Food Lipids: Chemistry, Nutrition, and Biotechnology

C. �-Oxidation of Specific Fatty Acids
Plant fatty acids often possess other functional structures in their molecules, including
double bonds in the Z configuration, hydroxyl or oxo groups at even-numbered
carbon atoms, and methyl branching at certain positions of the carbon chain. These
structures appear to form barriers to continuous passages of these fatty acids through
the �-oxidation reaction sequence. To complete the degradation of these fatty acids,
such barriers must be circumvented, and a modified �-oxidation pathway is required
to allow this to happen.
1. Linoleic Acid
Naturally occurring unsaturated fatty acids usually have the double bond(s) in the Z
configuration. This configuration appears to form a barrier to the degradation by the
�-oxidation pathway up to the point of the double bond, yielding an intermediate
that cannot be metabolized by enzymes of the regular �-oxidation reaction sequence.
Linoleic acid (18:2) is 9Z,12Z-octodecadienoic acid (18:2,9Z,12Z) and contains Z
double bonds at both even- and odd-numbered of fatty acid carbon atoms. The oxidation
of 18:2 by plant peroxisomes is shown in Fig. 3.
The basic degradative reactions of 18:2 are the same as those for saturated
fatty acids but with some additional enzymes needed. After three repetitive passages
by the �-oxidation pathway, a 3Z-enoyl-CoA intermediate (3Z-dodecenoyl-CoA) is
yielded when the position of the double bond at C-9 has been reached. The enoyl
CoA formed by acyl-CoA oxidase and functioning as intermediate in the �-oxidation
reaction sequence has, however, the 2E configuration. Conversion of 3Z-enoyl-CoA
intermediate into the 2E-enoyl-CoA is accomplished by action of a � 2 ,� 3 -enoyl-CoA
isomerase (EC 5.3.3.8).
The � 2 ,� 3 -enoyl-CoA isomerase of glyoxysomes from cucumber cotyledons
has been purified and characterized. The enzyme is a homodimer (Mr, 50 kDa) and
catalyzes the reversible conversion of 3Z-enoyl-CoA to 2E-enoyl-CoA. Neither 2Zenoyl-CoA,
which is also a substrate for the enoyl-CoA hydratase activity of the
multifunctional protein, 4Z-enoyl-CoA, nor 2E,4Z-dienoyl-CoA, a substrate of 2,4dienoyl-CoA
reductase, is accepted as a substrate by the isomerase. Besides 3Zenoyl-CoA,
3E-enoyl-CoA is also converted to 2E-enoyl-CoA by the isomerase.
Comparison of the activity of the enzyme toward 3Z- and 3E-hexenoyl-CoA indicates
that the activity with the Z isomer is approximately 30 times higher. Increase of the
chain length of 3E-enoyl-CoA from C6 to C12 increased the relative activity of the
enzyme [46].
Further degradation of 2E,6Z-dodecadienoic acid yields a 2E,4Z-dienoyl-CoA,
which is further catabolized by either 2,4-dienoyl-CoA reductase pathway or dehydratase
pathway.
2,4-Dienoyl-CoA reductase (EC 1.3.1.34) reduces 2E,4Z- as well as 2E,4Edienoyl-CoA
to 3E-enoyl-CoA in an NADPH-dependent reaction. To return to the
�-oxidation pathway, the 3E-enoyl-CoA formed by the 2,4-dienoyl-CoA reductase
must subsequently be isomerized to 2E-enoyl-CoA by the � 2 ,� 3 -enoyl-CoA isomerase
(see above). With 2E,4E-decadienoyl-CoA serving as substrate, 2,4-dienoyl-
CoA reductase activity has been demonstrated in glyoxysomes from cucumber cotyledons
[26] and pineapple (Ananas comosus Merr.) fruit tissue [47].
In the dehydratase pathway, 2E,4Z-dienoyl-CoA is first degraded by �-oxidation
to produce a 2Z-enoyl-CoA intermediate (2Z-octoenoic acid) when the position
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.