100 THE BIOCHEMISTRY OF B VITAMINS 



treatise on enzyme chemistry and hence cannot be undertaken here. How- 

 ever, before going into the detailed account of the specific reactions in 

 which the vitamins are involved, it seems advantageous to discuss a few 

 topics about the general processes and mechanisms of enzyme reactions 

 which have particular bearing upon points to be treated individually in 

 the chapters which follow. 



A biochemical reaction, like any chemical reaction, involves changes 

 in the chemical constitution of one or more molecules with an accompany- 

 ing transformation of energy. These two changes — the material and the 

 energy — take place simultaneously and cannot be divorced from each 

 other. There has been an unwarranted tendency on the part of some 

 workers in the biological sciences to separate biochemical reactions 

 (including those mediated by derivatives of B vitamins) into energy- 

 producing reactions and reactions utilized for the synthesis of cellular 

 components. Such an idea is conveyed in the categorical statements often 

 made to the effect that "carbohydrates and fats are used for the storage 

 and production of energy while proteins are used for building cell struc- 

 ture." In the chemical reactions by which energy is obtained from carbo- 

 hydrates and lipides, cells are at the same time forming, in addition to 

 "waste" or excretory products, a large number of compounds which are 

 either incorporated directly into the structure of cells or are converted 

 into other compounds which are essential units of cellular constituents. 

 On the other hand, the energy liberated by the chemical changes occurring 

 during protein metabolism is utilized in a manner identical to that energy 

 made available during carbohydrate and lipide metabolism. 



Before an enzyme reaction can be considered as well characterized, at 

 least three things must be known: 



(a) the net energy transformations taking place, 



(b) the exact chemical changes occurring (i.e., the specific reactants 



and products) , and 



(c) the components of the catalyst mediating the reaction. 

 These will be considered in turn. 



Energy Transformations in Biological Systems. The role of the bi- 

 ological catalysts involved in producing energy transformations can be 

 better appreciated after comparing the conditions involved in cellular 

 reactions with those employed to effect comparable changes without the 

 aid of enzymes. In nonbiological systems, the production of utilizable 

 energy by the oxidation or degradation of organic compounds (wood, 

 petroleum, alcohol, etc.) almost always involves some kind of combustive 

 process wherein the chemical energy of the organic compounds is first 

 converted into heat with an attending production of high temperatures. 



