HYDROLYSIS OF PEPTIDE BONDS 



peptide linkage adjacent to the carboxyl end of the chain. The 

 amino- and carboxypeptidases cannot spUt Unkagcs that are centrally 

 located in the peptide chain; for this reason they arc referred to as 

 exopeptidases. 



All the protein-splitting enzymes whose backbone requirements 

 have been determined belong to group III. These enzymes are 

 capable of hydrolyzing central peptide bonds and, therefore, are re- 

 ferred to as endopeptidases. They were found lo require, in their 

 substrates, a peptide bond in close proximity to the carbonyl group of 

 the peptide bond which is hydrolyzed by the enzyme. Although no 

 known proteolytic enzyme has been identified as belonging to group 

 IV, the suggestion was made recently that an enzyme which hydrolyzes 

 leucylglycylglycine and which is found in intestinal mucosa may belong 

 to this group (31). 



The presence of the indispensable groups in the backbone of a 

 substrate in itself is insufficient to render the substrate susceptible to 

 the action of an enzyme. It has been found that each of the proteolytic 

 enzymes tested thus far also requires the presence, in the substrate, of 

 a certain type of side chain (R) in a precisely defined location. In the 

 second column of Table II, the recjuired location of side chain R is 

 indicated for each of the enzymes mentioned; and the chemical nature 

 of these R groups is given in the third column. The enzymes which 

 have been listed require in their substrates one of the following side 

 chains: isobutyl as in leucine, benzyl or /*-hydroxybenzyl as in phenyl- 

 alanine or tyrosine, aminobutyl or guanidopropyl as in lysine or argi- 

 nine. The side chains mentioned here represent only a few of those 

 which jut out from the peptide chain of proteins. It will be a task of 

 the future to determine precisely the specificity of proteolytic enzymes 

 which require, in their substrates, the side chains of amino acids such 

 as glycine, glutamic acid, histidine, tryptophane, etc. 



Homos pecific Proleolytic Enzymes 



In the course of the systematic study of the specificity of proteo- 

 lytic enzymes, it was noted that several different enzymes exhibited 

 the same backbone and side-chain requirements in their substrates. 

 For example, as will be seen from Table II, an enzymic component of 

 papain and an enzymic component of beef spleen cathepsin have the 



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