Serine and Threonine

Reduction of a S-aminoketone resulting from the addition of an equivalent of a glycinate anion on ethyl difluoro- or trifluoroacetate is stereoselective and leads to ethyl di- or trifluorothreoninate threo (syn). Release of the acid, performed by saponification, is accompanied by a partial epimerization into an alio compound. However, the amino acids are obtained in enantiopure forms by using a lipase. . It s important to note that (25, 35)-difluorothreonine exhibits activity toward the growth of leukemia cell hnes comparable to 5-fluorouracil.  

A straightforward enantioselective synthesis of aZ/o-difluorothreonine is based on a three component Petasis reaction (enantiopure difluorolactic aldehyde. 

---

The phenolic hydroxyl group of tyrosine, the imidazole moiety of histidine, and the amide groups of asparagine and glutamine are often not protected in peptide synthesis, since it is usually unnecessary. The protection of the hydroxyl group in serine and threonine (O-acetylation or O-benzylation) is not needed in the azide condensation procedure but may become important when other activation methods are used. 

CF3CO2H, PhSCH3, 25°, 3 h. ° The use of dimethyl sulfide or anisole as a cation scavenger was not as effective because of side reactions. Benzyl ethers of serine and threonine were slowly cleaved (30% in 3 h complete cleavage in 30 h). The use of pentamethylbenzene had been shown to increase the rate of deprotection of 0-Bn-tyrosine. ... 

Figure 4.4 Schematic diagram of the structure of the a/p-barrel domain of the enzyme methylmalonyl-coenzyme A mutase. Alpha helices are red, and p strands are blue. The inside of the barrel is lined by small hydrophilic side chains (serine and threonine) from the p strands, which creates a hole in the middle where one of the substrate molecules, coenzyme A (green), binds along the axis of the barrel from one end to the other. (Adapted from a computer-generated diagram provided by P. Evans.)...

Now, it is seen that polar groups dominate the molecular structure, resulting from hydroxyl groups from the two serine and threonine fragments in addition to the peptide bonds themselves. Only weak dispersive interactions will be contributed by glycine fragments (CH2 groups). 

FIGURE 5.5 (a) The hydroxy amino acids serine and threonine are slowly destroyed during the course of protein hydrolysis for amino acid composition analysis. Extrapolation of the data back to time zero allows an accurate estimation of the amonnt of these amino acids originally present in the protein sample, (b) Peptide bonds involving hydrophobic amino acid residues snch as valine and isolencine resist hydrolysis by HCl. With time, these amino acids are released and their free concentrations approach a limiting value that can be approximated with reliability. 

Lipase from Aspergillus niger, 0.2 M phosphate buffer, acetone, pH 7, 37°, 50-96% yield. This lipase was used in the cleavage of phosphopeptide heptyl esters. These conditions are sufficiently mild to prevent the elimination of phosphorylated serine and threonine residues." ... 

These water-sensitive derivatives can be used to form cleanly the t-butyl ethers of serine and threonine. They are cleaved with aqueous acid or base. ... 

Phosphorylation is a common method of regulation. As described above, SH2 domains bind to phosphorylated tyrosine residues. Conversely, phosphorylation of serines and threonines proximal to SH3 and PDZ domains uncouples them from their target motifs. Therefore modulation of protein kinase activity in cells regulates interactions between adaptor proteins and their target proteins. 

Histone phosphorylation is a common posttranslational modification fond in histones, primarily on the N-terminal tails. Phosphorylation sites include serine and threonine residues, tyrosine phosphorylation has not been observed so far. Some phosphorylation events occur locally whereas others occur globally throughout all chromosomes during specific events like mitosis. Histone phosphorylation is catalyzed by kinases. Removal of the phosphoryl groups is catalyzed by phosphatases. 

Phosphorylation is the reversible process of introducing a phosphate group onto a protein. Phosphorylation occurs on the hydroxyamino acids serine and threonine or on tyrosine residues targeted by Ser/Thr kinases and tyrosine kinases respectively. Dephosphorylation is catalyzed by phosphatases. Phosphorylation is a key mechanism for rapid posttranslational modulation of protein function. It is widely exploited in cellular processes to control various aspects of cell signaling, cell proliferation, cell differentiation, cell survival, cell metabolism, cell motility, and gene transcription. 

