Phosphatases are enzymes capable of dephosphorylating a corresponding substrate by removing a phosphate group on a substrate molecule by hydrolyzing a phosphate monoester to form a phosphate ion and a free hydroxyl group. The action of phosphatase is the opposite of that of a kinase, which is a phosphorylating enzyme that can use an energy molecule, such as ATP, to add a phosphate group to a corresponding substrate molecule. A phosphatase that is ubiquitous in many organisms is alkaline phosphatase.
Phosphatase can be divided into two classes: acid phosphatase and alkaline phosphatase.
Figure 1. Structure of phosphatase protein.
The phosphatase catalyzes the hydrolysis of the phosphate monoester to remove the phosphate moiety from the substrate. Water decomposes in the reaction, the -OH group attaches to the phosphate ion, and H+ protonates the hydroxyl group of the other product. The end result of this reaction is the destruction of the phosphate monoester, while producing phosphate ions and molecules with free hydroxyl groups. Phosphatase is capable of dephosphorylation of seemingly different sites on the substrate with high specificity. The identification of "phosphatase codes" is still in progress, but it is now possible to conduct a first comparative analysis of all protein phosphatases encoded in the nine eukaryotic "phosphorus" genomes. Studies have shown that the so-called "docking interaction" plays an important role in substrate binding. The phosphatase recognizes and interacts with various motifs (secondary structural elements) on the substrate. These motifs bind to the docking site on the phosphatase with low affinity, and this site is not contained within its active site. Although each docking interaction is weak, many interactions occur simultaneously, with a cumulative effect on binding specificity. Docking interactions can also allosterically regulate phosphatase, thereby affecting its catalytic activity.
Figure 2. The general reaction catalyzed by a phosphatase enzyme.
Protein phosphatases (PPs)
Protein phosphatases (PPs) are structurally and functionally diverse enzymes which are represented by three distinct gene families. Two of these, the PPP and PPM families, dephosphorylate phosphoserine and phosphothreonine residues, whereas the protein tyrosine phosphatases (PTPs) dephosphorylate phosphotyrosine. The most abundant protein serine/ threonine phosphatases of the eukaryotes, PP1, PP2A and PP2B, belong to the PPP family whereas PP2C and the related mitochondrial pyruvate dehydrogenase phosphatase are members of the PPM family. Within each family, although the catalytic domains are highly conserved, suggesting similarities in tertiary structure and catalytic mechanisms, considerable structural and functional diversity of individual protein phosphatases is created as a result of a combination of associated regulatory domains and subunits.
Protein serine/threonine phosphatases of the PPP family play numerous roles in mediating intracellular signaling processes. In addition to PP1, PP2A and PP2B, related novel protein phosphatases have recently been characterized that occur in low abundance and in a tissue and developmental specific manner. PP1 and PP2A are specifically inhibited by a variety of naturally occurring toxins such as okadaic acid, a diarrhetic shellfish poison and strong tumor promotor, and microcystin, a liver toxin produced by blue green algae. Whereas PP2B is only poorly inhibited by the toxins that affect PP1 and PP2A, it was recently defined as the immunosuppressive target of FK506 and cyclosporin in association with their major cellular binding proteins, the cis-trans peptidyl propyl isomerases FKBP12 and cyclophilin, respectively. The structural complexity of PP1 and PP2 holoenzymes in vivo resolves the seemingly paradoxical situation that a relatively small number of protein phosphatase catalytic subunits are responsible for the specific dephosphoiylation of a variety of cellular proteins, and that both PP1 and PP2A have been implicated in regulating many diverse cellular functions, including glycogen metabolism, muscle contraction, control of the cell cycle and RNA splicing. As the individual catalytic subunits of PP1 and PP2A catalyze the dephosphoiylation of a broad and overlapping range of substrates in vitro, specificity in vivo is generated either by altering the selectivity of the enzyme towards a particular substrate or by targeting the phosphatase to the subcellular location of its substrates. This is achieved by regulatory or targeting subunits that bind to the phosphatase catalytic subunits. In addition, the regulatory subunits allow the activity of the PPPs to be modulated by reversible protein phosphorylation and second messengers.
Interactions between kinases and phosphatases
Protein kinases phosphorylate proteins, and protein phosphatases dephosphorylate proteins. Protein phosphorylation and dephosphorylation are common pathways for eukaryotic signal transduction, and their dynamic changes involve almost all processes from embryonic development to individual maturation, including canceration and apoptosis of cells. The balance between phosphorylation and dephosphorylation is mainly regulated by protein kinases (PK) and protein phosphatases (PPs). Phosphorylation and dephosphorylation, as molecular switches, are the easiest and fastest way of signal transduction, usually activated by phosphorylation, dephosphorylation and inactivation. Numerous studies have shown that protein phosphorylation and dephosphorylation processes play an important role in a variety of signal recognition and transduction, and it is a regulatory process prevalent in organisms. Protein kinases are a class of regulatory enzymes that promote the phosphorylation of target proteins by transferring the phosphate group of the ATP gamma group to the amino acid residues of the substrate. By promoting the phosphorylation of functional proteins, the cells make corresponding responses to various stimulation.
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