196 



UNITY AND DIVTERSITY IN BIOCHEMISTRY 



Pyruvic decarboxylase is a complex enzyme system the coenzymes 

 of which are lipoic (or thioctic) acid, thiamine pyrophosphate (TPP), 

 coenzyme A and DPN. 



Some workers maintain that a compound of thiamine pyrophosphate 

 and lipoic acid is present, lipothiamide pyrophosphate. The oxidative 

 decarboxylation of pyruvate actually occurs in three stages. In the first, 

 pyruvate is condensed with lipothiamide pyrophosphate (LTPP) with loss 

 of CO 2- The accompanying dehydrogenation results in the formation 

 of a thioester link (energy-rich bond). Here the dehydrogenation is not at 

 the start of the respiratory chain but leads to an energy-rich thioester 

 bond. The LTPP which has been acetylated in this first step and contains 

 the thioester bond reacts with the -SH group of coenzyme A an exchange 

 of thioacyl takes place producing a molecule of acetyl-CoA (containing 

 the energy rich thioester bond) and a molecule of LTPP in the sulphy- 

 dryl form. In a third reaction, the sulphydryl form is converted to the 

 disulphide form with loss of two hydrogen to DPN+ with formation of 

 DPNH + H+. The DPNH on entering the respiratory chain gives the 

 usual three ATP's. 



CH3CO COOH + 



LTPP 



CH.CO 



LTPP + CO. 



HS 



CH,CO ~ S 



HS 



LTPP + CoA — SH -> LTPP + CH3CO - S — CoA 



/ / 



HS HS 



HS 



LTPP + DPN 



+ 



HS 



/ 



LTPP + DPNH + H+ 



/ 



Fig. 41 — Oxidative decarboxylation of pyruvate. 



II. ANOTHER PATHWAY FOR THE PRODUCTION OF 

 ACETYL-CoA : THE FATTY ACID CYCLE 



The sulphur of CoA plays the same role for the introduction of fatty 

 acids into the metabolic cycle as inorganic phosphate does for molecules 

 of the sugars. The key to what has long been known as the Knoop 



