THE CHEMISTRY OF PHOTOSYNTHESIS 113 



2. Forma tW7i of an acetyl-lipoic acid-complex 



CH3CO---TPP + L< I -> CH3CO— Sx + TPP + 



^S >L 



3. Transfer of the acetyl group to coenzyme A 



CH3-CO— S. + A— S— H -* CHsCO-'S— A + L^ 



_ /^ ace ty [coenzyme A SH 



4. Oxidation of the reduced lipoic acid by DPN+ 



L<f + DPN+-> DPNH + l/| 



Hence, the over-all reaction is 



CH3COCOOH + A— S— H + DPN+ > 



CH3-CO~S— A + CO2 + DPNH + H + 



According to Reed and De Busk (29), the true coenzyme of decarboxylation 

 is lipothiamide, a compound of lipoic acid with TPP+. The reaction scheme 

 of these investigators corresponds basically with that of Gunsalus. The pos- 

 sible importance of lipoic acid in photosynthesis is discussed in § 55. 



Hence, the product obtained by oxidative decarboxylation of pyruvic acid 

 is not free acetic acid but acetylcoenzyme A. It is the primary product which 

 enters the tricarboxylic acid cycle where it reacts with oxaloacetic acid. It 

 may be stressed here that Wood, Werkman, Hemingway and Nier (39) found 

 direct carboxylation of pyruvic acid to oxaloacetic acid in bacteria and ani- 

 mal tissues 



CH3COCOOH + CO>^ COOHCH2COCOOH 



pyruvic acid oxaloacetic acid 



It is possible that this reaction may help to supply the oxaloacetic acid neces- 

 sary for the first step in the tricarboxylic acid cycle. 



§ 46 The Tricarboxyb'c AcM Cycle 



The first step in the tricarboxylic acid cycle is the condensation of acetyl- 

 coenzyme A with oxaloacetic acid to form citric acid (21). Many investi- 

 gators contributed to our present knowledge of this most important cycle ac- 

 cording to which the acetyl group of acetylcoenzyme A is oxidized to CO2 

 and H2O. Due to the work of Krebs (18), the cycle depicted in Figure 44 is 

 well established and universally recognized. More recently, a dicarboxylic 

 acid cycle has been discovered by Romberg et al. (15, 16, 17), in different 

 microorganisms and mammalian tissues effecting the oxidation of glycolate 

 or glycine via glyoxylate and formate to COo and H.O. 



The tricarboxylic acid cycle proceeds with the aid of enzymes localized in 



