Chemical Cleavage of Proteins at Cysteinyl-X Peptide Bonds

Abstract

Cysteine is a significant amino acid residue in that it can form a disulfide bridge with another cysteine (to form cystine). Such disulfide bridges are important determinants of protein structure. No known endoproteinase shows specificity solely for cysteinyl, or cystinyl, residues, although endoproteinase Asp-N is able to hydrolyze bonds to the amino-(N)-terminal side of aspartyl or cysteinyl residues. Modification of aspartyl residues (1) can generate specificity for cystei-nyl residues. Again, as discussed by Aitken (2), modification of cysteinyl to 2-aminoethylcysteinyl residues makes the bond to the carboxy-(C)-terminal side susceptible to cleavage by trypsin, or the bond to the N-terminal side sensitive to Lys-N. However, specific cleavage of bonds to the N-terminal side of cysteinyl residues may be achieved in good yield by chemical means [(3); seeNote 1]. The cleavage generates a peptide blocked at its N-terminus as the cysteinyl residue is converted to an iminothiazolidinyl residue, but peptide sequencing can be carried out after conversion of this residue to an alanyl residue (4).