FATTY ACIDS ] 23 



heightened perspective from which to evaluate their systematic 

 significance. For example, it is pertinent to consider whether all fatty 

 acids are synthesized via one or via several basic biosynthetic routes. 

 Very little is known about the genetics of fatty acid synthesis, so one 

 potentially valuable aspect of the subject is temporarily obscured. 

 Finally, certain examples will be selected to illustrate the association 

 of specific fatty acids with particular genera or families. Table 7-2 in- 

 cludes a list of fatty acids. 



Fatty acid biosynthesis 



In recent years the pathways involved in fatty acid metabo- 

 lism have been rather well established, and it is now evident that 

 fatty acid metabolism is linked directly to the oxidative breakdown 

 of carbohydrate at the point of the formation of acetyl coenzyme A. 

 Presumably all aerobic organisms possess the ability to form acetyl 

 CoA. Since all fatty acids appear to be constructed from two-carbon 

 units supplied from acetyl CoA, this mechanism accounts for the 

 overwhelming predominance of even-numbered fatty acids found in 

 nature. The first step in the building up of fatty acids is assumed to 

 be a condensation between two molecules of acetyl CoA: 







2 CH3C— S CoA — > CH3CCH2C S CoA + CoA SH 



Within the last few years a slight modification of this step has 

 been recognized as a result of the demonstration that CO2 is essential 

 for fatty acid build up, yet CO2 does not become incorporated into the 

 fatty acid. To satisfy these requirements CO2 is postulated to combine, 

 with acetyl CoA to form malonyl CoA as an intermediate: 



O 

 C— OH 



(1) CH3CSC0A + CO2 — . CH2CSC0A 



o 



acetyl CoA malonyl CoA 





 O COH 



CHo 



(2) CH3CS CoA + CH2CS CoA * CH3CCH2CS CoA + CoA SH + CO2 



O 



acetyl CoA malonyl CoA acetoacetyl CoA 



