BIOSYNTHESIS 235 



(c) Passage from Free Acetate to Active Acetate 



We have seen how, in the priming reactions, active acetate or acetyl-CoA 

 is produced starting from various types of nutrient. Acetyl-CoA can also be 

 produced by activation of free acetate and this operation presents certain 

 curious features. In yeast and animal tissues the acetate is activated by 

 ATP and a special type of splitting of the molecule occurs, resulting in the 

 formation of AMP and pyrophosphate. 



ATP 4- CoA— SH -^ CoA— S-PP + AMP 



CH3— COOH + CoA— S^PP ^ CoA— S-'COCHs + PP 



ATP + CoA— SH 4- CH3COOH ^ CoA— S-COCH3 + AMP -f PP 



{d) Condensation of C^ Fragments to Form Acetoacetate 



This is a modified Claisen condensation between two molecules of 

 acetyl-CoA resulting in the appearance of acetoacetyl-CoA and separation 

 of CoA. The reaction is reversible and the enzyme catalysing the conden- 

 sation also catalyses the reverse thiolysis. If a specific deacylase is present, 

 the acetoacetate is liberated and the equilibrium is distributed until the 

 acetyl-CoA is completely transformed into acetoacetate. 



o o 



II II 



CH3— C— S— CoA + CH3— C— S— CoA 

 condensation \ \ thiolysis 



OH O 



CHs— C— CH2— C— S— CoA 



S— CoA 



o jr o 



II II 



CH3— C— CH2— C— S— CoA + HS— CoA 



hydrolysis | -\- HoO 



CH3— CO— CH2— COOH -f HS— CoA 



{e) Biosynthesis of Isoprene and Carotenoids 



Their biosynthesis is from Cg fragments by the intermediary of aceto- 

 acetyl-CoA. In isoprene, two carbon atoms are derived from the carboxy- 

 of acetate and three from its methyl, as shown below. 



The carotenoids synthesized by plants also derive from acetyl-CoA. 



