12 - The Sodium Pump

Abstract

This chapter reviews that a sodium pump is required to maintain the chemical and electrical gradients for ions across the cell membrane. This Na + pump extrudes three Na + ions and accumulates two K + ions against their respective chemical gradients at the expense of one adenosine triphosphate (ATP) molecule. The enzyme that carries out this transport process is Na + , K + -ATPase. The Na + , K + -ATPase enzyme requires the presence of both Na + and K + for optimal activity. The Na + , K + -ATPase belongs to a specialized class of enzymes called the P-type ATPases, all of which form an acid stable phosphoenzyme complex. Thus, in the presence of Na+ and MgATP, the Na + , K + -ATPase is phosphorylated. The addition of K + causes dephosphorylation. Cardiac glycosides, such as ouabain inhibit ion transport by binding to the Na + pump on the extracellular side of the cell membrane, therefore, increasing the internal Na + ion concentration. The chapter also discusses that Na + , K + -ATPase is composed of two subunits in a one-to-one stoichiometry: the α or catalytic subunit, and the β or glycoprotein subunit.