108 THE BIOCHEMISTRY OF B VITAMINS 



definite enzymes. Is such an assumption still necessary? This question, 

 as well as the related question concerning whether or not there are many 

 undiscovered B vitamins, is certainly debatable. The thesis that there are 

 only a small number of B vitamins yet to be found may have substantial 

 foundation. 



Components of Enzyme Systems. Once the relationship of a vitamin 

 to a particular type of chemical reaction has been established, there still 

 remain many questions to be considered regarding the relationship of 

 the vitamin to the enzyme system itself. 



Although thiamine, riboflavin, and nicotinic acid are as effective agents 

 as can be found for the treatment of the corresponding nutritional 

 deficiencies, none of the three is effective as such in the biochemical 

 reactions for which they are required; each has to be incorporated into 

 a more complex molecule before it can take part in its reactions. All the 

 other typical vitamins whose functions have been completely elucidated 

 have been shown to behave in an analogous manner — they act as cat- 

 alysts in enzymatic reactions only after they have been built up into 

 coenzymes of higher molecular weight than the vitamin itself. 



Enzyme systems vary considerably in their complexity. The com- 

 ponent parts of the system acting upon a substrate include: 



A. Substance required to produce a suitable environment in which the 

 reaction can be carried out. This group includes water, and the com- 

 pounds necessary to establish the proper pH, a suitable redox potential, 

 and appropriate ionic concentrations. 



B. A protein. 



C. Co-factors. 



(1) Specific inorganic ions. 



(2) Specific organic compounds — the coenzymes. 



Some of the simpler systems do not require any cofactors; others require 

 only an activating inorganic ion, or a coenzyme; many enzymes, however, 

 require both. 



There is little specificity about the substances needed to establish a 

 favorable environment for a reaction, and one has considerable leeway 

 in his choice of buffering agents, poising agents, and salts which can be 

 used. These factors should not be confused with or classified as coenzymes. 

 The concentration of the nonspecific components can usually be varied 

 over a considerable range. It is impossible to set down a single optimum 

 value for the pH, redox potential, and salt concentrations, since the most 

 effective levels will depend upon the other conditions imposed upon the 

 system. In contrast, the substances listed as cofactors are much more 

 specific, and during a reaction are combined with the protein component 

 in stoichiometric amounts. The divalent ions — magnesium, calcium, man- 



