Modification 5 -phosphate group

After their synthesis (translation), most proteins go through a maturation process, called post-translational modification that affects their activity. One common post-translational modification of proteins is phosphorylation. Two functional classes of enzymes mediate this reversible process protein kinases add phosphate groups to hydroxyl groups of serine, threonine and tyrosine in their substrate, while protein phosphatases remove phosphate groups. The phosphate-linking... 

As mentioned above, many transcription factors are not always active. Rather the activity of transcription factors is often achieved by induced reversible modification. Most frequently is the addition of phosphate groups (phosphorylation) to Ser, Thr, or Tyr residues. For the AP-1 component c-Jun the phosphorylation at Ser63 and Ser73 enhances activity when cells are subjected to stress, e.g. radiation. Phosphorylation is, however, dispensable for c-Jun-dqDendent tissue homeostasis in the liver, indicating that certain activities do not require the regulatory enhancement. Jun-N-teiminal kinase and a kinase called RSK or p38 catalyze the phosphorylation of AP-1. 

Protein tyrosine kinases (PTKs) are enzymes (EC 2.7.1.112) that catalyze the transfer of the y-phosphate group of ATP to tyrosine residues of protein substrates. The activity of PTKs is controlled in a complex manner by posttranslational modifications and by inter- and intramolecular complex formations. 

The phosphoryl group in phenylphosphate is derived from the -phosphate group of ATP. The free energy of ATP hydrolysis obviously favors the trapping of phenol K, 0.04 mM), even at a low ambient substrate concentration. The reaction is stimulated several fold by another protein, subunit 3 (24kDa). The molecular and catalytic features of phenylphosphate synthase resemble those of phosphoenolpyruvate synthase, albeit with interesting modifications. ... 

Figure 2.7. Identification ofphosphoproteins by site-specific chemical modification. A. Method of Zhou et al. (2001) involves trypsin digest of complex protein mixture followed by addition of sulfhydryl groups specifically to phosphopeptides. The sulfhydryl group allows capture of the peptide on a bead. Elution of the peptides restores the phosphate and the resulting phosphopeptide is analyzed by tandem mass spectrometry. B. Method of creates a biotin tag in place of the phosphate group. The biotin tag is used for subsequent affinity purification. The purified proteins are proteolyzed and identified by mass spectrometry.

This same type of modification strategy also can be used to create highly reactive groups from functionalities of rather low reactivity. For instance, carbohydrate chains on glycoproteins can be modified with sodium periodate to transform their rather unreactive hydroxyl groups into highly reactive aldehydes. Similarly, cystine or disulfide residues in proteins can be selectively reduced to form active sulfhydryls, or 5 -phosphate groups of DNA can be transformed to yield modifiable amines. 

Figure 1.112 Phosphate groups can be modified with adipic acid dihydrazide in the presence of a carbodi-imide to produce hydrazide derivatives. This is a common modification route for the 5 -phosphate group of oligonucleotides.

This modification is not directly reversible, but the phosphate group can be removed from the protein by the action of protein phosphatases. 

CV-6209 (3), is an analogue of CV-3988 where the phosphate group has been replaced by an acetylcarbamate group. This modification produces a marked... 

After donating its phosphate group to ADP, 1,3-diphosphoglycerate is converted into 3-phospho-glycerate. This reaction is followed by enzymic modification to 2-phosphoglycerate. 

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-... 

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