128 THE BIOCHEMISTRY OF B VITAMINS 



222); isocitric acid «=± as-aconitic acid; etc. Each of these reactions 

 involves the dehydration of a /^-hydroxy acid, a type of reaction often 

 encountered in organic chemistry. When catalyzed enzymatically, no 

 vitamin-containing coenzyme has been shown to be required. 



The isomerases which act upon phosphohexoses and phosphotrioses 

 constitute a class of enzymes essential for most, if not all, living organisms 

 that metabolize carbohydrates. These two enzymes catalyze structural 

 rearrangements by establishing an equilibrium between: 



(1) glucose-6-phosphate and fructose-6-phosphate; 



(2) glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. 

 The conversion of an aldose phosphate to the corresponding ketose phos- 

 phate may take place as a result of the intermediate formation of an enol 

 form common to both. A coenzyme requirement for such systems has not 

 been demonstrated. These reactions produce isomers having the molecular 

 configuration required for the subsequent aldol condensation or cleavage 

 by which trioses and hexoses are interconverted (p. 219). 



Coenzymes Mediating Biological Oxidations and Reductions 



Biological oxidations and reductions constitute a class of enzymatic 

 processes which has been intensively and thoroughly investigated during 

 the past half century. Probably no other biochemical phenomenon has 

 received the amount of attention that has been devoted to the study of 

 the mechanisms by which organic substances are oxidized and reduced 

 under the conditions imposed by an environment compatible with life. 



It was early recognized that most of the reactions classified as oxida- 

 tions do not involve molecular oxygen or any of the oxygen-donating 

 types of compounds commonly employed as oxidizing agents in laboratory 

 syntheses. The reactions in which metabolic substrates are "oxidized" 

 can more properly be designated dehydrogenations, for the mechanism is 

 in most instances one involving the transfer of hydrogen atoms; indeed, 

 only rarely are oxygen atoms exchanged during the transformation. Even 

 in aerobic processes, which require molecular oxygen, the particular com- 

 ponent reaction in which the oxygen is utilized is usually one in which the 

 oxygen molecule acts as an acceptor for hydrogen atoms which have been 

 transported from the initial substrate by a series of enzymatic reactions. 

 The oxygen molecule is, in fact, hydrogenated, forming hydrogen peroxide, 

 and the oxygen atoms of the oxygen molecule are not incorporated into 

 any of the organic molecules undergoing oxidation. 



A more exact interpretation of the mechanism of hydrogenation- 

 dehydrogenation reactions can be set forth by explaining the phenomenon 

 in terms of the donation and acceptance of electrons rather than hydrogen 

 atoms. Excellent discussions of the current theories regarding the mecha- 



