Special problems: oxidation of unsaturated fatty acids

The types of special problems encountered in oxidation of unstaurated fatty acids can be illustrated using linoleic acid as an example. The first problem arises when a 3,4 cis double bond is encountered as a result of partial oxidation of linoleoyl CoA. An isomerase changes the position and configuration of this double bond to produce an intermediate that can be further processed by normal b-oxidation.

After a cycle of b-oxidation reactions starting and ending at the trans-D² enoyl CoA stage, another problem is encountered - a dienoyl CoA with a 4,5 cis double bond. Instead of conversion to a 2,3 cis (or a,b cis) form, the dienoyl CoA is reduced to form a 3,4 trans enoyl CoA. This is the case with mammalian fatty acid metabolism, and this results in the need to isomerize the latter product to the usual 2,3 trans form that in this case can then be completely degraded by normal b-oxidation. In E. coli, the dienoyl CoA can be reduced and isomerized by a single enzyme.