ABSTRACT
Ions of sodium and potassium represent the cationic electrolytes of living organisms. Of the two ions, K+ has the greater role in biology [1]. The cytosol in cells from animals, plants, and bacteria contains high concentrations of K+—typically >0.1 M. Plants require potassium for growth, and potassium is one of the three major inorganic ingredients of commercial fertilizers. On the other hand, plants and most microbes exhibit little or no requirement for sodium. However, sodium does have important roles in animals, where it is usually found in highest concentrations in extracellular fluids. In animals, an ATP-dependent extrusion or pumping of Na+ from cells is coupled to the inward movement of K+. A substantial amount of ATP utiliza tion by animal cells is devoted to operation of the Na+/K+ ATPases, which are trans membrane proteins that are responsible for this ion pumping [2,3]. At the same time, the gradient of [Na+] created by the active pumping facilitates the inward movement of sugars and amino acids by sodium-dependent cotransporters [4]. The active pump ing of K+ into and Na+ out of cells occurs against a back-diffusion of the ions down their concentration gradients. The structure and function of channel proteins that control diffusion of the ions across biological membranes is a topic in Chapter 3 of this volume. The roles of K+ and Na+ as electrolytes in numerous physiological processes are of critical importance in many aspects of human health and, therefore, medicine [5]. One expects Na+ and K+ to bind nonspecifically to proteins, nucleic acids, negatively charged head groups of phospholipids, and anionic metabolites. However, in addition to the Na+/K+ ATPases and other membrane proteins involved in ion transport, various other enzymes and proteins exhibit specific requirements for K+ or Na+ [6]. This chapter deals with structural and functional aspects of the specific interactions of Na+ and K+ with proteins.
