Most fatty acids occur as esters of glycerol called triglycerides. These are important in energy storage, as edible oils or fixed oils. Mostly in seeds and a few fruits.
Primary fatty acids
energy storage in plants and animals
Plant vegetative lipids
Leaves - mostly associated with chloroplasts composition relatively constant
chloroplast membranes -lots of glycolipids linoleic and linolenic acids common
leaves about 7% lipids, of that, about 30% phospholipids, glycolipids, waxes
phosphatidyl glycerol is the most common phospholipid (about 22%), mono and digalactosyl diglycerides often make up from 20-45% of the total leaf lipids.
Biosynthesis of fatty acids
similar to biosynthesis in animals, fungi (yeasts) and other organisms, but synthetases are soluble and act as discrete proteins
enzymes are associated with chloroplasts, plastids, ER, and cytoplasm
Fatty acids do not appear to be transported ...
C-18 fatty acids
2 systems are involved in their formation
de novo system first makes C-16 fatty acids
type 1, ACP, NADPH, NADH, acetyl-CoA, malonyl-CoA
acetyl-CoA plus ATP, CO2, H2O and acetyl-CoA carboxylase makes malonyl-CoA then, malonyl-CoA is converted to malonyl-SACP, which condenses with acetyl-CoA to synthesize fatty acid chains
elongation system type 2
NADPH, palmityl-ACP, malonyl-ACP (palmityl-CoA won't serve)
"ACP-track" and "CoA-track"
ACP-derivatives converted to CoA derivatives by thioesterases in subsequent reactions
Biosynthesis of triglycerides
this doesn't occur in the chloroplasts which are the primary site of FA's in the plant.
microsomal fractions
glycerol and ATP plus glycerokinase (a soluble enzyme), thiokinases, acylating enzymes associated with microsomal particles (and ER)
glycerol-3-phosphate with acyl-CoA is converted to phosphatidic acid
phosphatidic acid is dephosphorylated to diacylglycerols
diacylglycerols are converted to triglycerides.
Unsaturated Fatty Acids
stearyl-desaturase (C-18), microsomal
stearyl-ACP is converted to oleic acid with O2, NADPH or NADH
Z-double bonds are formed
in anerobic bacteria, another mechanism occurs
on the CoA track
oleic is converted to linoleic which is converted to α-linolenic acid
Degradation of fatty acids
germination - triacylglycerols by lipases give fatty acids
both β- and α-oxidation processes occur
the latter pathway is less common and requires O2
β-oxidation
glyoxysomes contain β-oxidation enzymes
glycerides --- acetyl-CoA ---- TCA cycle
basically the reverse of synthesis, but different enzymes etc.
this process appears to be identical in animals
lipoxygenases
Z,Z-interrupted system - adds O2 - leads to breakdown of multiply unsaturated fatty acids
Steps of Fatty Acid Biosynthesis
Triglyceride Stereochemistry
© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department of
Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois
61801, USA. 217-333-7577. seigler@life.uiuc.edu.
