation and dephosphorylation was demonstrated. ? і ? subunits of Na+, K+-ATPase isoforms were considered in brief. The basic attention was given to kinetics of the transport cycle partial reaction. The contribution of partial reaction to electrogenicity of the Na+, K+-ATPase transport cycle was demonstrated. It was analyzed the rate limitation of the pump cycle on the separate stages. It was demonstrated, that (Na3) E1-P ? P-E2 (Na2) transition is the slowest step of the (Na3)E1-P?P-E2(Na3)?P-E2(Na2)?P-E2(Na)?P-E2 reaction sequence. In the research with use of fluorescent label and mathematical simulation was demonstrated that the major rate limitation step of whole Na+, K+-pump work cycle is E2 ? E1 transition. The next steps electrogenisity in the Na+, K+-ATPase transport cycle was demonstrated: cytoplasmic binding and extracellular release of Na+ ions, K+ ions extracellular binding.Na+ binding to the highly specific uncharged site is electrogenic (with dielectric coefficients ~ 0,25). ATP binding, enzyme phosphorylation, and ion occlusion are electroneutral steps.The subsequent conformational change is slow and of low electrogenicity (~ 0,1) The major electrogenic process is the fast release of the first Na+ ion to the extracellular side (dielectric coefficients consist 0,65-0,75), most probably from the neutral site. The other two Na+ ions are also released with high rate constant, although with small dielectric coefficients (about 0,1 for every ion). Extracellular binding of K+ ions is electrogenic with dielectric coefficients comparable to that of binding of second and third Na+ ion. The following reaction steps, including release of K+ ions to intracellular side, are electro neutral.

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