Peptide hydrolysis, exchange reaction

There are very few examples in which masked N-termini of polymer-supported peptides for practical purposes are deprotected by exchange reactions like thiolysis [ 113] of nitro-sulfenyl groups and hydrolysis [114] of trifluoracetyl residues. 

Peptide Synthesis in an Exchange Reaction during Hydrolysis (Trans-... 

The exchange in peptides of one amino acid or ammonia (as in an amide) for another amino acid or peptide is a reaction catalyzed by proteolytic enzymes and has been termed Transpeptidation. In transpeptidation the total number of peptide bonds remains the same there is no net hydrolysis or synthesis. 

Proteins, peptides, and other polymeric macromolecules display varying degrees of chemical and physical stability. The degree of stability of these macromolecules influence the way they are manufactured, distributed, and administered. Chemical stability refers to how readily the molecule can undergo chemical reactions that modify specific amino-acid residues, the building blocks of the proteins and peptides. Chemical instability mechanisms of proteins and peptides include hydrolysis, deamidation, racemization, beta-elimination, disulfide exchange, and oxidation. Physical stability refers to how readily the molecule loses its tertiary and/or sec-... 

Hydrolysis of peptides,84 amides,85 phosphate esters,86 sulfonate esters87 and acetals88 can also be metal catalyzed. The hydrolysis of a phosphate ester coordinated to cobalt(III) also occurs at an increased rate (Scheme 19).89 A rather similar reaction occurs in the amine exchange of coordinated dithiocarbamates (equation 21).90 The conversion of imidates to amidines has been mentioned previously and is a similar type of reaction (see Section 7.4.2.2.1). 

Once correct positioning occurs, and the match is made between the anticodon of the met-tRNA and the start codon, the GTP molecule bound to eIF-2 is hydrolyzed in a reaction promoted by eIF-5. The physical nature of this reaction remains controversial. There are thought to be two forms of eIF-5 with molecular masses of 125 kDa and 60 kDa without, however, any differences in their biological properties (Hershey, 1991 Merrick, 1992). The hydrolysis of GTP causes the release of the initiation factors from the surface of the 40S ribosomal subunit, and allows attachment of the 60S subunit by triggering the release of eIF-6 from it. The formation of the SOS initiation complex culminates in the formation of the first peptide bond at the ribosomal P site. The initiation factor eIF-4D is required for the formation of the first peptide bond. eIF-4D is a small protein (about 16 kDa), and has a unique posttranslational modification of its lysine-50 residue by the action of a polyamine, spermidine, to form a hypusine residue essential for its activity (Hershey, 1991 Merrick, 1992). Furthermore, in order to allow efficient and catalytic use of eIF-2 after GTP hydrolysis and its release from the complex, another factor, eIF-2B, facilitates the exchange of eIF-2 bound GDP for GTP. 

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