104 THE BIOCHEMISTRY OF B VITAMINS 



a carbohydrate or fat through the series of steps by which it is believed 

 to be normally utilized. These same types of reactions will also ade- 

 quately account for the synthesis of polysaccharides and fats from the 

 intermediary compounds arising during metabolism (Chapter IIIB). Un- 

 fortunately, it is not possible to make this statement general for all 

 carbohydrates and lipides, since at present one can do nothing more 

 than speculate about the biosynthesis of sterols. 



There still remain to be identified a number of the chemical pathways 

 by which the individual amino acids, purines and pyrimidines are con- 

 nected with each other and with the intermediates which they have in 

 common with carbohydrates and lipides. At the present time this problem 

 is receiving a great deal of attention and interest, and pertinent findings 

 appear in almost every current issue of biochemical journals. The 

 mechanism by which these component units are elaborated into protein 

 molecules is still an enigma and represents the biggest gap in our knowl- 

 edge of fundamental biochemical processes. 



Below is presented an outline of the general types of enzymatic reactions 

 necessary to account for the basic processes by which carbohydrates are 

 known to be utilized and by which fats are presumably synthesized and 

 degraded. Also included are the fundamental reactions associated with 

 nitrogen metabolism which appear to be reasonably well characterized 

 and of general biological importance. A discussion of the specific reactions 

 and their role in general metabolic processes can be found in the following 

 chapters. 



In this outline, the vitamin or group of vitamins associated with each 

 particular type of reactions has been indicated. On the basis of present 

 knowledge, it can be assumed that the specified vitamin or a member of 

 the specified group will generally be required for the particular type of 

 enzymatic action irrespective of the specific substrate used or the 

 biological source furnishing the corresponding enzyme. 



Types of Enzymatic Reactions Utilized 

 in Essential Metabolic Processes 



Types oj Reactions Vitamins required 



I. Reactions in which acetal, ester, amide, none 



and acid anhydride linkages are 

 formed or cleaved by the addition or 

 removal of the elements of water or 

 phosphoric acid. 

 II. Intramolecular hydrations and dehy- none 



drations. 

 III. Intramolecular isomerizations. none 



