PTP, Protein tyrosine phosphatase

Insulin binding to the extracellular side of cell membranes initiates the insulin cascade , a series of phosphorylation/dephosphorylation steps. A postulated mechanism for vanadium is substitution of vanadate for phosphate in the transition state structure of protein tyrosine phosphatases (PTP).267,268 In normal physiological conditions, the attainable oxidation states of vanadium are V111, Viv and Vv. Relevant species in solution are vanadate, (a mixture of HV042-/ H2VOO and vanadyl V02+. Vanadyl is not a strong inhibitor of PTPs, suggesting other potential mechanisms for insulin mimesis for this cation. 

Since then, a plethora of tyrosine-phosphorylated proteins has been discovered. Originally, tyrosine phosphorylation was believed to be involved primarily in regulating cell proliferation, since many oncogene products and growth factor receptors are protein tyrosine kinases (PTKs). However, it has become clear that tyrosine phosphorylation is involved in regulating a variety of cellular processes. In fact, the nervous system contains a large variety of PTKs and protein tyrosine phosphatases (PTPs), and some of these are exclusively expressed in neuronal tissues. Figure 24-1 shows the... 

FIGURE 24-2 Tyrosine phosphorylation and dephosphorylation. Protein tyrosine kinases (PTK) catalyze the transfer of the y-phosphate group from ATP to the hydroxyl group of tyrosine residues, whereas protein tyrosine phosphatases (PTP) remove the phosphate group from phosphotyro-sine. R, protein. 

A variant of Tethering with extenders, termed breakaway Tethering, has been developed for enzymes with catalytic sites that do not tolerate the insertion of a cysteine residue due to disruption of structural or functional integrity. Protein tyrosine phosphatases (PTPs)... 

On the other hand, a particular protein function can be realized with different protein folds, and an example of this are protein phosphatases. Protein phosphatases feature two distinctively different catalytic mechanisms for hydrolytically cleaving phosphorylated amino acid residues. The active sites of serine/threonine protein phosphatases (PPs) contain two metal centers that directly activate a water molecule for nucleophilic attack of the phosphate ester bond. In contrast, protein tyrosine phosphatases (PTPs) [105] possess a Cys residue present in the active site loop containing the conserved PTP signature motif HCXXXXXRS. The Cys sidechain acts as the attacking nucleophile in the formation of a phosphocysteine intermediate, which is eventually hydrolyzed by a water molecule [106], The same catalytic mechanism is also shared by dual-specificity phosphatases (see below). 

Fig. 3.8 The protein tyrosine phosphatases (PTPs) with the C(X)gR motif are divided into tyrosine-specific PTPs the VHl-like dual specificity, serine/threonine- and tyrosine-specific phosphatases the CDC25 phosphatase and the low molecular weight (LMW) phosphatases. The low molecular weight phosphatases are acid phosphatases without distinct regulatory or targeting domains. Their function is not known. The tyosiiie-speoifio phosphatases are further subdivided into receptor-like and non-reoeptor-like phosphatases. (This scheme is reproduced with permission of the authors and Trends Biochem. Sci. from ref. 76.)...

Shen Y, Schneider G, Cloutier JE, Veillette A, Schaller MD. Direct association of protein-tyrosine phosphatase PTP-PEST with paxillin. J. Biol. Chem. 1998 273 6474—6781. Angers-Loustau A, Cote JE, Charest A, Dowbenko D, Spencer S, Lasky LA, Tremblay ML. Protein tyrosine phosphatase-PEST regulates focal adhesion disassembly, migration, and c)dokinesis in fibroblasts. J. Cell. Biol. 1999 144 1019-1031. 

Protein tyrosine phosphatases (PTPs) cleave phosphate groups from phosphotyrosine residues. As is the case with protein kinases, these enzymes are well characterized structurally. 

Not so much is known about the significance of protein tyrosine phosphatases (PTP). hihibition of PTPs (by vanadate) leads to increase in tyrosine phosphorylation, secretion and aggregation (biazuer ail, 1990 Pumigliae/a/., 1992). At least two PTPs appear to be involved in platelet aggregatioa PTPIB is proteolytically cleaved and activated by calpain and released into the cytosol during aggregation. SH-PTPl associates with the... 

VI-IX are structurally characterised, vanadate-inhibited phosphatases. VI, Rat prostat acid phosphatase VII, bovine phosphotyrosyl phosphatase VIII, mammalian protein tyrosine phosphatase PTP-IB (mutant Cys215Ser) IX, E. coli alkaline phosphatase. For comparison, the active centre of vanadate-dependent haloperoxidases (VHPO) (V), is also shown. The structures Xa and Xb have been proposed, based on EPR, for the vanadyl complexes formed with the PTP-IB active site peptide Val-His-Cys-Ser-Ala-Gly. 

Cui L, Yu WP, DeAizpurua HJ et al (1996) Cloning and characterization of islet cell antigen-related protein-tyrosine phosphatase (PTP), a novel receptor-Hke PTP and autoantigen in insulin-dependent diabetes. J Biol Chem 271 24817-24823... 

Carton AJ, Flint AJ, Tonks NK (1996) Identification of pl30(cas) as a substrate for the cytosolic protein tyrosine phosphatase PTP-PEST. Mol Cell Biol 16 6408-6418... 

Key words High-throughput screening, HTS, Protein tyrosine phosphatase, PTP, Inhibitors, pNPP,... 

Protein phosphatases are classified into four superfamilies based upon the structural conservation and mechanism of action of their catalytic domain [7-11]. The protein tyrosine phosphatase (PTP) superfamily includes the classical receptor and non-receptor... 

Fig. 8.22 Regulation of protein tyrosine phosphatases (PTPs). The specific activity of PTPs can be either increased or decreased by phosphorylation of serine, threonine or tyrosine residues. Reversible or irreversible oxidation of the active-site cysteine residue (denoted C) also inactivates PTPs. For receptor-like PTPs (RPTPs), dimerization of the catalytic domains has also been proposed as an inhibitory regulatory mechanism. Furthermore, ligand binding to the extracellular domains has been proposed as an inhibitory mechanism. Modulation of specific activity following ligand binding could involve changes in the phosphorylation, oxidation state of the active-site cysteine or dimerization.

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