Threonine residues modification

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. 

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. 

A high proportion of the positiveiy charged basic amino acids lysine and arginine within these flexible tails are frequent targets for extensive posttranslational modifications (Berger, 2002). Such modifications include the acetylation of lysine residues, the methylation of lysine and arginine residues, the ubiquitination of lysine residues, the phosphorylation of serine and threonine residues, the sumoylation of lysine residues, and the poly ADP-ribosylation of glutamic acid residues. 

The most important modification that occurs to proteins is their phosphorylation. Phosphorylation results in the addition of a phosphate group to the hydroxyl group of a serine or threonine residue and, less frequently, to a tyrosine. The phosphate group intro-... 

For glycoproteins, the formation of anhydro rings from hexosamine residues is important in chromogen formation, especially in relation to the Ehrlich reaction and its modifications (3). / -Elimination is involved in the removal of oligosaccharide or polysaccharide chains from the protein core of glycoproteins when they are attached by O-glycosidic linkages to serine or threonine residues. A subsequent -elimination may also... 

A posttranslational modification of major significance in IFs relates to the phosphorylation and dephosphorylation events that occur at serine and/or threonine residues within specific recognition sites. These sites are located entirely in the head and tail domains. It has now been widely established that the action of kinases which lead to phosphorylation, and... 

Hydroxyproline and hydroxylysine occur most noticeably in collagen. These are formed by modification of proline and lysine residues by specific enzymes after synthesis of the collagen chains. It is interesting to note that proly/hydroxylase, which hydroxylates proline, requires ascorbate (vitamin C) as a coreactant. Other chemical modifications known to occur commonly are the attachment of sugars (glycosylation) to asparagine, serine, and threonine residues and the phosphorylation of serine. Chemical modifications are also associated with the transport of proteins out of the cells in which they are synthesized. 

Finally, because this modification occurs on serine and threonine residues, it can also compete directly with an O-phosphory lation event at an identical or nearby site to provide an additional level of regulation on a specific protein. To date, all described O-GlcNAc-modified proteins have been found to be phospho-proteins (28, 29, 30). Such competition between O-GlcNAc and O-phosphate binding occurs with many examples listed above as well as with others. 

The unsaturated residues Dha and Dhb are formed by dehydration of serine and threonine residues, respectively, and the thioether linkages Lan and MeLan are generated by intramolecular Michael-type addition of cysteine thiols to the unsaturated sites (e.g., Fig. 3b). These modifications can be performed by either two separate enzymes (LanB and LanC) in class I lantibiotics or a single bifunctional enzyme (LanM) in class II lantibiotics. Typically, proteolysis of the leader sequence is performed by a dedicated protease, either a LanP serine protease (class I) or the cysteine protease domain of a LanT protein (class II). The lanB genes encode large ( 1000 residues) predominantly hydrophilic dehydratases that may be membrane associated. To date, the dehydratase activity of a LanB protein has not been reconstituted in vitro and little is known about the mechanism of catalysis of this group of enzymes. 

Figure 3 (a) The nisin biosynthetic gene cluster, (b) Posttranslational modifications during the biosynthesis of nisin. Dehydration of serine and threonine residues in the structural region of the precursor peptide NisA is performed by the dehydratase NisB. Then, the (Me)Lan rings are installed by the cyclase NisC. After secretion, the unmodified leader sequence is removed by the serine protease NisP, which generates the biologically active species, (c) The proposed cyclization mechanism for NisC. 

Protein kinases catalyze the transfer of the 7-phosphate moiety of ATP to the corresponding protein substrate. More than 30% of all eukaryotic proteins are phosphorylated, with the majority of the modifications occurring on serine or threonine residues 89 Kinases constitute the largest enzyme class in eukaryotic proteomes with more than 500 members encoded in the human genome90 and play a central role in most signal transduction pathways... 

---