der Johann Wolfgang Goethe – Universität in Frankfurt am Main von

The Na +,K +-ATPase was discovered more than 50 years ago, but even today the pumpcycle and its partial reactions are still not completely understood. In this thesis, Voltage Clamp Fluorometry was used to monitor the conformational changes that are associated with several electrogenic partial reactions of the Na +,K +-ATPase. The conformational dynamics of the ion pump were analyzed at different concentrations of internal Na + or of external K + and the influences on the conformational equilibrium were determined. To probe the effect of the internal Na + concentration on the Na + branch of the ion pump, oocytes were first depleted of internal Na + and then loaded with Na + using the epithelial sodium channel which can be blocked by amiloride. The conformational dynamics of the K + branch were studied using different external K + concentrations in the presence and in the absence of external Na + to yield additional information on the apparent affinity of K +. The results of our Voltage Clamp Fluorometry experiments demonstrate that lowering the intracellular concentration of Na + has a comparable effect on the conformational equilibrium as increasing the amount of K + in the external solution. Both of these change