Cyanogen bromide, peptide bond

The most commonly utilized chemical cleavage agent is cyanogen bromide (it cleaves the peptide bond on the carboxyl side of methionine residues). V8 protease, produced by certain staphylococci, along with trypsin are two of the more commonly used proteolytic-based fragmentation agents. 

Cytochrome c(l-65) and -(66-104), derived by cyanogen bromide cleavage, form a stable complex in which the 65-66 peptide bond spontaneously slowly re-forms to give a protein with full biological activity.1124 ... 

The new selective cleavage of methionine peptide bonds with cyanogen bromide offers more promise of success. There are about twenty methionine residues in 7-globulin and three to four methionines in the 50,000 molecular-weight fraction which has retained the power to bind antigen (Porter, 1957). 

Fig. 19. Topography of the NBS cleavage of the six tyrosyl peptide links of native and

Treatment of proteins with cyanogen bromide results in cleavage of the peptide chain COOH-terminal to methionyl residues with concomitant conversion of the methionine to homoserine lactone which is in equilibrium with homoserine. In certain instances (e.g. -Met-Ser-or -Met-Thr-) some of the methionine is converted to homoserine without peptide bond cleavage (see Schroeder et al. 1969). Homoserine and its lactone are also products of the breakdown of the carboxymethylsulfonium salts of methionine ( 2.5.10). 

Gross and Witkop found gel filtration over Sephadex G-25 invaluable for the separation of fragments resulting from the cleavage of methionine-peptide bonds in ribonuclease by reaction with cyanogen bromide. In this case 0.2 N acetic acid was used as solvent. 

Cyanogen bromide cleaves peptide bonds at specific point too - on carboxyl side of methionines (Figure 5.13)... 

Cyanogen bromide (BrC=N) causes the hydrolysis of the amide bond on the C-side of a methionine residue. Cyanogen bromide is more specific than the endopeptidases about what peptide bonds it cleaves, so it provides more reliable information about the primary structure (the sequence of amino acids). Because cyanogen bromide is not a protein and therefore does not recognize the substrate by its shape, cyanogen bromide will still cleave the peptide bond if proline is at the cleavage site. 

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