Ser/Thr protein phosphatases

Numerous protein phosphatases are targeted to their substrates and regulators through the interaction with specific scaffolding proteins. Some of these anchoring proteins bind both, kinases and phosphatases. This applies to ser/thr protein phosphatases as well as to tyrosine phosphatases. 

Ser/Thr-protein phosphatases are ubiquitous enzymes which constitute the catalytic domains of multiprotein complexes. They are responsible for the dephosphorylation of a range of phosphoproteins. Several protein phosphatases have been characterized by X-ray crystallography and display an active site structure similar to purple acid phosphatase. 

Wipf, P., Cunningham, A., Rice, R.L. and Lazo, J.S. Combinatorial synthesis and biological evaluation of library of small-molecule ser/thr-protein phosphatase inhibitors. BioOrg. Med. Chem., 1997,5,165-177. 

Fig. 7.2 Tlie crystal structure of mammalian Ser/Thr protein phosphatase-1, complexed with the toxin mycrocystin was determined at 2.1 A resolution. PPl has a single domain with a fold, distinct from that of the protein tyrosine phosphatases. The Ser/Thr protein phosphatase-1, is a metalloenzyme with two metal ions positioned at the active site with the help of a p-a-p-o-p scaffold. A dinuclear ion centre consisting of Mn2+ And Fe2+ g situated at the catalytic site that binds the phosphate moiety of the substrate. Ser/Thr phosphatases, PPl and PP2A, are inhibited by the membrane-permeable ocadaic acid and by cyclic hexapeptides, known as microcystins. The toxin molecule is depicted as a ball-and-stick structure. On the left and on the ri t, two different views of the same molecule are shown. Microcystin binds to three distinct regions of the phosphatase to the metaLbinding site, to a hydrophobic groove, and to the edge of a C-terminal groove in the vicinity of the active site. At the surface are binding sites for substrates and inhibitors. These ribbon models are reproduced vnth permission of the authors and Nature from ref. 9.

Scheme 2 Three possible roles for metal-assisted phosphoryl transfer to water by PAPs, and related Ser/Thr protein phosphatases, which contain similar binuclear metal centers. In 1, an Fe3 +-coordinated hydroxide is the nucleophile. In 2, a bridging hydroxide (or oxide) is the nucleophile, forming an intermediate that is subsequently opened by attack of water. In 3, an Fe3 + -coordinated hydroxide acts as a general base to facilitate attack by water.

YTXs often coexist with diarrhetic shellfish toxins (DSP) but their effects are different. YTX did not induce diarrhea or intestinal fluid accumulation in adult mice after i.p. or oral administration [3,4]. DSP toxins are specific and potent inhibitors of Ser/Thr protein phosphatases PPl and PP2A. These enzymes play a critical role in phosphorylation/dephosphorylation processes within eukaryotic cells. YTX also inhibited phosphatases but with a very low potency, lower than four orders of magnitude than DSP toxins [3]. In addition, the cytotoxicity of both toxin groups was compared and YTX was less toxic, by three orders of magnitude [3]. Therefore, it was concluded that YTX mechanism of action was not mediated by these enzymes inhibition. 

Inhibitors of ser/thr protein phosphatases I and 2A, calyculin A and okadaic acid, which each inhibit both phosphatases [236], have been shown to inhibit N-formyl peptide-induced neutrophil migration without... 

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