COMPLEX LIPID METABOLISM

COMPLEX LIPID METABOLISM Phospholipids are: – major constituents of all cell membranes – components of bile – anchor some proteins in membranes – signal mediators – components of lung surfactant PHOSPHOLIPIDS – components of lipoproteins Properties of phospholipids • Phospholipids are amphipathic molecules • Head group = alcohol attached via phosphodiester linkage to either: – diacylglycerol (glycerophospholipid) or – sphingosine (sphingophospholipid = sphingomyelin). PHOSPHOLIPIDS Cellular membranes are composed of phospholipids and sphingolipids • Glycerophospholipids and sphingolipids spontaneously selfassociate in water to form bilayer vesicles (i.e., closed membranes) • Bilayers are permeability barriers that enclose cells and cell organelles, and “dissolve” intrinsic membrane proteins PHOSPHOLIPIDS Types of phospholipids • The simplest glycerophospholipid is phosphatidic acid (PA) PHOSPHOLIPIDS • It consists of glycerol, phosphate, and 2 fatty acyl chains in ester linkages Types of phospholipids Other glycerophospholipids derived from PA include: PHOSPHOLIPIDS Cardiolipin is found in mitochondrial membranes PHOSPHOLIPIDS Phospholipids are distributed asymmetrically in the plasma membrane Outside Inside Plasmalogens • Plasmalogens have an ether-linked hydrocarbon chain at C-1 of glycerol, instead of ester-linked fatty acid PHOSPHOLIPIDS Plasmalogens • Platelet-activating factor (PAF) is a plasmalogen (a phosphatidalcholine) with an acetyl group at C-2 of glycerol • It has potent physiologic actions (platelet activation; inflammatory responses; bronchoconstriction) PHOSPHOLIPIDS Sphingolipids • Sphingomyelin contains sphingosine with a long-chain fatty acid attached in amide linkage ( = ceramide) PHOSPHOLIPIDS • Ceramide plus a phosphocholine group constitutes a sphingomyelin • Ceramide is also the core component of glycosphingolipids Sphingomyelin • Sphingomyelin is present in plasma membranes and in lipoproteins • It is very abundant in myelin PHOSPHOLIPIDS • Sphingomyelin is abundant in specialized plasma membrane microdomains called lipid rafts Lipid rafts • Lipid rafts are specialized microdomains in the plasma membrane that are rich in sphingomyelin and cholesterol • GPI-linked proteins accumulate in lipid rafts • Lipid rafts appear to function in signaling Phospholipid synthesis PHOSPHOLIPIDS • Recall synthesis of PA as an intermediate of TG synthesis • It involves glycerol-P and two fatty acyl CoA molecules Phospholipid synthesis • Glycerophospholipid synthesis involves activated intermediates: – CDP-alcohol + diacylglycerol or – CDP-diacylglycerol + alcohol • Synthesis occurs in the ER of almost all cells PHOSPHOLIPIDS Synthesis of PC • Choline can be made from ethanolamine by transfer of 3 methyl groups from S-adenosyl-methionine • Choline is an essential nutrient • De novo synthesis of PC from PS involves a decarboxylation to give PE followed by three methylation steps PHOSPHOLIPIDS Synthesis of PS & PI • PS is made by a base exchange reaction: PE + serine PS + ethanolamine PHOSPHOLIPIDS • PI is synthesized from CDP-diacylglycerol and myoinositol • PI often has arachidonate in the C-2 glycerol position Roles of phosphatidylinositol - I • PI can provide arachidonate for eicosanoid synthesis PHOSPHOLIPIDS Roles of phosphatidylinositol - II • Phosphatidylinositol 4,5-bisphosphate (PIP2) participates in hormonal signal transduction via activated phospholipase C formation of inositol-P3 and diacylglycerol, followed by mobilization of Ca+2 and activation of protein kinase C PHOSPHOLIPIDS Roles of phosphatidylinositol - III • PI anchors some enzymes to the plasma membrane through a glycan chain • Examples include alkaline phosphatase and acetylcholine esterase PHOSPHOLIPIDS Synthesis of sphingomyelin • Sphingomyelin is made from: – palmitoyl CoA + serine sphingosine PHOSPHOLIPIDS – sphingosine + FA CoA ceramide – ceramide + CDP-choline sphingomyelin • FA are commonly 18:0, 24:0, and 24:1 (15) Phospholipid degradation • Glycerophospholipid degradation occurs by phospholipases present in tissues (membrane bound or free), pancreatic juice, and venoms • Phospholipases are specific for ester bonds in the glycerophospholipids: phospholipases A1, A2, C, and D PHOSPHOLIPIDS Phospholipases • Phospholipases A1 and A2 are also important in the remodeling of phospholipids PHOSPHOLIPIDS • FA CoA is then used in reesterification, e.g., to form the dipalmitoylphosphatidylcholine found in lung surfactant or arachidonic acid in PI Sphingomyelin degradation • Sphingomyelin is degraded in lysosomes by sphingomyelinase to give ceramide, PHOSPHOLIPIDS • and ceramidase to give sphingosine • Niemann-Pick disease is due to sphingomyelinase deficiency Glycolipids • Glycolipids are derivatives of ceramides and sphingosine with carbohydrate directly attached to ceramide • In contrast to sphingomyelin they do not have a phosphocholine group • Glycolipids are essential components of cell plasma membranes (outer leaflet), but are most abundant in nervous tissues GLYCOLIPIDS