Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

II. Long Chain Fatty Acids
83
FIGURE 4-1 Fatty acid synthesis. Acetyl-CoA
is generated in the mitochondria from pyruvate
but cannot penetrate the mitochondrial membrane
to reach fatty acid synthesizing enzymes in the
cytosol. Citrate is formed from acetyl-CoA and
oxaloacetate and migrates to the cytosol where
it is cleaved to regenerate acetate and oxaloacetate.
The acetyl-CoA is converted into malonyl-
CoA and used for fatty acid synthesis. The oxaloacetate
cannot penetrate the mitochondrial
membrane but must be converted to malate or
pyruvate, which can penetrate the membrane and
be converted back to oxaloacetate in the mitochondria.
NADPH needed for fatty acid synthesis
is generated by the pentose phosphate pathway
and malic enzyme.
fatty acid synthase. Fatty acid synthase uses malonyl-CoA
to add two carbon units at a time to a growing LCFA chain
that is attached to the enzyme itself, and it uses NADPH
to reduce the oxygen that was attached to what was the
end carbon of the old LCFA chain. The reaction proceeds
in a series of distinct steps, which all occur on the same
enzyme complex. The overall reaction is as follows:
CH3-(CH 2) n -CO-enzyme
OOCCH2CO-CoA
2 NADPH 3 H
⎯⎯⎯→
CH3-(CH 2) n2-CO-enzyme CO2
H O 2 NADPH
2
The subscript, n , in the structural formula for the growing
LCFA is an even number ranging from zero (i.e., the
starting acetyl group) to usually no more than eight (stearate).
The process begins when an acetyl group binds to
the enzyme complex and usually ends when a palmityl
(16-carbon) group has been formed on the enzyme, at
which point the LCFA is detached from the enzyme. New
carbons are added to the carboxyl end, not the methyl end,
of the growing LCFA. The carbon atom in the carbon dioxide
produced in the fatty acid synthase reaction is the same
carbon atom in the carbon dioxide used to form malonyl-
CoA from acetyl-CoA.
Cellular synthesis of the enzymes directly involved in
LCFA synthesis (acetyl-CoA carboxylase and fatty acid
synthase) and the enzymes involved in the generation
of NADPH and acetyl-CoA translocation is stimulated
by diets that are high in carbohydrate and low in fat and
suppressed by fasting, high-fat/low-carbohydrate diets,
and diabetes. These changes appear to be brought about,
in part, by alterations in plasma insulin and glucagon that
accompany diet changes or diabetes ( Gurr et al ., 2002 ;
Rangan and Smith, 2002 ).
Fatty acid synthesis is expensive energetically. To add
a single acetyl-CoA to a growing LCFA chain, one ATP
is used directly and six more are used indirectly (each of
the two NADPH is equivalent to three ATP). Because fatty
acid synthesis occurs in the cytosol and requires NADPH,
there must be a generous source of that cofactor when fatty
acid synthesis is active. The main source of NADPH for
fatty acid synthesis is the hexose monophosphate pathway
in the cytosol. This pathway utilizes plasma glucose in the
case of adipose or mammary tissue, whereas in the liver, it
can use plasma glucose, glycogen, or gluconeogenesis as
the hexose source. Another source of NADPH in the cytosol
is the malic enzyme reaction. These sources of NADPH
are illustrated in Figure 4-1 .
Although the most common length for nascent LCFA
when they are released from fatty acid synthase is 16 carbons,
they can be 18 carbons or, in the case of fat synthesis
in the mammary gland, as short as 4 carbons. When LCFA
are detached from fatty acid synthase, they are rapidly
thioesterified to CoA by LCFA-CoA synthetase, an enzyme
found in the endoplasmic reticulum and outer mitochondrial
membrane. Most of the palmitate produced by fatty
acid synthase will be elongated to produce stearate by fatty
acid elongase, an enzyme found mainly in the endoplasmic
reticulum but also in mitochondria. This enzyme adds