Calcium pumping ATPase enzyme. Computer model of the ATPase enzyme that pumps calcium in and out of muscle cells and controls muscle contractions. The name ATPase refers to this enzyme's ability to break up the ATP (adenosine triphosphate) molecule and release energy. This ATPase uses the energy to pump calcium ions in and out of muscle cells, and it is found in muscle cell membranes. Calcium ions are central to the process of muscle contraction. Enzymes are catalysts and so promote reactions and are not used up. They are also proteins. Proteins are long chains of amino acids. This protein has around 1000 amino acids and about 15,000 atoms.