Phosphoserine phosphatase and

Protein phosphatases are classified according to their activity toward phospho-amino acids they act on (Fig. 1). Nomenclature is independent of regulation simply because stimuli were unknown. Protein phosphatases hydrolyzing O-phospho-monoesters are currently subdivided into two major classes (i) phosphatases acting on phosphoserine (pSer) and phosphothreonine (pThr), and (ii) the second class... 

Figure 10.9 Active site of Methanococcus phosphoserine phosphatase with Mg2+ and phosphoserine in the active site (a) and of human phosphoserine phosphatase with Ca2+ bound and the modelled substrate in the active site (b). (From Peeraer et al., 2004. Reproduced with permission of Blackwell Publishing Ltd.)...

Peeraer, Y., Rabijns, A., Collet, J.-F., Van Scafdngen, E. and De Ranter, C. (2004) How calcium inhibits the magnesium-dependent enzyme human phosphoserine phosphatase, Eur. J. Biochem., 271, 3421-3427. 

For dephosphorylation reactions, many enzymes with different specificities are available There are phosphatases, specific for phosphotyrosyl residues, for Ser/Thr-bound phosphates, and dual-specificity phosphatases, recognizing both phosphotyrosyls and phosphoserines Tyrosine phosphatases and dual-specificity phosphatases have already been introduced (Chapter 3). Here, the properties of serine/threonine phosphatases will be described and their regulation by cellular relocation introduced. Much of what we know about the regulation of this class of phosphatases we owe to the work of P. Cohen and his colleagues. Table 7.1 lists common phosphoserine/phosphothreonine phosphatases of eukaryotes. 

D. Serine is synthesized from glucose. The pathway branches from glycolysis at phospho-glyceric acid, which is reduced, transaminated, and dephosphoiylated by phosphoserine phosphatase. Pyruvate kinase is a glycolytic enzyme that functions beyond the branch point for serine synthesis. 

WW domains are signaling modules of ca. 40 amino acids that bind short proline-rich sequences such as PPLP or PPR motifs (review Macias et al., 2002). A subset of WW domains, found, e.g., in the proline cis-trans isomerase Pinl, however, specifically binds to phosphoserine-Pro and phosphothreonine-Pro motifs. The Pin 1 protein has an essential role in mitosis. It is thought that Pinl binding to phosphorylated mitotic proteins facilitates proline cis/trans isomerizations and subsequent conformational changes. For the Pinl substrate Cdc25 phosphatase it has been shown that proline isomerization facilitates the subsequent dephosphorylation of phosphorylated Cdc25 protein by the protein phosphatase PP2A. 

Forkhead-associated domains (FHA domains) were originally identified as conserved sequence elements within a subset of forkhead transcription factors and were subsequently found, e. g., in other transcription factors, in protein kinases, protein phosphatases, and kinesin motors (review Durocher and Jackson, 2002). FHA domains comprise up to 140 amino acids, exhibit binding specificity toward phosphothreonine residues, and efficiently discriminate against phosphoserine residues. 

Serine is synthesized in a direct pathway from glycerate-3-phosphate that involves dehydrogenation, transamination, and hydrolysis by a phosphatase (Figure 14.6). Cellular serine concentration controls the pathway through feedback inhibition of phosphoglycerate dehydrogenase and phosphoserine phosphatase. The latter enzyme catalyzes the only irreversible step in the pathway. 

Raf phosphoserine-259 then is dephosphorylated (by an unknown phosphatase) and other serine or threonine residues on Raf become phosphorylated by yet other kinases. These reactions Incrementally Increase the Raf kinase activity by mechanisms that are not fully understood. 

The cellular and in vivo use of natural phosphoamino acid-containing peptides has severe limitations because, in addition to the poor chemical stability of the 0-phosphate group, they are subjected to dephosphorylation by phosphatases and are poorly absorbed into cells. These circumstances and the potential therapeutic use of compounds containing enzymatically and chemically stable phosphoamino acid isosteres have motivated the synthesis of phosphoamino mimetics. Figure 20 shows some selected examples of phosphotyrosine (44 [175-177], 45 [176], 46 [178], 47 [179], 48-49 [177], 50 [180], 51 [177], 52 [181], 53 [182], 54 [183], 55-56 [184,185], 57 [186]), phosphoserine (58 [187], 59 [188], 60 [189,190], and phosphothreonine (61 [191]) mimetics. These building blocks have been incorporated into peptides and peptidomimetics by solid-phase synthesis using standard procedures [175,179,182,183,189,192-198]. 

In the biosynthesis of serine from glucose, 3-phosphoglycerate is first oxidized to a 2-keto compound (3-phosphohydroxypyruvate), which is then transaminated to form phosphoserine (Fig. 39.5). Phosphoserine phosphatase removes the phosphate, forming serine. The major sites of serine synthesis are the liver and kidney. 

Regulatory mechanisms maintain serine levels in the body. When serine levels fall, serine synthesis is increased by induction of 3-phosphoglycerate dehydrogenase and by release of the feedback inhibition of phosphoserine phosphatase (caused by higher levels of serine). When serine levels rise, synthesis of serine decreases because synthesis of the dehydrogenase is repressed and the phosphatase is inhibited (see Fig. 39.5). 

Residue type and sequential assignments obtained from specifically labeled samples, when combined with 3D heteronuclear data, can significantly increase the efficiency and accuracy of the assignment process, the first step in structure determination by NMR. A protocol for the design of specifically labeled samples with high information content has been developed by Shi et aV. In vitro protein synthesis methods were used to produce four specifically labeled samples of the 23.5 kDa protein phosphoserine phosphatase (PSP) from Methanoccous jannas-chii. Each sample contained two C/ N-labeled amino acids and one N-... 

Fig. 1. Main routes involved in the synthesis and interconversion of glycine and serine in plants. The various steps are numbered, and the necessary enzymes are as follows 1, glycolate oxidase, E.C. 1.1.3.1 2, aminotransferases, serine, E.C. 2.6.1.45, and glutamate, E.C. 2.6.1.4, glyoxylate aminotransferases 3, enzyme complex in mitochondria (see Fig. 2) 4, serine-glyoxylate aminotransferase, E.C. 2.6.1.45 5, glycerate dehydrogenase, E.C. 1.1.1.29 6, glycerate kinase E.C. 2.7.1.31 7, D-3-phosphoglycerate phosphatase, E.C. 3.1.3.38 8, d-3-phosphoglycerate dehydrogenase, E.C. 1.1.1.95 9, phosphoserine aminotransferase, E.C. 2.6.1.52 10, phosphoserine phosphatase, E.C. 3.1.3.3 11, serine hydroxymethyltransferase E.C. 2.1.2.1 12, nonenzymatic decarboxylation 13, formyl tetrahydrofolate synthetase, E.C. 6.3.4.3 14, isocitrate iyase, E.C. 4.1.3.1.

Hanford and Davies (1958) showed that a partly purilied extract from pea epicotyls converted phosphoglycerate to phosphoserine. The reaction was dependent on NAD, glutamate, and pyridoxal phosphate. The first step was presumably catalyzed by phosphoglycerate dehydrogenase and the second by glutamate phosphohydroxypyruvate aminotransferase. Such a phosphoserine aminotransferase was present in extracts of pea seeds, leaves, and apical meristems (Cheung et al., 1968). Little is known of phosphoserine phosphatase in plants, but its presence in spinach leaves is reported by Larsson and Albertsson (1979). 

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