PROTEOLYTIC ENZYMES II3 



which have been crystallized, and a summary of the more 

 important results of this work provides a background against 

 which the limited information concerning microbial pro- 

 teases can be considered. 



Hydrolysis of proteins and peptides by animal enzymes [36] 



Until 1935 the animal proteases were classified according 

 to molecular size of substrate and pH for optimum activity, 

 i.e. the emphasis was on physical properties. Proteins were 

 regarded as being the substrates of the proteinases (pepsin, 

 chymotrypsin and trypsin), whilst peptides, substances with 

 a relatively small number of peptide bonds, were the sub- 

 strates of the peptidases (aminopolypeptidases, carboxypoly- 

 peptidases and dipeptidases). The introduction by Berg- 

 mann of a new and relatively simple method for the chemical 

 synthesis of small peptides of known composition stimulated 

 a detailed inquiry into the specificity of enzymes capable of 

 hydrolysing peptide bonds. By using these synthetic sub- 

 strates it was shown that the main factors affecting whether 

 a proteolytic enzyme hydrolysed a peptide bond were, 

 firstly, the nature of the amino-acids linked together by the 

 bond, and secondly, the absence or presence of free amino 

 or carboxyl groups in the vicinity of susceptible bonds. 

 Bergmann proposed that the proteases be grouped into the 

 endopeptidases and the exopeptidases according to whether 

 they hydrolysed peptide bonds remote from or near to the 

 ends of peptide chains in natural substrates. Endopeptidases 

 were typified by pepsin, trypsin and chymotrypsin: the 

 activity of pepsin and trypsin is inhibited by free amino 

 groups near to susceptible bonds whilst chymotrypsin, and 

 possibly trypsin as well, is inhibited by neighbouring car- 

 boxyl groups. The exopeptidases comprise the dipeptidases 

 and the amino- and carboxy-polypeptidases. The latter two 

 groups of enzymes attack peptide bonds adjacent to, and in 

 some cases penultimate to, terminal amino-acid residues 

 with free amino groups and free carboxyl groups respec- 

 tively. Whereas both proteins and peptides can serve as sub- 

 strates for the endopeptidases, the exo_peptidases only 

 attack peptides. The endopeptidases, unlike some of the 



