Synthesis of Eicosanoids, Glycerolipids and Isoprenoids

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..)

Eicosanoids

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

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

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

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

Glycerolipid Biosynthesis Phosphatidic acid is the precursor for all other glycerolipids

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

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

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

Two Fates of HMG-CoA

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.

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

Condensation of IPP into Polyprenyl Compounds isomerase Dimethylallyl pryophosphate

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

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

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

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!