Protein, acetylated disulfide group

The mechanisms of modification are different and depend on the chemical molecules used for the reactions. The anhydride of succinic acid reacts with lysine residues, a-amino groups of proteins and free thiol groups. During succinylation, proteins lose their globular structure and undergo aggregation by disulfide cross-binding with other whey proteins. Acetyl anhydride can modify tyrosine residues. 

FIGURE 3-26 Breaking disulfide bonds in proteins. Two common methods are illustrated. Oxidation of a cystine residue with performic acid produces two cysteic acid residues. Reduction by dithiothreitol to form Cys residues must be followed by further modification of the reactive —SH groups to prevent re-formation of the disulfide bond. Acetylation by iodoacetate serves this purpose. 

The reaction [Eq. (7)] requires a disulfide-reducing system such as dithiothreitol or disulfide reductase and a reducing agent such as NADPH or reduced ferredoxin. It is proposed [Eq. (5)] that carbon monoxide oxidoreductase binds CO as a one-carbon intermediate [C,], which can be either oxidized to C02 or condensed with the methyl group of a methylated corrinoid protein and CoA in the final step of acetyl-CoA synthesis. 

Breaking cystine residue disulfide cross-links [13], or breaking peptide bonds in the protein main-chains, increases the equilibrium water content of wool at RHs above 80%. Acetylation of amino and hydroxyl groups, or esterification of carboxylic add groups in amino add side-chains, decreases the EWC of wool at low RHs because each treatment replaces hydrophilic groups with less hydrophilic groups. 

Post-translational modification, which extends the range of functions of the protein, is one of the later steps in the biosynthesis of many proteins. Post-translational modification of proteins occurs by methylation, hydroxylation, acetylation and phosphorylation of the protein functional groups, by attaching various lipids and carbohydrates to the protein molecule and by making structural changes such as the formation of disulfide bonds from cysteine residues by oxidation. 

Some of the modifications are almost universal. The formyl group of the N-terminal N-formyl-Met in bacterial polypeptides is removed by the action of deformylase. The resulting N-terminal Met may or may not be removed by the action of Met aminopeptidase, as may several other amino acid residues near the N terminus in a process likely to be general in all organisms. The processing of N-terminal Met is particularly important in recombinant proteins that are expressed in E. coli. Often the additional Met comes from the AUG initiator codon that has been engineered into the protein. The N-terminal Met of such recombinant proteins can be cleaved in vitro by aminopeptidase M. The new N-terminal amino acids may be subject to N -acetylation as on most eukaryotic cellular proteins. Cys residues in a protein often form intramolecular as well as intermolecular disulfide linkages. 

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