Selective Cleavage and Modification

Withop, 13. Nonenzymatic methods for the preferential and selective cleavage and modification of proteins. Advances in Protein tlhemistry 16, 221 321 (1961). 

Nonenzymatic Methods for the Preferential and Selective Cleavage and Modification of Proteins B. WiTKOP... 

Selective Cleavage and Modification of Peptides and Proteins T. F. Spande, B, Witkop, Y. Decani, AND A. PaTCHORNIK... 

NONENZYMATIC METHODS FOR THE PREFERENTIAL AND SELECTIVE CLEAVAGE AND MODIFICATION OF PROTEINS... 

Fig. 13. Selective cleavage ol OPS ol Shewanellaputrefaciens A6 (29) and modification and cleavage of the resultant disaccharide (49).39...

Proteins are typically made as pro-proteins and are then subsequently modified by post-translational processing involving selective proteolysis ( trimming ) and addition of other groups. Thus, nascent polypeptides commence with N-formylmethionine (bacteria) or methionine (eukaryotes). However, N-terminal sequences are often removed in proteolytic processing. In many eukaryote proteins, the final N-terminal amino acid of the processed protein is N-acetylated. The C-terminus may also be changed by peptide cleavage and other covalent modification. 

Chemical and enzymatic methods for the selective cleavage of peptide bonds are not completely separate either-or propositions. In addition chemical modification may well be combined with enzymatic degradation in a manner which promises new usages and more selectivity for proteinases and peptidases in sequence studies. 

Random incorporation of modified residues in in vitro transcribed RNA is of particular interest in analogue interference experiments (Section 4.6.3) where randomly modified RNA molecules with altered properties are selected and compared to the original pool of RNA molecules. A crucial prerequisite for the interference analysis is the ability to identify the modified residues. A convenient and efficient method is the use of nucleotides which contain both the modification of interest (e.g. 2 -0-methyl NTP, dNTP, inosinetri-phosphates) and an a-phosphorothioate group (one of the nonbridging oxygens is replaced with a sulphur).14-16 The concurrent incorporation of the phosphorothioate renders the modified nucleotides sensitive to a selective cleavage by iodine. 

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