A phosphatase is an enzyme that hydrolyses phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl group. This action is directly opposite to that of phosphorylases and kinases, which attach phosphate groups to their substrates by using energetic molecules like ATP. A common phosphatase in the body is alkaline phosphatase.

The presence or absence of the phosphate group on proteins, especially enzymes, is known to play a regulatory role in many biochemical pathways and signal transduction pathways. Hence together, specialized kinases and phosphatases regulate enzymatic activity.

Phosphatases can be categorised into two main categories: metalloenzymes (which are dependent on the presence of two or more metal ions in their active sites for activity), and non-metalloenzymes. These categories can then be divided into further sub-categories.

Best known of the non-metalloenzymes are the protein tyrosine phosphatases, which hydrolyse phospho-tyrosine residues. However, the metalloenzymes by far comprise the greatest bulk of phosphatases, and contain such enzymes as alkaline phosphatase (three metal ions, only two of which are catalytically active), the serine threonine phosphatases and inositol monophosphatase (a key enzyme in manic depression).


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