100 PHOTOSYNTHESIS 



oxidation of pyruvate, suggesting that the initial product 

 from pyruvate was not acetic acid but some other acetyl 

 compound. Lipmann had discovered previously that py- 

 ruvic acid could be broken down by a bacterial system 

 to yield acetyl phosphate. He also discovered that as well as 

 cocarboxylase another coenzyme was necessary for the 

 acetylation process: this was called coenzyme A. Later it 

 became clear that the initial product of the oxidative 

 decarboxylation of pyruvate must be an acetyl compound 

 of coenzyme A. Hence the process had to be written 



CH3COCOO-+C0A+2H2O-2H 

 Pyruvate -^CHaCOO— C0A+HCO3-+H + 



acetyl CoA 



Coenzyme A is then a substance concerned in group transfer 

 (acetyl group) in a way analogous to hydrogen transport by 

 coenzymes I and H. 



Ochoa has shown how the oxidative decarboxylation of 

 pyruvate can lead to the synthesis of citric acid. He has 

 isolated a condensing enzyme, in a crystalline state, which 

 catalyses the reaction: — 



Acetyl CoA+oxalacetate -' citrate + CoA 



Stadtman was able to separate an acetyl transferring 

 enzyme which catalysed the reaction 



Acetyl phosphate+CoA=acetyl CoA+inorganic phosphate 



The total reaction measured was: — 



Acetyl phosphate +oxalacetate= citrate + inorganic phosphate 



O'Kane and Gunsalus (1948) discovered that a strain of 

 Streptococcus faecalis required an unknown factor connected 

 with the oxidation of pyruvate. The factor was found to be 

 a sulphur-containing fatty acid, a disulphide of «-octanoic 

 acid, called lipoic acid. This factor appears to have a wide 



CH2 



/ \ 

 CH2 CH— CH2— CH2— CH2— CH2— COOH 



s s 



a-Lipoic acid (Thioctic acid) 



