152 - The Cell 



of fermentation — all the way down to pyruvic 

 acid, in fact — is essentially the same as in 

 glycolysis, except that fermentation ordi- 

 narily starts with glucose rather than with 

 glycogen. Each molecule of the 3-carbon 

 compound, pyruvic acid, then is degraded 

 to the 2-carbon compound ethyl alcohol, by 

 a complex decarboxylation reaction, which 

 is shown in Figure 8-8. In fermentation, ac- 

 cordingly, each original elucose molecule 

 gives rise to 2 molecules of alcohol (CH 3 - 

 CFL-OH) and 2 molecules of carbon dioxide 

 (CO_,); and in the process 2 molecules of 

 ADP are transformed into ATP, at the ex- 

 pense of inorganic phosphate present in the 

 cell. 



CH 3 -COCOOH + DPN-H 2 



pyruvic acid 



coenzyme, 



thiamine 



pyrophosphate 



enzyme, 

 carboxylase 



CH,-CH 2 OH + C0 2 + 



DPN 



ethyl alcohol 



Fig. 8-8. Formation of alcohol from pyruvic acid, 

 by decarboxylation. Double arrow indicates two-stage 

 reaction; intermediary stages not shown. Decarboxy- 

 lation reactions are very important in cellular me- 

 tabolism generally. 



Carboxylation and Decarboxylation. The 



complete catabolism of many organic sub- 

 strates involves decarboxylation (Fig. 8-8). 

 Thus the carbon chain of an organic mole- 

 cule may be shortened not only by hydrolysis 

 (or phosphorolysis) (Fig. 8-6) and by frag- 

 mentation (Fig. 8-5) — processes that break a 

 molecule into fairly large pieces — but also 

 by decarboxylation which shortens the chain 

 just one carbon unit at a time. The cell con- 

 tains a number of carboxylases, which cata- 

 lyze the decarboxylation of various substrates. 

 Also present are the necessary coenzymes, 



especially thiamine pyrophosphate, which co- 

 operate in such catalyses. 



The importance of the carboxylases ex- 

 tends beyond catabolism, however, many of 

 these enzymes can act reversibly. Thus the) 

 can bring about carboxylation (carbon diox- 

 ide fixation), instead of decarboxylation 

 (Fig. 8-9). Anabolically, therefore, the car- 

 boxylases enable the cell to lengthen the 

 carbon chains of various organic substrates 

 (Fig. 8-9). 



H,0 



Fig. 8-9. Fixation of CO., (carboxylation). Note 

 that the 3-carbon compound, pyruvic acid, is con- 

 verted into the 4-carbon compound, oxaloacetic acid. 

 Reverse of this reaction, of course, is decarboxylation. 

 Other reactions show interconversion of some com- 

 mon organic acids, by reduction (with DPN-Ho) and 

 by dehydration (— H 2 0). 



Amination and Deamination (Fig- 8-10). 

 The primary and most important role ful- 

 filled by amino acids is to provide compo- 

 nents of protein structures (p. 134). However, 

 some cells particularly animal cells, may ab- 

 sorb amino acids in excess of the require- 

 ments of protein synthesis. Such excesses may 



