ATPase

Definition
ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and a free phosphate ion.

Discovery
Na+/K+-ATPase was discovered by Danish biologist Jens Christian Skou in 19571.

Classification
There are different types of ATPases which differs in their functions, structure and in the type of ions they transport viz., F-ATPases, V-ATPases, A-ATPases, P-ATPases and E-ATPases. F-ATPases are found in mitochondria, chloroplasts and bacterial plasma membranes produce ATP, using the proton gradient generated by oxidative phosphorylation (mitochondria) or photosynthesis (chloroplasts). V-ATPases are found in eukaryotic vacuoles, catalyzing ATP hydrolysis to transport solutes and lower pH in organelles. A-ATPases are found in Archaea and their functionality is similar to F-ATPases. P-ATPases are found in bacteria, fungi and in eukaryotic plasma membranes and organelles, and function to transport a variety of different ions across membranes. E-ATPases are cell-surface enzymes that hydrolyse a range of NTPs, viz., extracellular ATP.

Structural characteristics

ATPases are integral membrane proteins and move solutes across the membrane. Na+-K+-ATPase is composed of two subunits. The alpha subunit (~113 kDa) binds ATP and both sodium and potassium ions, and contains the phosphorylation site. The smaller beta subunit (~35 kDa glycoprotein) is necessary for activity of the complex. It appears to be critical in facilitating the plasma membrane localization and activation of the alpha subunit2.

Mode of action

The sodium-potassium exchanger (Na+/K+ATPase) enzymatic activity is initiated by binding to ATP and three intracellular Na+ ions. ATP is hydrolyzed, leading to phosphorylation of a cytoplasmic loop of the pump and release of ADP. A conformational change in the pump exposes the Na+ ions to the outside, where they are released. The pump binds two extracellular K+ ions, leading to dephosphorylation of the alpha subunit. ATP binds and the pump reorients to release K+ ions inside the cell, which results in electrical gradient across the cell membrane.

Functions

ATPases import many of the metabolites like glucose, amino acids and other nutrients into the cell necessary for cell metabolism and export toxins, wastes, and other solutes that can hinder cellular processes. The ionic transport conducted by sodium pumps (Na+/K+ATPase) creates both an electrical and chemical gradient across the plasma membrane. It helps in electrolyte movement across epithelial cells. The cell's resting membrane potential is a manifestation of the electrical gradient, and the gradient is the basis for excitability in nerve and muscle cells. The hydrogen potassium ATPase (H+/K+ATPase or gastric proton pump) acidifies the contents in the stomach3.

References

1.Boldyrev AA (2000). Na+,K+-ATPase: 40 years of investigations. Membr Cell Biol., 13(6):715-719.
2.Lingrel JB, Kuntzweiler T (1994). Na+, K+-ATPase. J Biol Chem., 269:19659.
3.Schubert, Mitchell L (2007). Gastric secretion. Current Opinion in Gastroenterology., 23(6) 595-601.