Protein phosphatases are enzymes that remove phosphate groups that have been attached to amino acid residues of proteins by protein kinases. Whereas a kinase enzymatically adds a phosphate to a protein, a phosphatase's purpose is phosphate removal. It should be noted that phosphate addition and removal do not necessarily correspond to enzyme activation or inhibition, and that several enzymes have separate phosphorylation sites for activating or inhibiting functional regulation. CDK, for example can be either activated or deactivated depending on the speicific amino acid residue being phosphorylated. The phosphates are important in signal transduction by regulating the proteins they are attached to. To reverse the regulatory effect, the phosphate has to be removed. This occurs on its own by hydrolysis or is mediated by protein phosphatases.

Serine/threonine-specific protein phosphatases

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Serine and threonine phosphates are stable under physiological conditions, so a phosphatase has to remove the phosphate to reverse the regulation. There are four known groups:

1. PP1 (α, β, γ1, γ2)
2. PP2A
3. PP2B (AKA calcineurin)
4. PP2C
5. PP4
6. PP5

The first three have sequence homology in the catalytic domain, but differ in substrate specifity.

Ser/Thr-specific protein phosphatases are regulated by their location within the cell and by specific inhibitor proteins.

Enzymes