An attractive and useful method for the preparation of aziridine-2-carboxylic esters makes use of the readily available amino acids serine and threonine. Essentially, this synthesis involves the ring closure of 1,2-amino alcohols. 

Single protein kinases such as PKA, PKC, and Ca +-calmodulin (CaM)-kinases, which result in the phosphorylation of serine and threonine residues in target proteins, play a very important role in hormone action. The discovery that the EGF receptor contains an intrinsic tyrosine kinase activity that is activated by the binding of the hgand EGF was an important breakthrough. The insuhn and IGF-I receptors also contain intrinsic... 

Polgar, L. Catalytic mechanisms of serine and threonine peptidases. In Handbook of Proteolytic Enzymes, 2nd edition Barrett, A. J. Rawlings, N. D. Woessner, J. F., Eds. Elsevier Academic Press Amsterdam, Boston, Heidelberg, London, New York, Oxford, Paris, San Diego, San Francisco, Singapore, Sydney, Tokyo 2004, pp 1441-1448. 

Removal of the oligosaccharide chain from the glycoprotein usually involves the use of one of three main methods treatment with NaOH-NaBH4, hydrazinolysis, or proteolysis.34-36 The NaOH-NaBH4 treatment is used to release, somewhat specifically, oligosaccharides O-linked to serine and threonine. Hydrazinolysis is used to break N-linkages, and proteolysis, to isolate glycopeptides. Each method apparently still has some drawbacks. 

Cytoplasmic serine/threonine protein kinases catalyze the transfer of phosphate groups to serine and threonine residues of target proteins. Serine/threonine kinases have been recognized as the products of protooncogenes (e.g., c-mos, c-raj) or as kinases intimately involved with the regulation of serine/threonine kinase activity by cAMP. Some of these kinases specifically phosphorylate cellular structural proteins, such as histone, laminins, etc. Others phosphorylate still more kinases, resulting in either the activation or deactivation of downstream protein kinases. Specific examples in which serine/threonine kinases elicit specific cellular responses are discussed in this chapter. 

Test results for a series of reactions of polyfunctional amino acids with CDI in aqueous solution are reported in reference [51]. Serine and threonine did not poly-condense. Instead, via the iV-imidazolyl-carbonyl amino acids, L-2-oxooxazolidine-4-carboxylic acid or L-(+)-fraw,s-5-methyl-2-oxooxazolidine-4-carboxylic acid were obtained. 

Serine and threonine Halomethyl ketones, peptidic aldehydes... 

The second method also relies on site-specific chemical modification ofphosphoproteins (Oda et al., 2001). It involves the chemical replacement of phosphates on serine and threonine residues with a biotin affinity tag (Fig. 2.7B). The replacement reaction takes advantage of the fact that the phosphate moiety on phosphoserine and phosphothreonine undergoes -elimination under alkaline conditions to form a group that reacts with nucleophiles such as ethanedithiol. The resulting free sulfydryls can then be coupled to biotin to create the affinity tag (Oda et al., 2001). The biotin tag is used to purify the proteins subsequent to proteolytic digestion. The biotinylated peptides are isolated by an additional affinity purification step and are then analyzed by mass spectrometry (Oda et al., 2001). This method was also tested with phosphorylated (Teasein and shown to efficiently enrich phosphopeptides. In addition, the method was used on a crude protein lysate from yeast and phosphorylated ovalbumin was detected. Thus, as with the method of Zhou et al. (2001), additional fractionation steps will be required to detect low abundance phosphoproteins. 

The catechol-type ligand appears to be restricted to siderochromes derived from prokaryotic microorganisms. Klebsiella oxytoca, an organism closely related to members of the genus Aerobacter, forms the 2,3-dihy-droxy-N-benzoyl derivates of serine and threonine in three day cultures (72). It is not known if the latter amino acid occurs in trimers but examination of space-filling CPK models does indicate that enterobactin could accomodate a methyl substituent on the carbon of the serine residue. Catechols occur in higher protist organisms but their formation... 

---