ИСТИНА

According to recent data, transport of Na and K ions by the Na,K-ATPase includes their exchange competition with protons in the binding sites of the protein. This process can be studied by measuring the membrane admittance (capacitance and conductance). The small changes of the membrane admittance initiated by fast pH steps were measured in an experimental system consisting of membrane fragments with Na,K-ATPase attached to a bilayer lipid membrane (BLM). pH jumps were performed by release of protons from a photosensitive compound (caged H+) triggered by a UV light flash. The capacitance increments depended on initial pH and the concentration of sodium and potassium ions. The observed dependencies were explained by a model of competitive binding of Na or K ions and protons present on cytoplasmic side of Na,K-ATPase, assuming that the sites can bind either Na (or K) ions or protons [1,2]. Binding of Na or K ions was affected by Mg ions and ATP. The Mg ions decreased the apparent affinity for Na ions of the binding sites. In the case of K ions the presence of Mg ions essentially influenced the kinetics of the pH-driven change of the capacitance including fast and slow steps which probably is caused by a shift of the E1/E2 conformation equilibrium of the enzyme. In the presence of 0.5 mM of ATP the effects of Na ions on the pH-driven change of the capacitance were attenuated, which can be explained by a shift of the protein conformation to a state with low K-ion affinity initiated by binding of ATP to the low affinity site. Supported by RFBR project 16-04-01162 References 1. Tashkin, V.Yu. et al Biochemistry (Moscow), 7, 113–121, 2013 2. Tashkin, V.Yu. et al Biochemistry (Moscow), 9, 92–99, 2015