223 



C0 2 + 2H + +2e 



a-Ketoglutarate 



Figure 5. Schematic and simplified outline of the citric acid cycle. Abbreviations: 

 DPN + = diphosphopyridine nucleotide, DPNH = reduced diphosphopyridine nucleo- 

 tide, TPN + = triphosphopyridine nucleotide, TPNH — reduced triphosphopyridine 

 nucleotide. Enzymes are numbered as follows: 



1. Condensing enzyme 5. Complex system 



2. Aconitase 6. Succinic dehydrogenase 



3. Isocitric dehydrogenase 7. Fumarase 



4. Oxalosuccinic decarboxylase (?) 8. Malic dehydrogenase 



Free acetate may enter the cycle and be oxidized, by 2 moles of oxygen. 



The occurrence of the cycle is well established in animal cells, and 

 there is good evidence that it is at least potentially functional in 

 yeast, bacteria, and higher plants. There is, however, a real question 

 whether its major metabolic importance is in supplying the carbon 

 skeleton of amino acids (167, 248) or whether it has an additional role 

 in providing energy to the cell (74, 283). With the probable exception 

 of the oxidative decarboxylation of a-ketoglutaric acid, the phos- 

 phorylations associated with the citric acid cycle are at the cofactor 

 level. 



The organic acids of the citric acid cycle which are formed in sub- 

 stantial amounts by fungi include succinic, citric, fumaric, and acetic 



