| Chem 432 |
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Spring 2002 |
| Lecture Notes:: 4 February |
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| PREVIOUS |
As we've seen fats are only used in biosynthesis for lipids in animals. Plants, on the other hand can use fats for biosynthesis of carbohydrates, amino acids, etc. Of course plants don't generally store lots of energy as fat, except in their mobile forms, such as seeds. Seeds then use this fat, which is a dense form of energy storage, to manufacture the carbohydrate and protein needed to sprout. So how do seeds use fat for biosynthesis?
Plants adds two new enzyme activities to the set seen in the TCA Cycle to create a new pathway, the Glyoxylate Cycle or Pathway. The stoichiometry of this pathway is:
The pathway can be represented by a simple cycle with two acetyl-CoA's added with succinate as the product, the Glyoxylate Cycle.
In actuality, the pathway is broken up into two parts by being compartmentalised in the mitochodria and a specialized organelle, the Glyoxysome. The two new reactions occur in this organelle:
The acetyl-CoA required for this synthesis is generated in the glyoxysome from fatty acids via a modified b-oxidation pathway using NAD+ molecular oxygen (instead of FAD) as oxidants.
Triglycerides are synthesized from DHAP (cytosol) or glycerol-3-P (mitosol) and fatty acyl CoA (activated fatty acid) via the pathway outlined in Figure 19.25 on p 589 of your text. Note that either starting reactant leads first to the monacylglycerol phosphate, Lysophosphatidic acid:
A second fatty acyl CoA is then added to give Phosphatidic acid, which is then hydrolyzed to the diacylglycerol and a third fatty acyl CoA to give the final product. Note that 2-monoacylglycerol from the intestinal track may also be used to make triglycerides by adding two fatty acyl CoA's.
These phospholipids generally tend to have a saturated fatty acyl group on the C1 position while they mostly have an unsaturated fatty acyl group on the C2 position. The biosynthesis of phosphatidylethanolamine and phosphatidylcholine (lecithin) is outlined in Figure 19.29 on p 594 of your text.
Phosphtidylserine is then made by exchanging serine (using the side chain -OH group) for ethanolamine on a phosphatidylethanolamine.
Finally note the synthesis of ceramide, a sphingolipid, as outlined in Figure 19-31 on p 597 of your text.
Cholesterol is an essential molecule, not only in its own right where it modifies membrane fluidity (e.g. red blood cells), but also as a precursor for the various steroids (e.g. estrogens and androgens for determining secondary sexual charecteristics, corticosteroids to promote healing etc., salt balance etc.), vitamin D, and bile salts.
Cholesterol and the steroids are all based on the so-called Steroid Nucleus shown below:
Cholesterol is ultimately constructed from acetyl CoA units, as shown by the red cicles on the cholesterol molecule below:
 Pathway Diagrams |
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Last modified 5 February 2002
