Practice question

Cyanogen bromide chemical cleavage exquisite selectivity methionine residues enabling precise excision fusion partner. Mechanism involves nucleophilic attack thioether sulfur methionine electrophilic carbon CNBr generating cyanosulfonium intermediate undergoing intramolecular cyclization carbonyl oxygen forming iminolactone ring subsequently hydrolyzed acidic conditions homoserine lactone C-terminal cleavage site while N-terminal fragment released. Specificity exploited recombinant insulin manufacturing where fusion proteins engineered single methionine codon junction between beta-galactosidase carrier and insulin A or B chain, while insulin sequences devoid internal methionine due codon optimization avoiding ATG inside coding preserving integrity upon CNBr treatment. Reaction performed tenfold molar excess CNBr 70 percent formic acid 20-24h room temperature dark quantitatively liberating insulin chains retaining S-sulfonated cysteines protecting thiols random oxidation. Cleavage efficiency >90 percent, but side reactions oxidation tyrosine dibromotyrosine cleavage tryptophan strong conditions modification serine requiring careful control pH temperature. Modern mammalian production favors enzymatic cleavage trypsin carboxypeptidase B avoiding toxic chemical reagents improving specificity environmentally friendly process.