Oxidation of unsaturated fatty acids:

Unsaturated fatty acids need some additional processing before they can be degraded completely through β-oxidation.  The Unsaturated fatty acyl CoAs with double bonds at odd-numbered  carbon atoms example among,  for instance,  C-9 and C-10 as in palmitoleate are acted on in the common way through the degradation  mechanism  until the acyl CoA dehydrogenase  encounters  the cis- Δ³-enoyl  CoA created at the end of the third round. The presence of the double bond  among  C-3  and  C-4  avoids  the  formation  of  another  double  bond among C-2 and C-3. To overcome this problem an isomerase converts the cis- Δ³   bond into a trans-Δ²    twice bond, and the resulting trans-Δ²-enoyl CoA can then continue down the β-oxidation pathway.

Another enzyme, in further to the isomerase, is needed for the oxidation of polyunsaturated fatty acids that have a double bond at an even-numbered carbon atom. In that case the 2 and4-dienoyl  intermediate  resulting  from the action of acyl CoA dehydrogenase  is acted on through 2,4-dienoyl  CoA reductase  to form cis-Δ³-enoyl  CoA. This is then converted through the isomerase into the trans form that continues down the pathway.  These reactions are important while over half the fatty acids of plant and animal lipids are unsaturated (and frequently polyunsaturated).