17-8 : B- oxidation pathway in saturated fatty acids

Step 1

Dehydrogenation of fatty acyl-CoA forms a trans double bond between the 2nd and 3rd carbons, creating trans-Δ²-enoyl-CoA. Three enzymes handle this step based on chain length: VLCAD (12–18 carbons), MCAD (4–14 carbons), and SCAD (4–8 carbons). These enzymes use FAD to transfer electrons.

Step 2

1. H20 added across double-bond of trans-delta2-Enoyl-CoA >>> L-Beta-hydroxyacyl-CoA (3-hydroxyacyl-CoA) 

   • Catalyzed by enoyl-CoA hydratase 

Step 3

1. Dehydrogenation of L-Beta-hydroxyacyl-CoA >>> Beta-ketoacyl-CoA 

   • Catalyzed by Beta-hydroxyacyl-CoA dehydrogenase (specific for L stereoisomer) 

   • Electron flow:  

     • NADH formed in reaction (2 e-) >>>> NADH dehydrogenase >>> respiratory chain 

     • ATP >>> ADP as e- pass to O2 

Step 4

. Thiolysis reaction: Beta-ketoacyl-CoA is cleaved by the reaction with the thiol group of Coenzyme A >>> Acetyl-CoA + Acyl-CoA (Myristoyl-CoA)  

• Catalyzed by acyl-CoA acetyltransferase (thiolase)  

 

 

 

How are the last three steps catalyzed ?

12 or more fatty acyl chains : trifunctional (TFP)

12 or less fatty acyl chains : catalyzed by 4 soulube wnzymes in matrix