154 - The Cell 



NH, 



>"! 



COENZYME A 



Fig. 8-11. Molecular structure of coenzyme A. Note the sulfhydryl ( — SH) group and the vitamin (pantothenic 

 acid), very important constituents of the CoA molecule. 



and a vitamin, pantothenic acid). This ter- 

 minal sulfur group is very active. In the 

 series of reactions (Fig. 8-12) by which 

 CoA-SH transmits 2-carbon acetyl units to 

 the Krebs cycle, a high-energy sulfur bond 

 is generated (Fig. 8-12). This bond conserves 

 energy from the oxidative decarboxylation 

 of pyruvic acid, and the energy is then uti- 

 lized in launching activated acetyl units into 

 the citric acid cycle. Moreover, CoASH is 

 regenerated for further work each time such 

 a transmission is completed (Fig. 8-12). 



ft should also be noted (Fig. 8-5) that the 

 active acetyl units may originate not only 

 from carbohydrate (via pyruvic acid) but 

 also from the derivatives of fat and protein 

 catabolism. Furthermore, since coenzyme A, 

 under proper conditions, can act reversibly, 

 it also can initiate the synthesis of protein, 

 carbohydrate, and fatty compounds, from 

 simpler intermediary metabolites (Fig. 8-5). 



Essentially the Krebs cycle consists of a 



series of oxidation (dehydrogenation) and 

 decarboxylation reactions by which the 6- 

 carbon compound, citric acid, is converted 

 to the 4-carbon compound, oxaloacetic acid. 

 Then acetylation occurs and citric acid is 

 regenerated once more. The intermediary 

 compounds and reactions are now well 

 known. They are very complex, however, 

 and will not be considered in detail. For 

 each acetyl unit (CH H CHO — ). obviously, 

 two decarboxylations must occur, and 2 pairs 

 of electrons {\e~ -(-4H+) are passed over to 

 the primary acceptors of the cell for final 

 transmission through the cytochrome system 

 (Figs. 8-4 and 8-5). 



The quantity of useful energv generated 

 by the Krebs cycle, working in conjunction 

 with the primary acceptors and the cyto- 

 chrome systems, is very impressive. Most of 

 the energy is conserved by the cell in the 

 form of high-energy phosphate (ATP), which 

 stands ready for use when the cell must per- 



