280 



ENZYMES 



(Table 10-1, class G.l.a.). Its chemical formula has been given on 

 p. 243. 



7. Other Coenzymes. Three definite chemical substances are known 

 to serve as coenzymes for the interconversion of several organic phos- 

 phates during carbohydrate metabolism. Glucose-l,6-diphosphate is a 

 coenzyme for phosphoglucomutase, which catalyzes the migration of a 

 phosphate group between the 1 and 6 positions of glucose. Glyceric acid- 

 2,3-diphosphate acts in an entirely analogous manner in catalyzing the 

 migration of phosphate groups between the 2 and 3 positions of 

 glyceric acid. A coenzyme necessary for the enzymatic conversion of 

 galactose-1-phosphate to glucose-1-phosphate (reaction II, Fig. 13-1) has 

 been purified by Caputto and co-workers. The suggested formula is 

 given below: 



N=C-OH 

 I I 

 0=C CH 



I II 

 N— CH 



•0- 



H H 



I I 

 



H 











■0- 



H H 



I I 



OHO 



-C— C— C— C— C— 0— P— 0— P— 0— C— C — C— C— C-CH2OH 



III I I I I I I I I 



H H H H OH OH H H OH H H 

 ^ ^ ■' 



Uridine part Glucose part 



Coenzyme for conversion of galactose-1-phosphate to glucose-1-phosphate 



A number of other compounds or their derivatives are suspected to be 

 coenzymes on the basis of their chemical properties or their gross metabolic 

 effects. The tripeptide glutathione (GSH, p. 130) can be oxidized to 

 form a double molecule, the parts of which are held together by a disulfide 

 ( — S — S — ) linkage. Specific pyridinoproteins have been studied which 

 catalyze this reaction. Although the role of glutathione in oxidation- 

 reduction reactions is not fully understood, it is known to be a cofactor 

 in the glyoxalase reaction (Table 10-1, class D.l.a.) and to be a func- 

 tional part of glyceraldehyde phosphate dehydrogenase. Ascorbic acid 

 is also capable of undergoing alternate oxidation and reduction, but the 

 mechanism of its metabolic function has not been explained. Biotin has 

 been implicated in certain carbon dioxide-fixation reactions, e.g., the con- 

 densation of carbon dioxide and pyruvate to form oxalacetate (reaction 14, 

 Fig. 13-4) , but to date no enzyme has been purified which has been proven 



