260 UNITY AND DIVERSITY IN BIOCHEMISTRY 



3. Coezyme A 



Pantothenic acid is not synthesized by the body and therefore must be 

 obtained from the diet. It is condensed with a molecule of cysteine and 

 the resulting pseudopeptide is decarboxylated to form a compound of 

 pantothenic acid and thioethanolamine, pantetheine. The latter, in the 

 presence of ATP and a phosphokinase, becomes phosphopantatheine, 

 which, in the presence of ATP and a pyrophosphorylase, is converted to 

 dephospho-CoA, which, in turn, is phosphorylated on the 3' position of 

 ribose in the presence of ATP and a phosphokinase to form CoA (Fig. 78). 



(/) Biosynthesis of Peptide and Amide Bonds 



In animals hippuric acid is formed from benzoic acid and glycine which are 

 joined together by a secondary amide linkage similar to a peptide link. The 

 reaction has been well studied, ATP is required as an energy-donor and the 

 benzoic acid must be activated by being first combined with coenzyme A. 



The stages of the synthesis are as follows: (E = enzyme) 



(1) E + ATP ^ E— AMP + PP 



(2) E— AMP + HS— CoA ^ E— S— CoA— AMP 



(3) E— J— CoA + HOOC— CeHa ^ CoA— S— OC— CeHs + 



E + H2O 



(4) CoA— S—OC— CeHs + H2N— CH2— COOH^ 

 CeHs— CONH— CH2— COOH + CoA— SH 



(1+2+3+4) 



CeHoCOOH + H2N— CHo— COOH + ATP ^ 



CeHs— CONH— CH2— COOH + AMP + PP 



It is clear that during the synthesis ATP is split into AMP and PP. 



A second type of synthesis of the secondary amide bond is found to occur 

 during the synthesis of pantothenic acid, a constituent of coenzyme A. 

 Here, we likewise have a splitting of ATP into AMP and PP, but not 

 through activation by CoA. 



In the presence of an enzyme extract of E. coli we get 

 CH3 



HO-CH,-C-CHOH-COOH + H2N-CH2-CH2-COOH + ATP 



I 

 CH3 



pantoic acid j3-alanine 



CH3 



-^HO— CH2— C— CHOH— CONH— CH2— CH0-COOH+ AMP + PP 



I 

 , CH3 



pantothenic acid 



