250 ORIGIN OF STRUCTURES AND FUNCTIONS 



which also appear to combine directly with the molecules of 

 the substrate, like the prosthetic groups in enzymes which 

 have two components/'^ 



In a number of cases it is quite evident that the groups in 

 question are very small in comparison with the size of the 

 whole molecule of the enzyme. The molecular weight of the 

 enzyme urease, for example, is almost 10,000 times that of 

 its substrate, urea, and it must have a reactive centre which 

 is, relatively, extremely small. By inhibiting urease with 

 silver ions it has been shown that each molecule of urease 

 must contain three or four such centres. ^^ 



We can form an opinion as to the chemical nature of the 

 amino acid radicals at these centres by blocking them with 

 some substance which has a specific activity." By this means 

 it has become clear that it is not just one particular radical 

 which is responsible for the activity of the enzyme, but 

 several amino acids arranged close together in the protein 

 molecule. For example, the catalytic activity of lysozyme 

 depends on the presence of the following free (unblocked) 

 groups: amino, amido, carboxyl, guanidine, hydroxyl and 

 disulphide groups.^" 



R. M. Herriott^^ gives some interesting facts about the 

 chemical structure of chymotrypsin and pepsin. Comparative 

 study by blocking the free functional groups in active 

 enzymes and their inactive precursors enables one to estab- 

 lish the relationship between the catalytic activity of a given 

 protein and the presence of one or another amino acid 

 radical. For example, it has been shown that di-?5opropyl 

 fluorophosphate combines with chymotrypsin but not with 

 its precursor. When this happens, the hydroxyl group of 

 serine is blocked. In the precursor this hydroxyl group forms 

 part of an ester linkage and is only liberated from this by 

 hydrolysis. Besides the hydroxyl group of serine, the imidazole 

 ring of histidine is also necessary for the activity of chymo- 

 trypsin. In just the same w^ay the proteolytic activity of 

 pepsin depends on the presence of the free carboxyl and 

 phenolic groups of tyrosine. 



I. B. Wilson^^ has to some extent succeeded in elucidating 

 the structure of the active centres of cholinesterase. One 

 should not, hoAvever, suppose that it is only the groups within 



