J. S. FRUTON 



further evidence for the "dual-affinity" hypothesis. More recently, 

 the finding of synthetic substrates for the protein-spUtting enzymes 

 (pepsin, trypsin, chymotrypsin, papain, etc.) has led to the extension 

 of this hypothesis to all representative proteolytic enzymes (10). 



Of the two essential points in the substrate, one, of necessity, 

 must be the sensitive CO — NH group or some part thereof. The other 

 requisite point of contact lies in the "backbone"* of the substrate and 

 varies with the nature of the enzyme. The nature of this second 

 group and its position in the backbone relative to the sensitive peptide 

 bond provide the basis for the classification of proteolytic enzymes into 

 four groups as given in Table I (the requisite groups are italicized and 

 the sensitive peptide linkage is indicated by means of a dotted line). 



Table I 

 Classification of Proteolytic Enzymes 



In groups I and II are included the enzymes restricted in their 

 action to peptide bonds at the end of a peptide chain. The peptidases 

 belonging to group I (aminopeptidases) selectively attack the chain 

 at the peptide linkage adjacent to the amino end of the chain while 

 the peptidases of group II (carboxypeptidases) attack the chain at the 



* In order to describe the structural setting of a peptide bond, it is desirable 



to speak of a "backbone" of the substrate, i. e., the sequence of — NH — CH — CO — 

 groupings linked through peptide bonds, and the "side chains," :. e., the groups 

 attached to the CH groups of the backbone. 



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