PHYSIOLOGICALLY ACTIVE COMPOUNDS 247 



with these substrates. This is due to the fact that the same 

 prosthetic group is combined with proteins which have 

 different compositions and structures. For example, there 

 exist no less than 15 different enzymes which have as 

 their prosthetic group phosphopyridoxal (vitamin Bg). But, 

 depending on the protein component, one will catalyse the 

 transamination of amino acids, others their decarboxylation, 

 still others the formation of indole from tryptophan, hydro- 

 gen sulphide from cysteine, etc.^^ 



As well as the enzymes with t\vo components ^ve no^v kno^v 

 a large number of enzymes which can be prepared in crystal- 

 line form and which, on hydrolysis, break doAvn completely 

 to amino acids. They therefore cannot contain prosthetic 

 groups and would appear to be simple proteins.''^ Enzymes 

 of this kind, having only one component, cannot enter into 

 combination with their substrates otherwise than by means 

 of the free functional groups of the amino acid residues com- 

 prising their polypeptide chains (Fig. 16). 



Unfortunately, present-day protein chemists only know the 

 complete amino acid composition of a very limited number 

 of crystalline enzymic preparations. We give the facts for 

 four proteins of comparatively low molecular ^veight which 

 have enzymic actions, and for five with higher molecular 

 weight (Table 3). 



A knowledge of these figures for the amino acid composi- 

 tion is, however, of very little help in determining the causes 

 of the activity of any particular enzyme. On the contrary, 

 many contemporary authors emphasise the fact that proteins 

 having similar amino acid compositions may have very dis- 

 similar enzymic activities while, on the other hand, two 

 preparations of the same enzyme isolated from different 

 sources are often very different in amino acid composition.'^^ 



This should not surprise us. As we have shown above, 

 only a certain number of the amino acid residues in its 

 polypeptide chain play a part in determining the specific 

 biological activity of /S-ACTH. w^hile others are relatively 

 unimportant in this respect. There are a number of facts 

 which suggest that among enzymes, too. their activities are 

 associated with particular parts of the molecule. Centres of 

 activity may be found in them, groups of amino acid radicals 



