36 BACTERIAL ENZYMES 



NATURE OF ENZYME CATALYSIS 



Knowledge of the nature and function of the prosthetic 

 groups becomes of importance in understanding the nutrition 

 of exacting bacteria (see Chap. V). Several enzymes may 

 have the same prosthetic group but, nevertheless, have 

 different substrate specificities. It is probable that the 

 prosthetic group plays an active part in the decomposition 

 of the substrate, but that the protein moiety of the enzyme 

 is responsible for the specificity towards the substrate. It is 

 thought that a type of loose combination takes place between 

 the substrate and the enzyme protein before catalysis occurs. 

 For instance we often find that the enzyme action is dependent 

 upon the presence, not only of the chemical group whose 

 alteration is catalysed, but also of other groups in the substrate 

 molecule. Lysine decarboxylase cannot catalyse the decar- 

 boxylation of lysine unless both a and e — NHg groups of 

 the lysine molecule H2N.CH2.CH2.CH2.CH0.OHNH2.COOH 

 are intact and unsubstituted. Further, the enzyme cannot 

 attack the dextro-is,om.ei of lysine nor can it attack L-ornithine, 

 which differs from L-lysine in having one less C atom in 

 the carbon chain. Before L-lysine decarboxylase can attack 

 its substrate, this must possess : 



1. an unsubstituted — COOH group, 



2. an intact alpha — NHg group in the laevo- position, 



3. an intact — NHg group in the terminal position, 



4. the distance corresponding to — CHg . CHg . CH2 . CHg . CH— 

 between the two amino-groups. 



The — COOH and — NHg groups are chemically reactive 

 or " polar " groups, and it is thought that a combination 

 between these groups and corresponding groups on the 

 surface of the enzyme protein must take place before the 

 decarboxylation is catalysed. L-Lysine decarboxylase can 

 thus be thought of as combining with the two amino-groups 

 of lysine as a preliminary to the removal of the — COOH 

 group. Substitution of — CH3, etc., in either — NHg would 



