1690 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



CHjOH-CHOH-CHO 



(trio&e) 

 C3 



^ CHa-CO-COOH 



(pyruvate) 

 C3 



CoA 



COOH-CHj-CO-COGH 



(oxaloacetate) " 



*" CHjCO-CoA 

 (acetyl-CoA) 



OH 



I 

 COOH-CH^-C-COOH 



CH2COOH 



2H 



(citrate) 



COGH-CHrCHOH-COGH 



(malate) 



H2G 



CGOH-CH=CH-CGGH 



(fumarate) 



2H 



H2O 



CGOH-CH=C-CGOH 



CHCGGH 



(aconitate) 

 Ce 



H2O— ^ 



CGGH-CHGH-CH-CGOH 

 I 

 CHj-COOH 



(iso-citrate) 



CGGH-CH^-CH.-CGOH 



(succinate) 

 C, 



CGOH-CO-CH-COOH 

 I 

 CHrCGGH 



(oxalosuccinate) 

 Ca 



2H 



C0GH-CH2-CH,-C0-CG0H 



(a-ketoglutarate) 

 Ca 



Net reaction : 



Scheme 36.1. Krebs cycle. 



Before cycle CHeOs + H2O -* C0H4O2 + CO2 + 4[H] 

 In cycle C2H4O2 + 2H2O -* 2CO2 + 8[H] 



Total 



C3H6O3 + 3H2O -* 3CO2 + 12 [H] 



lipoic acid as hydrogen acceptor) . The fact that pyruvate and oxalacetate 

 are in a reversible carboxylation-decarboxylation equiHbriiim does not alter 

 the fact that the Krebs cycle revolves upon oxalacetate rather than upon 

 pyruvate as a pivot. It is the oxalacetate that takes up the acetyl group, 

 merges with it in citrate, loses CO2 molecules and H atoms to decarboxyl- 

 ases and dehydrogenases, and is regenerated at the end of the cycle. 

 We now turn to the mechanism of carbon dioxide reduction. 



