246 ORIGIN OF STRUCTURES AND FUNCTIONS 



methylamine, carboxyl, phenyl and other groups which do 

 not in themselves have carboxylase activity but considerably 

 augment this activity of the amino group. 



It has further been shown*^ that natural carboxylases, that of 

 yeast, for example, have as a prosthetic group a phosphorylated 

 derivative of vitamin Bj. In this compound the amino group 

 is combined with the heterocyclic pyrimidine and thiazole 

 rings, which confer on it a very high catalytic activity. Not 

 only that, but when the thiamine pyrophosphate is combined 

 with a specific protein the complex acquires a catalytic activ- 

 ity as a decarboxylase** nearly 10,000 times greater than that 

 of the most efficient of Langenbeck's artificial models. 



A similar situation is found when we study other enzymes 

 having two components, in particular the oxidising enzymes 

 cytochrome oxidase, *^^ peroxidase,''" etc. 



Catalase,'^^ which catalyses the breakdown of hydrogen 

 peroxide to oxygen and water, is also a compound of a specific 

 protein with a prosthetic group, haem." The combination 

 of this enzyme with its substrate is brought about through 

 the agency of the iron in the haem. Even ions of inorganic 

 iron have a weak catalytic activity. If, however, the iron is 

 combined with a pyrrole nucleus its catalytic activity is 

 increased several fold. Haem, in which the iron is combined 

 specifically with four pyrrole nuclei, has a specific catalytic 

 activity about 1,000 times greater than that of inorganic iron. 

 In the natural enzyme the haem is combined with a specific 

 protein. As a result of this, its activity is increased ten million 

 times more. One milligramme of iron combined in the cata- 

 lase complex manifests a catalytic activity which it would 

 require ten tons of inorganic iron to produce. 



Thus the presence of a particular group in the prosthetic 

 part of an enzyme with two components seems to be a pre- 

 requisite for its activity, because without it the enzyme 

 cannot combine with its substrate. The essential strength 

 and specificity of enzymic catalysis is, nevertheless, associated 

 with the protein component of the enzyme. 



We often find the same prosthetic group in a number of 

 different enzymes. Nevertheless, there is a fundamental 

 qualitative difference between them, both as regards the 

 substrates on which they act and the nature of their reactions 



