Food Lipids: Chemistry, Nutrition, and Biotechnology

A. Carbon Source and De Novo Synthesis
The biosynthesis of fatty acids is largely similar among plants and animals. Both are
capable of producing fatty acids de novo from acetyl CoA via the concerted action
of acetyl CoA carboxylase and fatty acid synthase. The first step in the de novo
synthesis of fatty acids involves the production of malonyl CoA from acetyl CoA,
a reaction catalyzed by acetyl CoA carboxylase. Acetyl CoA carboxylase carries out
two partial reactions, each catalyzed at distinct sites, which first carboxylate the
reaction cofactor biotin and then transfer the carboxyl group to acetyl CoA [4]. The
net reaction is shown below:
�
Acetyl CoA � HCO3 � ATP → malonyl CoA � ADP � Pi
In animals, this enzyme is soluble in the cytosol and appears to be regulated by a
number of factors, including long chain acyl CoA, providing sensitivity to both de
novo production of acyl chains and diet. The second general step in the production
of fatty acids is to activate both malonyl CoA and the primary unit of condensation,
acetyl CoA, by transferring the acyl groups to an acyl carrier protein (ACP). These
reactions are catalyzed by malonyl CoA-ACP transacylase and acetyl CoA-ACP
transacylase, respectively. The malonyl ACP complex then enters a cycle of elongation
catalyzed by the soluble enzyme complex fatty acid synthase. Fatty acid synthase
lengthens the acyl chain by two carbons per cycle of activity, using acetyl CoA
as the condensing unit. This series of reactions culminates in the production of
palmitic and stearic acids. The cycle is terminated when acyl ACP thioesterase hydrolyzes
the acyl ACP thioester and releases a fatty acyl CoA [5]. Because the
products of fatty acid synthase are consistently palmitic and stearic acids, acyl ACP
thioesterase is likely to be specific for the hydrolysis of 16- and 18-carbon acyl ACP
complexes. Fatty acid synthesis is extensively reviewed by Goodridge [4] and Wakil
et al. [6].
Although the general mechanisms of fatty acid synthesis are similar, there are
several specific differences between plant and animal fatty acid synthesis. Plant fatty
acid synthase products are complexed in acyl ACP, whereas animals produce acyl
CoA. De novo synthesis of plant fatty acids occurs in the plastid where the products
of fatty acid synthase are predominantly palmitoyl and stearoyl APC. These products
are either utilized directly in the plastid as acyl ACP or translocated to the cytoplasm
and converted to an acyl CoA complex [5]. The de novo synthesis of macromolecules
in animals, including fatty acids, usually requires the transport of acetyl CoA into
the cytoplasm, as acetyl CoA carboxylase and fatty acid synthetase are soluble cytoplasmic
enzymes.
B. Desaturation
Fatty acid desaturation of nutritional importance to humans is largely similar in plants
and animals. The requirements for desaturation include molecular oxygen, a reduced
pyridine nucleotide, an electron transfer system, a terminal desaturase enzyme, and
a fatty acyl substrate [7]. A net reaction scheme for the �9 desaturation of stearoyl
CoA is as follows:
� �
NADH � H � stearoyl CoA � O2 → NAD � oleoyl CoA � HO 2
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