Histidine residues, posttranslational modification

Pathological conditions are also linked to posttranslational modifications such as oxidized histidine residues found in P-amyloid protein of Alzheimer s patients, or conformational variants in the case of prion-induced encephalopathies. The development of sensitive MS tools and proteomics techniques is playing an active role in the precise description of these mechanisms.97,98... 

A large number of a, 3-didehydro-a-amino acids have been identified as constituents of relatively low molecular weight cyclic compounds from microbial sources. However, the presence of a,p-didehydroalanine in bacterial as well as in mammalian histidine ammonia lyase and in phenylalanine ammonia lyase shows that the occurrence of a,p-didehydro-a-amino acids is not limited to small molecules alone 8 These residues are incorporated in natural sequences by posttranslation modification. a,p-Didehydro-a-amino acids have also been postulated to be precursors in the biosynthesis of several heterocyclic metabolites including penicillin and cephalosporin 9 Other well-known compounds containing ,( -di-dehydro-a-amino acids are nisin 10,11 (a food preservative112 ), subtilin (a broad spectrum antibiotic) 13 and some of the metabolites isolated from Streptomyces strains such as gri-seoviridin 14 ... 

Figure 29.35. Blocking of Translocation by Diphtheria Toxin. Diphtheria toxin blocks protein synthesis in eukaryotes by catalyzing the transfer of an ADP-ribose unit from NAD+ to diphthamide, a modified amino acid residue in elongation factor 2 (translocase). Diphthamide is formed by a posttranslational modification (blue) of a histidine residue.

Diphtheria toxin inactivates elongation factor 2, an enzyme required for protein synthesis (Pappenheimer, 1977) through catalyzing its ADP-ribosylation, thereby inhibiting protein synthesis and inducing cell death. Elongation factor 2 contains a unique amino acid, diph-thamide, which is formed by posttranslational modification of a histidine residue (Van Ness et ai, 1980). The ADP-ribose binds covalently to this unusual amino acid. 

Histidine and yS-alanine yield the dipeptide carnosine (present in muscle), and histidine and y-aminobutyrate yield homocamosine (found in brain). Methylhistidyl residues are found in some proteins (e.g., actin Chapter 21) as a result of posttranslational modification. Histamine is decarboxylated histidine. 

Upon entry into the eytoplasm, fragment A catalyzes the ADP ribosylation of the transfer faetor, EF-2, leading to its inaetivation and the interruption of protein synthesis. The ADP-ribose group is donated by NAD+. The ADP ribosylation reaction, catalyzed by the toxin, is specific for EF-2 of eukaryotic cells other proteins of eukaryotic and bacterial eells are not substrates. This speeificity is due to an unusual amino acid residue in EF-2, diphthamide, which is the acceptor of the ADP ribosyl group. Diphthamide derives from the posttranslational modification of histidine. The acute symptoms are treated with antitoxin. The bacteria, which are gram-positive, succumb to a variety of antibiotics, including penicillin. Diphtheria is effectively prevented by immunization with toxoid (inactivated toxin) preparations. 

Protein phosphorylation is a pervasive posttranslational modification in cells. It is reversible and can dramatically affect the activity of a modified protein. Protein phosphorylation is one of the most important mechanisms used for signal transduction by cells. In prokaryotic cells, the best-known reversible protein phosphorylations occur on histidine and aspartate in eukaryotes the best-known occur on the hydroxyl groups of serine, threonine, and tyrosine, although histidine can also be phosphorylated (Fig. 3.9). Other reversible modifications also occur, such as the acetylation of lysine residues in histone proteins. 

Pyruvoyl cofactor is derived from the posttranslational modification of an internal amino acid residue, and it does not equilibrate with exogenous pyruvate. Enzymes that possess this cofactor play an important role in the metabolism of biologically important amines from bacterial and eukaryotic sources. These enzymes include aspartate decarboxylase, arginine decarboxylase," phosphatidylserine decarboxylase, . S-adenosylmethionine decarboxylase, histidine decarboxylase, glycine reductase, and proline reductase. ... 

A common mechanism of regulating the state of a protein is via phosphorylation/ dephosphorylation (by kinases and phosphatases, respectively). Serine and threonine residues are the most commonly phosphorylated residues. Tyrosine residues can also be phosphorylated as can histidines. Other mechanisms of controlling the state of proteins include sulfonation and methylation (by sulfotransferases and methyltrans-ferases, respectively). Such posttranslational modifications to the protein are key to the behavior of the target in question and a vHTS study should target the state of interest. One should also be wary of engineered proteins, where one or more residues may have been mutated to alter the biological behavior of said protein. 

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