PHYSIOLOGICALLY ACTIVE COMPOUNDS 253 



ate parts of the peptide chains and closed rings seem to bear 

 a definite spatial relationship to one another, which is recipro- 

 cally strengthened by the drawing together of these parts by 

 means of covalent and ionic bonds, as well as by less stable 

 bonds such as hydrogen bonds. ^^ 



This sort of structure confers a definite size and shape on 

 the fundamental molecular unit of the protein. These units 

 may combine with one another to give discrete particles of 

 uniform size having a relatively low degree of association, 

 which are usually referred to as molecules of ' globular ' or, 

 more accurately, corpuscular proteins, although it would 

 have been more proper to have given them the name of 

 micelles or molecular complexes.®^ In many cases they may 

 be easily and reversibly dissociated into the fundamental 

 units, which demonstrates the fact that the bonds uniting the 

 fundamental units with one another in their polymers are 

 weaker than those within the fundamental units themselves. 

 It may easily be understood that the three-dimensional archi- 

 tecture of the molecule is of decisive significance in determin- 

 ing the chemical potentialities of a given protein, and thus 

 also its biological properties. Proteins having an identical 

 structure of their peptide chains but with different spatial 

 arrangements of them must obviously also have different 

 enzymic, hormonal or immunological properties. 



This is due to the fact that when the chains curl up and 

 form lateral linkages, separate parts of them are necessarily 

 brought into close approximation with one another. As a result 

 of this, amino acid radicals which are widely separated along 

 the peptide chain and even radicals belonging to different 

 chains may be brought together in the protein molecule into 

 the same reactive centre of an enzyme or into the three- 

 dimensional ' chemical relief ' of the surface of the molecule, 

 which forms the basis for the combination of the antibody 

 with the antigen in immunological reactions.®^ 



This type of configuration also means that, while some of 

 the active groups of the amino acid residues find themselves 

 on the surface of the protein molecules and therefore avail- 

 able for chemical activity, others are hidden in the depths of 

 the molecule, protected or ' screened ' by the groups which 

 happen to be near them, so that the chemical and even the 



