PRIMING REACTIONS 197 



^-oxidation has been provided by the fatty acid cycle, the elucidation of 

 which owes much to the researches of Lynen. This cycle is completely 

 reversible and it has been reproduced m vitro, in both directions, using 

 purified enzymes extracted from bacteria or animal tissues. 

 The cycle is made up of four parts. 



1. Condensation of acetyl-CoA (active acetate) with a second molecule 

 of acetyl-CoA to give acetoacetyl-CoA and free CoA. 



CHs— CO— S— CoA .. — CH.— CO— S— CoA 



HS— CoA A _CH,— CO— CHj— CO-S- CoA 



2. Reduction of the acetoacetyl-CoA to j8-hydroxybutyryl-CoA 



4-2H 



— CHo— CO— CHo— CO— S- CoA ^ 



— 2H 

 -CHo— CH-CH2— CO— S-^ CoA 



I • 



OH 



3. Dehydration of the /3-hydroxybutyryl-CoA to crotonyl-CoA. 



— H2O 

 ~CH2-CH— CH2— CO— S— CoA ^ 



I 4-H,0 



OH 



— CH2— CH=CH— CO— S— CoA 



4. Reduction of crotonyi-CoA to butyryl-CoA 



+ 2H 

 — CH2— CH=CH— CO— S— CoA ^ 



— 2H 



— CH2— CH.— CH2— CO— S- CoA 

 FADH, ^ FAD 

 We started with acetyl-CoA and we have lengthened the chain by 2 

 carbon atoms, at the price of one acetyl-CoA molecule and four hydrogen 

 atoms. The breakdown of sugar can give us both of these. If we repeat the 



REFERENCES 



Stumpf, p. K. (1954). Glycolysis. Chemical Pathivays of Metabolism, Greenberg 



(Editor) vol. 1, Academic Press, New York, 67-108. 

 Cohen, S. (1954). Other pathways of carbohydrate metabolism, Che?nical Pathzvays 



of Metabolism, Greenberg (Editor) vol. 1. Academic Press, 173-233. 

 Dickens, F. (1956). The hexosemonophosphate oxidative pathway of yeast and 



animal tissues. Proceedings of the 3'"'* Inter Jiational Congress of Biochemistry, 



Brussels 1955, Academic Press, New York, 170-179. 

 HORECKER, B. L. (1958). Le cycle des pentose et sa signification physiologique. 



Bull. Sac. Chim. Biol, 40, 555-578. 



