Calcium is a cofactor for a limited number of important enzymes apart from the more familiar actin-myosin complex in muscle; alpha amylase and thermolysin are two of the most familiar. As a free ion or working through calmodulin, calcium is better understood as an activator of enzymes in hormone-dependent cell signaling pathways. Enzymes that have been referred to as Ca-ATPases and H+/Ca-ATPases should not be mistaken for calcium-dependent enzymes. This is a misnomer in that the Ca2+ is the object of the enzyme's action rather than the cofactor for activity. The ATPases comprise a large group of membrane-bound enzymes that either pump Ca2+ from the cytosol into the endoplasmic reticulum or expel calcium from the cell through membrane channels.
Reactivity As a group IIa metal, calcium is limited to a 2+ valence state and serves primarily as a divalent cation in its interactions with enzymes. The role of Ca2+ is limited mainly to structure stability.
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