Outside Inside Roles of glycolipids • Glycolipids have important roles in cell interactions, growth, and development • They are very antigenic (e.g., blood group antigens); GLYCOLIPIDS • act as surface receptors for some toxins and viruses; • and undergo major changes during cell transformation Glycolipid structure — cerebrosides • The carbohydrate component is linked by an Oglycosidic bond to ceramide • Cerebrosides contain a single sugar (Glu or Gal) or few sugars; they are abundant in brain and myelin GLYCOLIPIDS Glycolipid structure — gangliosides • Gangliosides are acidic glycosphingolipids • They contain oligosaccharides with terminal, charged N-acetyl neuraminic acids (NANA) • Depending on the number of NANA sugars, gangliosides are designated M, D, T, Q (e.g., GM) GLYCOLIPIDS Ganglioside GM2 Glycolipid synthesis • Synthesis of glycosphingolipids takes place in the ER and Golgi by the sequential addition of sugars by specific glycosyltransferases • The sugars are activated: UDP-Glu, UDP-Gal, CMPNANA • Sulfate groups are added last by a sulfotransferase using PAPS (3'-phosphoadenosine-5'-phosphosulfate) GLYCOLIPIDS Glycolipid degradation • Degradation of glycosphingolipids occurs in lysosomes after endocytosis of membrane portions GLYCOLIPIDS • A series of acid hydrolases participate in the degradation • Degradation is sequential in the order: last on, first off Glycolipid degradation • Sphingolipidoses result from deficiencies of specific degradative enzymes GLYCOLIPIDS • They are diagnosed by accumulation of specific sphingolipid, enzyme activity measurements, and histologic examination of affected tissue Some sphingolipidoses GLYCOLIPIDS GLYCOLIPIDS Fabrazyme® = α-galactosidase A Eicosanoids • Eicosanoids are specialized FA • They include prostaglandins (PG), thromboxanes (TX), and leukotrienes (LT) EICOSANOIDS • Eicosanoids have strong hormone-like actions in the tissues where they are produced • Eicosanoids are not stored and are very unstable Eicosanoid synthesis • Dietary linoleic acid is the precursor. It is elongated and further desaturated to 20-carbon, 3, 4, or 5 double bond FAs • Arachidonate, 20:4 (5, 8, 11, 14), is the precursor of many eicosanoids • Arachidonate is normally part of membrane phospholipids (especially phosphatidylinositol). • Arachidonate is released by a specialized phospholipase A2 EICOSANOIDS Synthesis of prostaglandins from arachidonate • The free arachidonic acid is oxidized and cyclized in the ER by endoperoxide synthase ( = PGH2 synthase) • This enzyme has two activities – cyclooxygenase (COX) and peroxidase EICOSANOIDS • Initially yields PGH2 • Subsequent steps lead to thromboxane A2 and various prostaglandins Synthesis of leukotrienes from arachidonate EICOSANOIDS • Leukotrienes are produced from arachidonic acid via a different enzyme: lipoxygenase EICOSANOIDS Biological actions of eicosanoids • Biologic actions of eicosanoids are diverse in various organs: – vasodilation, constriction, platelet aggregation, inhibition of platelet aggregation, contraction of smooth muscle, chemotaxis of leukocytes, release of lysosomal enzymes EICOSANOIDS • Excess production symptoms: pain, inflammation, fever, nausea, vomiting Some major polyunsaturated fatty acids Name Linoleate Linolenate Arachidonate Structure 18:2(9,12) 18:3(9,12,15) Type ω-6 ω-3 Significance Essential FA Essential FA Prostaglandin precursor 20:4(5,8,11,14) ω-6 EICOSANOIDS Metabolism of linoleate versus linolenate into polyunsaturated fatty acids (PUFAs): Linoleate (18:2) (ω-6) arachidonate (AA) (20:4) (ω-6) Linolenate (18:3)(ω-3) eicosapentanoic acid (EPA) (20:5) (ω-3) and docosahexanoic acid (DHA) (22:6) (ω-3) EICOSANOIDS Omega-3 fatty acids • EPA & DHA are precursors for different eicosanoids than arachidonate • When we were evolving, dietary ratio of ω-6 FA (linoleate) to ω-3 FA (linolenate) was about 1:1 to 2:1 • Currently it is about 10:1 to 20:1 in Western diets • Fish oils have high content of ω-3 FA EICOSANOIDS Inhibitors of prostaglandin synthesis • Corticosteroids (e.g., cortisol) inhibit at the level of phospholipase A2 • Antiinflammatory drugs (NSAIDS) like indomethacin & ibuprofen reversibly inhibit COX • Aspirin irreversibly inactivates COX EICOSANOIDS Cyclooxygenase • There are at least two isozymes of PGH2 Synthase (COX-1 and COX-2) • COX-1 is constitutively expressed at low levels in many cell types • Specifically, COX-1 is known to be essential for maintaining the integrity of the gastrointestinal epithelium. EICOSANOIDS Cyclooxygenase • COX-2 expression is stimulated by growth factors, cytokines, and endotoxin • COX-2 levels increase in inflammatory disease states such as arthritis and cancer EICOSANOIDS • Up-regulation of COX-2 is responsible for the increased formation of prostaglandins associated with inflammation Next generation NSAIDs • Older NSAIDs inhibit both inhibit both COX-1 & COX-2: – acetylsalicylate (Aspirin®, Anacin®, etc.) – ibuprofen (Motrin IB®, Advil®, etc.) EICOSANOIDS • Newer generation drugs are specific COX-2 inhibitors: – Celebrex® – Vioxx®