1. Synthesis of Eicosanoids, Glycerolipids and Isoprenoids

2. Eicosanoids Eicosanoids are important regulatory molecules
Referred to as local regulators. Function where they are produced.
Two classes: Prostaglandins/thromboxanes, and Leukotrienes
Prostaglandins – mediate pains sensitivity, inflammation and swelling
Thromboxanes – involved in blood clotting, constriction of arteries
Leukotrienes – attract white cells, involved inflammatory diseases (asthma, arthritis, etc..)

3. Eicosanoids

4. Eicosanoid Synthesis
C20 unsaturated fatty acids (i.e. arachidonic acid (20:4D5,8,11,14) are precursors
Prostaglandins and Thromboxanes are synthesized by a cyclooxygenase pathway
Leukotirenes are synthesized by a lipoxygenase pathway
cyclooxygenase

5. Arachidonic acid present in membrane lipids are released for eicosanoid synthesis in the cell interior by phospholipase A2

6. Cyclooxygenase (COX) Inhibitors
Two COX isozymes: COX-1 and COX-2.
COX-1 – important in regulating mucin secretion in stomach
COX-2 – promotes pain and inflammation and fever (involved in prostaglandin synthesis).
Asprin (acetylsalicylate) non-specific COX inhibitor. Acts by acetylating an essential serine residue in the active site.
Because asprin inhibits COX-1, causes stomach upset and other side effects.
New drugs (Vioxx and Celebrex) specifically inhibit COX-2

8. Glycerolipid Biosynthesis
Important for the synthesis of membrane lipids and triacylglycerol
Synthesis occurs primarily in ER
Phosphatidic acid (PA) is the precursor for all other glycerolipids in eukaryotes
PA is made either into diacylglycerol (DAG) or CDP-DAG

9. Glycerolipid Biosynthesis
Phosphatidic acid is the precursor for all other glycerolipids

12. Isoprenoid Synthesis
Involves formation of isopentenyl pyrophosphate (IPP) monmers.
IPP is conjugated in a head to tail manner to generate polyprenyl compounds.

13. Formation of the isopentenyl pyrophosphate (IPP) via mevalonate pathway.
Primary pathway for isprenoid synthesis in animals and cytosolic isoprenoid synthesis in plants

14. Formation of the isopentenyl pyrophosphate (IPP)
Mevalonate
kinase
Phosphomevalonate
pyrohosphomevalonate
decarboxylase

15. Two Fates of HMG-CoA

16. Bacteria and Plants Synthesize IPP via Non-Mevalonate Pathway
In plants and most bacteria, IPP is synthesized from the condensation of glyceraldehyde-3-phosphate (3 carbons) and pyruvate (3 carbons).
Forms a 5 carbon intermediate through transketolase type reaction (transfer of 2 carbon aldehyde from pyruvate to G-3-P).
Occurs in chloroplast of plants. Involved in synthesis of chlorophyll, carotenoids, Vitamins A, E and K.

17. Very recent discovery (1996)
Pathway still not fully understood.
New pathway provides enzyme targets for new herbicidal and anti-microbial compounds

18. Condensation of IPP into Polyprenyl Compounds
isomerase
Dimethylallyl
pryophosphate

19. Cholesterol Synthesis from IPP
IPP Isomerase
prenyltransferase
Squalene synthase
Cholesterol Synthesis from IPP

20. cholesterol Squalene monooxygenase 2,3-oxidosqualene
lanosterol cyclase
20 steps
cholesterol

21. Regulation of HMG-CoA Reductase
As rate-limiting step, it is the principal site of regulation in cholesterol synthesis
1) Phosphorylation by cAMP-dependent kinases inactivates the reductase
2) Degradation of HMG-CoA reductase - half-life is 3 hrs and depends on cholesterol level
3) Gene expression (mRNA production) is controlled by cholesterol levels

22. Inhibiting Cholesterol Synthesis
HMG-CoA reductase is the key - the rate-limiting step in cholesterol biosynthesis
Lovastatin (mevinolin) blocks HMG-CoA reductase and prevents synthesis of cholesterol
Lovastatin is an (inactive) lactone
In the body, the lactone is hydrolyzed to mevinolinic acid, a competitive (TSA!) inhibitor of the reductase, Ki = 0.6 nM!