Protein tyrosine phosphatases Reaction mechanism

Studies of the oxidation of organic sulfides with amino acid-derived ligands in acetonitrile revealed very little difference between the mechanism of their oxidation and that of halides, except for one major exception. Despite the fact that acid conditions are still required for the catalytic cycle, hydroxide or an equivalent is not produced in the catalytic cycle, so no proton is consumed [48], As a consequence, there is no requirement for maintenance of acid levels during a catalyzed reaction. Peroxo complexes of vanadium are well known to be potent insulin-mimetic compounds [49,50], Their efficacy arises, at least in part, from an oxidative mechanism that enhances insulin receptor activity, and possibly the activity of other protein tyrosine kinases activity [51]. With peroxovanadates, this is an irreversible function. Apparently, there is no direct effect on the function of the kinase, but rather there is inhibition of protein tyrosine phosphatase activity. The phosphatase regulates kinase activity by dephosphorylating the kinase. Oxidation of an active site thiol in the phosphatase prevents this down-regulation of kinase activity. Presumably, this sulfide oxidation proceeds by the process outlined above. 

Reversible chemical modification of enzymes, which was discovered in 1955 by Edmond Fischer and Edwin Krebs [58], is a more prevalent mechanism for cellular signaling switching. Fischer and Krebs showed that enzymes can be turned from an inactive form to an active form via phosphorylation of certain residues of the protein. Enzymes that catalyze phosphorylation (addition of a phosphate group coupled with ATP or GTP hydrolysis) are called protein kinases. Enzymes that catalyze dephosphorylation (which is not the reverse reaction of the phosphorylation) are called phosphatases. For example, a protein tyrosine phosphatase is an enzyme that catalyzes the removal of a phosphate group from a tyrosine residue in a phosphorylated protein [57],... 

Fig. 8.17. Mechanism of hydrolysis of phosphotyrosine residues by tyrosine phosphatases. Cleavage of phosphate from phosphotyrosine residues takes place by an in-line attack of a nucleophilic cysteine thiolate of the tyrosine phosphatase at the phosphate of the phosphotyrosine residue. The negative charge on the thiolate is stabilized by the positive charge of a conserved Arg residue. In the course of the reaction, an enzyme-Cys-phosphate intermediate is formed, which is hydrolytically cleaved to phosphate and enzyme-Cys-SH. The figure shows selected interactions. Other interactions in the active center involved in substrate binding and catalysis are not shown. According to Tainer and Russel, (1994). R substrate protein.

Protein phosphatases that are specific for phosphoserine/ phosphothreonine have a distinct reaction mechanism from tyrosine phosphatases. Protein serine phosphatases are transition metal-dependent, and the reaction mechanism does not involve a phosphoenzyme intermediate as in the case of PTPs. Crystal structures of multiple protein serine phosphatases have revealed how the enzymes catalyze hydrolysis of phosphoserine (14). 

The Mn111 acid phosphatases have an intense band in the visible spectrum at 515-570 nm, assigned to tyrosine-Mn111 charge transfer. Unlike the iron proteins, they do not require mild reduction for maximum activity. The linear relation between the intensity of absorption at 515 nm and phosphatase activity indicates that the metal is essential for catalysis. Neither manganese protein shows a room temperature ESR spectrum, but on add denaturation they show a spectrum characteristic of high-spin Mn". Little is known at present about the mechanism of these reactions. 

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