Structure and mechanism of the (Na+, K+)-and (Ca2+)-ATPases

Abstract

The (Na+> and K+)-stimulated adenosine triphosphatase ((Na+, K+)-ATPase) and the (Ca2+)-stimulated adenosine triphosphatase ((Ca2+)-ATPase) are intrinsic membrane proteins present in eukaryotic cells whose functions are to couple the active transport of either Na+ and K+ or Ca2+ across the membrane to ATP hydrolysis. The (Na+, K+)-ATPase is present in the plasma membrane of essentially all animal cells and is responsible for the maintenance of Na+ and K+ gradients. The Na+ gradient is utilised in the generation of the action potential in nerve cells (Katz, 1966), in the uptake of sugars and amino acids by intestinal epithelia and renal tubules (Crane, 1965, Endou et al., 1975), in the readsorption of Na+ by the kidney (Jorgensen, P. L., 1980) and to assist in Ca2+> and proton efflux from the cell (Miyamato and Racker, 1980; Rindler and Saier, 1981). The K+ gradient (and K+ permeability) establish an internally negative membrane potential (Katz, 1966) and a high cytoplasmic K+ concentration is required for various enzymatic reactions (Evans and Sorges, 1966). The (Ca2+)-ATPase is also present in the eukaryotic plasma membrane (Schatzmann and Vincenzi, 1969) as well as various intracellular membranes, such as the sarcoplasmic reticulum of muscle (Hasselbach and Makinose, 1961), and serves to maintain the cytoplasmic Ca2+ concentration at a low level. This is essential to the process of muscle contraction, which is activated by an influx of Ca2+ into the cytoplasm (Mannherz and Goody, 1976).