COOH 



PRIMING REACTIONS 

 COOH COOH 



CH.. CHo CH, 



I " itH.O I ±HoO i 



C( OH) COOH ^ CHCOOH - CHCOOH 



I aconitase 1| amriitmr \ isocitric | 



QY\^ CH CHOH dehydrogenase CO 



203 



COOH 



I 

 CH. 



I 

 — CHCOOH ;=i 



COOH 

 citric acid 



COOH 



cis — aconitic acid 



COOH 



isocitric acid 



COOH 



oxalosuccinic acid 



±C02 



oxalosuccinic 

 decarboxylase 



COOH 



CKj 



i 

 CH2 



[ 

 CO 



— CO2 COOH 



+ I/202 I 

 ^ CYh 



oxidative ' 



decarboxylation ^^2 



COOH 

 a-ketoglutaric acid 



COOH 



succinic acid 



Following on these fundamental discoveries, Krebs formulated the cycle 

 in Fig. 43. 



malic 

 4C 



oxaloacetic 

 4C 



pyruvic 

 3C 



fu marie 

 4C 



succinic 

 4C 



V 



7C?) 



oc-ketcg'utaric -e--^/ 

 5C f 



CO2 



citric CO2 

 6C 



CO2: 



Fig. 43 (Krebs) — First formulation of the tricarboxylic acid cycle. 



The discovery of the mechanism for the oxidative decarboxylation of 

 pyruvate with formation of acetyl-CoA permitted the filling in of the miss- 

 ing parts of the cycle. 



The cycle (Fig. 44) begins with acetyl-CoA derived from the fatty acid 

 cycle or from the decarboxylation of pyruvate. The entry into the cycle 



