IX. Mitochondrial beta-oxidation
Additional Enzymes: trans- vs cis
Generating a trans- instead of a cis- double bond.
If there is also a double bond at an even-numbered carbon (e.g., the
second double bond in 18:2 9,12), the problem is to generate a
trans- double bond instead of a cis-. This occurs in an indirect
manner. Both activities occur in the mitochondrial matrix.
Three cycles of beta-oxidation occur normally. Beta-oxidation then continues as expected in the presence of the 9 double bond through the fourth cycle, generating a trans- double bond at the 2-position. Enoyl CoA isomerase is involved, as described previously. The fourth cycle completes, and the fifth cycle then begins normally, but proceeds only through the acyl CoA dehydrogenase step.
2,4-dienoyl CoA reductase reduces the compound, leaving
one trans- double bond, but in the wrong position. NADPH +
H+ is required.
- The product is a substrate for enoyl isomerase, the same
enzyme used for cis- double bonds at odd-numbered
carbons. It moves the double bond from the 3 to the 2
- Beta-oxidation now proceeds normally.