302 UNITY AND DIVERSITY IN BIOCHEMISTRY 



it is possible to split the macromolecule of an antibody with a protease and 

 the antibody activit\^ will remain localized in one of the hydrolysis products. 

 Substances capable of stimulating the production of antibody are called 

 complete antigens and are generally protein in nature. However, a much 

 smaller molecule attached to the antigen molecule may be the site for the 

 specificity of action of the antibody so formed. This small molecule is known 

 as a haptene. An example of this t^'pe is the antigenic activity of ovalbumin 

 coupled to an azobenzoate ion. The antibodies formed will be of several 

 types, certain of them reacting with the protein part of the molecule of 

 antigen and others with its haptene, the azobenzoate ion. The antiserum 

 prepared from a rabbit would precipitate the ovalbumin combined with, 

 the azobenzoate, but not ovalbumin alone. On the other hand, the antibody 

 will combine with the free haptene but without giving rise to a precipitate. 

 But the molecules of antibody which are combined with the free haptene 

 will no longer combine with the antigen carrying the haptene and this 

 competition between antigen and haptene for the site on the antibody 

 molecule which is specific for the haptene will result in a decrease in the 

 amount of precipitate formed by the combination of the antibody and the 

 antigen. 



The combination of the antigen with the antibody arises from the 

 complementary character of their structures, the two corresponding 

 regions of their molecules fitting each other in such a way that they are 

 bound together without the intervention of strong chemical bonds. The 

 complementary character of antigen and antibody is interpreted as due to 

 the antibody being formed by the folding of its protein chain against the 

 antigen acting as a template. According to Pauling the folding takes place so 

 that hydrogen bonds may be formed and that charge interactions can be 

 effective between the groups on the antigen and on the antibody. 



Specific antiserums permit various antigens to be distinguished from 

 each other and consequently also the various types of organisms which 

 these antigens characterize. 



Another aspect of diversity in marco molecules shows itself by the 

 insertion of the same enzyme unit into different systems. 



The various systems containing the phenolase complex in animals and 

 plants (heterotypical aspects of the same enzyme) are described in an 

 excellent review by H. S. Mason (1955) and it will suffice to quote the 

 conclusion of this review : 



"At the phylogenetic level of the plants, it appears to catalyze the for- 

 mation of intermediates in biosynthetic systems which produce the flower 

 pigments and related flavonoids, the lacs and lacquers, the simple and 

 polymeric tannins and their esters, the phenolic alkaloids, the quinones, 

 tropolones and simple plant melanins, and the lignins. At higher phylo- 

 genetic levels the phenolase complex catalyzes intermediate phases in the 



