FORM AND FUNCTION IN PROTEIN DEPHOSPHORYLATION

The phosphate ester is an extremely important chemical bond within the living cell, serving as an energy source, as a means of joining RNA/D NA molecules, and as an efficient mechanism to regulate the activity o f enzymes and proteins by modifying amino acid side-chains. Protein ph osphorylation is a highly regulated process by which information can b e shuttled from the cell surface to the nucleus. There are two classes of enzymes that regulate signalling through the phosphorylation and d ephosphorylation of proteins, namely protein kinases and protein phosp hatases. This review will focus on the structure and function of the p rotein phosphatases. Protein phosphatases are generally divided into t wo main groups based on substrate specificity. Protein Phosphatases (P Ps) specifically hydrolyze serine/threonine phosphoesters and Protein Tyrosine Phosphatases (PTPs) are phosphotyrosine-specific. A sub-famil y of PTPs, dual specificity phosphatases or dual specificity PTPs, are capable of efficient hydrolysis of both phosphotyrosine and phosphose rine/threonine. Recently solved X-ray structures of both PTPs and PPs have provided a wealth of new knowledge on the structure and catalytic mechanism of both enzyme families. Although both PPs and PTPs catalyz e phosphoester hydrolysis, they utilize completely different structure s and distinct catalytic mechanisms. These differences among the phosp hatases are in stark contrast to the serine/threonine and tyrosine pro tein kinases which are all predicted to have a common structure (revie wed by Johnson et al., 1996).