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The protein tyrosine phosphatases are a group of enzymes that remove the phosphate groups from tyrosine residues. These enzymes are important in cell signalling during growth and development and act antagonistically with the protein tyrosine kinases.

The PTP family is subdivided into several groups including the tyrosine specific receptor and non-receptor-like enzymes, the dual-specificity group, the low molecular weight PTPs and the cdc25 group. All members share the same mechanism of hydrolysis and are characterised by a CX5R sequence motif. The CX5R motif forms a loop that provides an oxyanion hole for the stabilisation of the phosphate group. Binding to this loop causes the movement of a conserved aspartic acid into the active site. However the cdc25 group is highly divergent. A cysteine-phosphate intermediate is formed during the reaction and is then hydrolysed. Although the different groups of PTPs are relatively divergent their active sites are highly conserved.

The Yersinia genus contains a plasmid encoding a virulence factor, YopH, a tyrosine phosphatase which translocates itself into the host cell using the N-terminus. YopH is hyperactive compared to human PTPases. The catalytic domains are structurally homologous to human PTP1B, which is also a tyrosine phosphatase. YopH cleaves the phosphate off phosphorylated tyrosine residues on host proteins, interrupting the control of many eukaryotic cellular processes, and contributes to the ability to resist phagocytosis by peritoneal macrophages.