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Chapter
10 |
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Lipids
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Lipids have diverse and essential roles
in plants. As the hydrophobic barrier of membranes, they
are essential for integrity of cells and organelles. In
addition, they are a major form of chemical energy storage
in seeds and are now recognized as a key component of
some signal transduction pathways. Most lipids, but not
all, contain fatty acids esterified to glycerol, and consideration
of this area of metabolism involves, first, the synthesis
of the fatty acid and, second, the synthesis of lipid
after esterification of the fatty acid to form phosphatidic
acid.
Fatty acid biosynthesis
in plants is very similar to that in bacteria and is carried
out in the plastid. Fatty acid synthesis begins with the
carboxylation of acetyl-CoA to mal-onyl- CoA by acetyl-CoA
carboxylase. This is the first committed step of fatty
acid synthesis and is a likely site for regulation of
the whole pathway. Acetyl-CoA and malonyl-CoA are subsequently
converted into fatty acids by a series of reactions that
add two carbons at a time to a growing chain. Acyl-carrier
protein is a 9-kDa protein that transports the intermediates
of fatty acid synthesis through the pathway. In plants,
each reaction is catalyzed by a separate gene product—in
contrast to fatty acid synthesis in animals, which depends
on a multifunctional protein.
Fatty acid biosynthesis
can be terminated by several different reactions, including
hydrolysis of the thioester bond, transfer of the acyl
group to a glycerolipid, and acyl desaturation. Plants
contain an unusual stearoyl-ACP desaturase that is soluble
and plastid-localized. Most fatty acyl desaturases are
membrane proteins localized in the endoplasmic reticulum
or the plastid.
Plants are capable of synthesizing
unusual fatty acids, and more than 200 different fatty
acids having been found in plants. Some of the enzymes
that synthesize unusual fatty acids bear close resemblance's
to common fatty acid enzymes, such as membrane- bound
desaturases. These unusual fatty acids are found almost
exclusively in seed oils, and it is thought that they
may serve a defense function.
There are two distinct
pathways for the synthesis of membrane glycerolipids.
The prokaryotic pathway, located in the chloroplast inner
envelope, uses 18:1-ACP and 16:0-ACP for the sequential
acylation of glycerol 3-phosphate and synthesis of glycerolipid
components of the chloroplast membranes. The eukaryotic
pathway involves (a) export of 16:0 and 18:1 fatty acids
from the chloroplast to the endoplasmic reticulum as acyl-CoAs
and (b) their incorporation into phosphatidylcholine and
other phospholipids that are the principal structural
lipids of all the membranes of the cell except the chloroplast.
In addition, the diacylglycerol moiety of phosphatidylcholine
can be returned to the chloroplast envelope and used as
a second source of precursors for the synthesis of chloroplast
lipids.
Membrane lipids serve as
a hydrophobic barrier, delimiting the cell and dividing
it into functional compartments. The membrane lipid composition
also affects plant form as well as many cellular functions.
For example, photosynthesis is impaired in plants lacking
polyunsaturated membrane lipids, and lipid composition
can affect chilling sensitivity and influence plant cell
responses to freezing. In addition, membrane lipids function
in signal transduction pathways and defensive processes.
Storage lipids play a distinctly
different role from membrane lipids. Storage lipids are
almost exclusively triacylglycerols and accumulate in
discrete subcellular organelles called oil bodies. The
mobilization and catabolism of triacylglycerols provides
energy for germination and pollination. The catabolism
of the released fatty acids occurs via ß-oxidation
in peroxisomes and glyoxysomes. |
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