458 ANIMAL BIOCHEMISTRY 



tions, the first 3 ol which are reversible reactions inchided in the 

 degradation scheme. These reactions can be illustrated by starting 

 with 2 moles of acetyl CoA and forming from them the 4-carbon in- 

 termediate, butyryl CoA. 



CH3COSC0A 



acetyl CoA 



+ CH3COSC0A 



thiolase 



CH3CH2CH2COSC0A + 



butyryl CoA 



TPN" 



TPNH + H" 

 ^ enoyi 



reductase 



CH3COCH2COSC0A + CoA 



DPNH + H"^ 



/3-hydroxyacyl 



dehydrogenase 



CH3CHCH2COSC0A + DPN + 

 OH 



enoyl 

 hydrase 



CH3CH = CHCOSCoA + H2O 



The above synthesis of fatty acids does not account for the ATP 

 requirement in the reaction. Furthermore, in in vitro studies this 

 synthesis does not operate efficiently beyond C-8 or C-10 fatty acids. 

 Recently evidence for the existence of a modified aldol condensation 

 has been presented by several laboratories. The suggested reactions 

 can be illustrated as follows: 







RC — S — CoA + TPNH + H3O+ 



Mn 



+ + 



acyl-S-CoA 

 reductase 



OH 

 i^ RC — SCoA + TPN"*" + H2O 

 H 



CO 2 + ATP 



O 



CO 2 activating 

 enzyme 



i^ HP2O7- + AMP--CO2 



1 1 carboxylase 



CH3C — SCoA + AMP— CO2 - AMP + COOH 



CH, 



C— SCoA 



malonyl-S-CoA 







R— CH— CH2C — SCoA + CO2 + CoASH 

 OH 



