PHYSIOLOGICALLY ACTIV^E COMPOUNDS 25I 



the active centres of the enzyme which are important for its 

 catalytic activities. As with the prosthetic gioups of enzymes 

 with two components, they are required only for the initial 

 step, the first stage of the enzymic activity, that is to say, for 

 the combination of the enzyme with its substrate. Thus, if 

 the active centre is absent or blocked no catalytic activity 

 of any sort can occur. However, if the reaction is to be accom- 

 plished, the mere combination of the enzyme with the sub- 

 strate is not enough. A further transformation is necessary, 

 as a result of which the appropriate changes take place in the 

 substrate and the enzyme is regenerated in its original form. 

 If this does not happen, not only is the enzyme unable to 

 accelerate the reaction, but it is itself bound up, immobilised 

 in a stable compound. 



It is still not clear which are the details of the structure 

 of the protein molecule associated with these final stages of 

 the catalytic activity of the enzyme. Contemporary scientific 

 literature on this subject consists only of various more or less 

 probable hypotheses (cf. H. Neurath and G. W. Schwert,^^ 

 P. V. Afanas'ev,^* S. E. Bresler^^ and others). There can, 

 however, be no doubt that the protein molecule as such takes 

 part in the catalytic process, not merely as the active centres 

 which enter into direct combination with the substrate. 



It is interesting to compare this suggestion with the results 

 obtained by M. Znamenskaya, P. Agatov and A. N. Belozer- 

 skii*® in their work on the mechanism of the biological 

 activity of gramicidin S. These workers showed that the 

 amino group is very important indeed in connection with 

 the activity of this antibiotic. This group should, however, 

 only be regarded as the active centre uniting the antibiotic 

 with the substrate. The nature and specificity of the activity 

 of gramicidin S depends on the structural features of the 

 molecule as a whole. 



Enough has been said to show that the order in which the 

 amino acid residues are arranged is of the first importance 

 in determining the specific biological functioning of enzymes. 

 This order, to some extent, includes both the structure of 

 the active centre and those details of the construction of 

 the protein molecule which are important for its catalytic 

 activity. 



