In Na+-K+ ATPase, K+ binds onto

The carrier protein is a complex of two separate globular proteins--a larger one called the a subunit, with a molecular weight of about 100,000, and asmaller one called the b subunit, with a molecular weight of about 55,000. The function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are impoant for the functioning of the pump:1. It has three binding sites for sodium ions on the poion of the protein that protrudes to the inside of the cell.2. It has two binding sites for potassium ions on the outside.3. The inside poion of this protein near the sodium binding sites has adenosine triphosphatase (ATPase) activity.When two potassium ions bind to the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is then believed to cause a chemical and conformational change in the protein carrier molecule, extruding the three sodium ions to the outside and the two potassium ions to the inside.Ref: Guyton; 13th edition; Chapter 4; Transpo of Substances Through Cell Membranes