The calcium ion is engaged in a series of fundamental physiological processes from skeleton stabilization, cell division, and blood coagulation to muscle contraction and immune responses.
The concentration of Ca2+ inside the cell is extremely small, about 10-7 M, while the extracellular concentration is in the mM range. In order to sustain this huge concentration gradient very effective, specific calcium pumps are required. Calcium is taken up in the small intestine bound to the active form of vitamin-D, in a yield of 50%.
Proteins with many acidic groups are particularly effective ligands for calcium ions, and a well-known example is parvalbumin (Figure 10.7), a protein located in smooth muscles. This protein is related to the extended family of structurally flexible calmodulins, which are calcium-binding proteins that can bind to and regulate a multitude of different protein targets, thereby affecting many different cellular functions.
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