COMPLEX LIPID METABOLISM

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Phospholipids are:

– major constituents of all cell membranes

– components of bile

– anchor some proteins in membranes

– signal mediators

– components of lung surfactant

– components of lipoproteins

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SProperties of phospholipids• Phospholipids are amphipathic molecules

• Head group = alcohol attached via phosphodiester linkage to either:

– diacylglycerol (glycerophospholipid) or

– sphingosine (sphingophospholipid = sphingomyelin).

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SCellular membranes are composed of

phospholipids and sphingolipids• Glycerophospholipids and sphingolipids spontaneously self-

associate 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

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STypes of phospholipids

• The simplest glycerophospholipid is phosphatidic acid (PA)

• It consists of glycerol, phosphate, and 2 fatty acyl chains in ester linkages

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STypes of phospholipids

Other glycero-phospholipids derived from PA include:

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SCardiolipin is found in mitochondrial membranes

Phospholipids are distributed asymmetricallyin the plasma membrane

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SPlasmalogens

• Plasmalogens have an ether-linked hydrocarbon chain at C-1 of glycerol, instead of ester-linked fatty acid

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SPlasmalogens

• 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)

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SSphingolipids

• Sphingomyelin contains sphingosine with a long-chain fatty acid attached in amide linkage ( = ceramide)

• Ceramide plus a phosphocholine group constitutes a sphingomyelin

• Ceramide is also the core component of glycosphingolipids

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SSphingomyelin

• Sphingomyelin is present in plasma membranes and in lipoproteins

• It is very abundant in myelin

• 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

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Phospholipid synthesis

• Recall synthesis of PA as an intermediate of TG synthesis

• It involves glycerol-P and two fatty acyl CoA molecules

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SPhospholipid synthesis

• Glycerophospholipid synthesis involves activated intermediates:

– CDP-alcohol + diacylglycerol or

– CDP-diacylglycerol + alcohol

• Synthesis occurs in the ER of almost all cells

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SSynthesis 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

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SSynthesis of PS & PI

• PS is made by a base exchange reaction:

PE + serine PS + ethanolamine

• PI is synthesized from CDP-diacylglycerol and myoinositol

• PI often has arachidonate in the C-2 glycerol position

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Roles of phosphatidylinositol - I

• PI can provide arachidonate for eicosanoid synthesis

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SRoles 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

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SRoles of phosphatidylinositol - III

• PI anchors some enzymes to the plasma membrane through a glycan chain

• Examples include alkaline phosphatase and acetylcholine esterase

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SSynthesis of sphingomyelin

• Sphingomyelin is made from:

– palmitoyl CoA + serine sphingosine

– sphingosine + FA CoA ceramide

– ceramide + CDP-choline sphingomyelin

• FA are commonly 18:0, 24:0, and 24:1 (15)

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SPhospholipid 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 glycero-phospholipids: phospholipases A1, A2, C, and D

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Phospholipases

• Phospholipases A1 and A2 are also important in the remodeling of phospholipids

• FA CoA is then used in reesterification, e.g., to form the dipalmitoylphosphatidylcholine found in lung surfactant or arachidonic acid in PI

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SSphingomyelin degradation

• Sphingomyelin is degraded in lysosomes by sphingomyelinase to give ceramide,

• and ceramidase to give sphingosine

• Niemann-Pick disease is due to sphingomyelinase deficiency

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SGlycolipids

• 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

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SRoles of glycolipids

• Glycolipids have important roles in cell interactions, growth, and development

• They are very antigenic (e.g., blood group antigens);

• act as surface receptors for some toxins and viruses;

• and undergo major changes during cell transformation

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SGlycolipid structure — cerebrosides

• The carbohydrate component is linked by an O-glycosidic bond to ceramide

• Cerebrosides contain a single sugar (Glu or Gal) or few sugars; they are abundant in brain and myelin

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SGlycolipid 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)

Ganglioside GM2

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SGlycolipid 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, CMP-NANA

• Sulfate groups are added last by a sulfotransferase using PAPS (3'-phosphoadenosine-5'-phosphosulfate)

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SGlycolipid degradation

• Degradation of glycosphingolipids occurs in lysosomes after endocytosis of membrane portions

• A series of acid hydrolases participate in the degradation

• Degradation is sequential in the order: last on, first off

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SGlycolipid degradation

• Sphingolipidoses result from deficiencies of specific degradative enzymes

• They are diagnosed by accumulation of specific sphingolipid, enzyme activity measurements, and histologic examination of affected tissue

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Some sphingolipidoses

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Fabrazyme® = α-galactosidase A

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SEicosanoids

• Eicosanoids are specialized FA

• They include prostaglandins (PG), thromboxanes (TX), and leukotrienes (LT)

• Eicosanoids have strong hormone-like actions in the tissues where they are produced

• Eicosanoids are not stored and are very unstable

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SEicosanoid 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

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SSynthesis 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

• Initially yields PGH2

• Subsequent steps lead to thromboxane A2 and various prostaglandins

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SSynthesis of leukotrienes from arachidonate

• Leukotrienes are produced from arachidonic acid via a different enzyme: lipoxygenase

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SBiological 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

• Excess production symptoms: pain, inflammation, fever, nausea, vomiting

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Some major polyunsaturated fatty acids

Name Structure Type Significance

Linoleate 18:2(9,12) ω-6 Essential FA

Linolenate 18:3(9,12,15) ω-3 Essential FA

Arachidonate 20:4(5,8,11,14) ω-6 Prostaglandin precursor

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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)

Metabolism of linoleate versus linolenate into polyunsaturated fatty acids (PUFAs):

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SOmega-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

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SInhibitors 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

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SCyclooxygenase

• 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.

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SCyclooxygenase

• COX-2 expression is stimulated by growth factors, cytokines, and endotoxin

• COX-2 levels increase in inflammatory disease states such as arthritis and cancer

• Up-regulation of COX-2 is responsible for the increased formation of prostaglandins associated with inflammation

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SNext generation NSAIDs

• Older NSAIDs inhibit both inhibit both COX-1 & COX-2:

– acetylsalicylate (Aspirin®, Anacin®, etc.)

– ibuprofen (Motrin IB®, Advil®, etc.)

• Newer generation drugs are specific COX-2 inhibitors:

– Celebrex®

– Vioxx®