THE BIOCHEMISTRY OF B VITAMINS 



sequently, pantothenate is not required for the synthesis of ethanol or 

 2,3-butylene glycol (formed by the reduction of acetylmethylcarbinol) . 

 Many of the thiamine-promoted processes, however, involve an "oxida- 

 tive" decarboxylation of pyruvic acid which produces the reactive acetyl- 

 ating intermediate, and hence the subsequent steps in these processes 

 all require the presence of a pantothenic acid coenzyme (p. 191). The 

 mechanism of transporting the two additional hydrogen atoms released 

 by each pyruvic acid molecule during the oxidative decarboxylation is 

 not known. In aerobic processes they are probably accepted by a ribo- 

 flavin-containing enzyme. 10 In anaerobic processes these hydrogen atoms 

 are used to reduce the products formed from the acetyl derivatives, pos- 

 sibly through the intermediation of flavoproteins or some other hydrogen 

 carrier. A possible mechanism for the formation of butyric acid and butyl 

 alcohol (based upon demonstrated reactions catalyzed by thiamine, 

 nicotinic acid, and pantothenic acid) is shown as an example of this 

 type of process: 



hexose unit 



DPN 



[2H] DP n [2H]di 



2 pyruvic acid 

 — I THIAMINE PYROPHOSPHATE 



[2H]? + [2H]? + 2 phosphorylated acetate + 2 C0 2 



I COENZYME A 



icetoacetyl phosphate 

 "4 DPN 



hydroxybutyryl phosphate 



crotonyl phosphate + H 2 

 -^(FLAVOPROTEIN?) j 



butyryl phosphate H 2 



^JDPN { 



butyraldehyde H 2 



—^(DPN?) j 



n-butanol H 2 



I 



2 CO, 



I 



2C0 2 



1 



2C0 2 



I 



2C0 2 



I 



2C0 2 



I 



2C0 2 



The synthesis of fatty acids from carbohydrates probably is carried 

 out by an analogous procedure. Hence, any conversion of carbohydrates 

 to fats or fat-like substances requires the coenzymes of thiamine, nico- 

 tinic acid, pantothenic acid, and probably riboflavin. In the synthesis of 

 fat from proteins, the same vitamins will be essential for the utilization of 

 those "anti-ketogenic" amino acids which on deamination are metabolized 

 via processes that involve carbohydrate intermediates. 



The Aerobic Utilization of Pyruvate. The first step in the aerobic 



