Calcium P-Type ATPases

Some sarco(endo)plasmic reticulum (SR) Ca +-ATPases (SERCA) are P-type ATPases that play a major role in muscle contraction-relaxation cycles and are responsible for transporting calcium into the lumen of the sarcoplasmic reticulum. Some definitions are useful in the discussion of Ca +-ATPases  

Every eukaryotic cell has three parts a membrane with channels to let materials into and out of the cell, cytoplasm containing organelles and the cytosol, and a nucleus, containing the cell s chromosomes, or genetic material. The cytoplasm contains everything within the cell, except the nucleus. 

The endoplasmic reticulum (ER) is responsible for the production of the protein and lipid components of most of the cell s organelles. The ER contains a large number of folds, but the membrane forms a single sheet enclosing a single closed sac. This internal space is called the ER lumen. The smooth endoplasmic reticulum (ER) in muscle cells contains the vesicles and tubules that serve as a store of calcium ions. These are released as one step in the muscle contraction process. Calcium pumps, Ca +-ATPases, serve to move the calcium from the cytoplasm to the ER or SR lumen. 

The sarcoplasmic reticulum (SR) functions to uptake calcium from the sarcoplasm (the cytoplasm of muscle fiber). The sarcoplasmic reticulum 

During the cycle, large conformational changes take place within domains of the enzyme to make sure that calcium ions do not leak back into the cytoplasm but are occluded at binding sites within the SR membrane portion of the enzyme until such time that the enzyme s lumenal gate opens to release the ions to the lumen. 

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