50 BACTERIAL ENZYMES 



— NH2 group arising from the broken peptide link. Pro- 

 teolytic enzymes differ in specificity towards tlie chemical 

 groups on either side of the — CO — NH — link, towards the 

 length of the peptide chain they can attack, and towards the 

 nature of the terminal groups of that chain. Some proteases 

 are able to attack large protein molecules in a native state, 

 others can attack only after the protein has been denatured, 

 others can attack relatively short polypeptide chains, and others 

 are specific for peptides of two, three, or four amino-acid 

 residues of definite structure. Some peptidases display speci- 

 ficity towards the nature of the particular amino-acids on 

 either side of the peptide link to be hydrolysed, and much 

 of our knowledge concerning stereo-specificity and the " lock 

 and key " nature of enzyme action has been deduced from 

 studies of particular peptidases and the structure of the 

 peptides they can attack. The proteases and peptidases of 

 animal tissues have been studied in considerable detail, but 

 our knowledge of the proteolytic enzymes of bacteria is so 

 far meagre. 



In order to digest large protein molecules which cannot pass 

 through the cell-wall, bacteria excrete extracellular proteases 

 into the surrounding medium, and the power to do this 

 seems to be restricted to comparatively few species. The 

 majority of bacteria can hydrolyse the simpler molecules of 

 peptone and polypeptides, but with a few notable exceptions 

 no detailed studies or separation of the enzymes involved has 

 been undertaken. Some organisms excrete an enzyme which 

 specifically hydrolyses gelatine and consequently these 

 organisms bring about a liquefaction of gelatine media on 

 which they are grown — this property is used as a diagnostic 

 test in systematic bacteriology — but the power to form 

 gelatinase is not necessarily accompanied by the ability to 

 excrete proteases. 



The ability to hydrolyse polysaccharides again involves the 

 excretion of extracellular enzymes, and the ability to do this is 

 subject to the same degree of species variation as any of the 

 fermentation reactions used for characterisation tests in 



