Complex lipid metabolism

Complex lipids Complex lipids on hydrolysis yield one or more fatty acids, an alcohol, and some other type of compounds They are categorized in: Phospholipids Glycolipids (also called cerebrosides due to their existence in cerebrum of the brain)

1. 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

Properties of phospholipids Phospholipids are amphipathic molecules Head group = alcohol attached via phosphodiester linkage to either: diacylglycerol (glycerophospholipid) or sphingosine (sphingophospholipid = sphingomyelin).

Cellular 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

Types of phospholipids The simplest glycerophospholipid is phosphatidic acid (PA) It consists of glycerol, phosphate, and 2 fatty acyl chains in ester linkages

Other glycero-phospholipids derived from PA include:

Cardiolipin is found in mitochondrial membranes

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. For example, 1. phosphatidalethanolamine (abundant in nerve tissue) 2. Phosphatidalcholine (abundant in heart muscle) is the other quantitatively significant ether lipid in mammals.

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)

Sphingolipids 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

Sphingomyelin 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

Phospholipid synthesis Recall synthesis of PA as an intermediate of TG synthesis It involves glycerol-P and two fatty acyl CoA molecules

Phospholipid biosynthesis Glycerophospholipid synthesis involves activated intermediates: CDP-alcohol + diacylglycerol or CDP-diacylglycerol + alcohol Synthesis occurs in the ER of almost all cells

Synthesis of PC Choline can be made from ethanolamine by transfer of 3 methyl groups from S-adenosyl-methionine (SAM) Choline is an essential nutrient De novo synthesis of PC from PS involves a decarboxylation to give PE followed by three methylation steps

SAM is S-adenosylmethionine: a methyl group donor for many biochemical reactions

Synthesis 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

Role of phosphatidylcholine in lung surfactant The synthesis of dipalmitoylphosphatidylcholine (DPPC, or dipalmitoyl lecithin) is very important in lung functioning. DPPC, made and secreted by type II pneumocytes , is a major lipid component of lung surfactant, which is the extracellular fluid layer lining the alveoli. Surfactant serves to decrease the surface tension of this fluid layer, reducing the pressure needed to reinflate alveoli, thereby preventing alveolar collapse (atelectasis).

Surfactant is a complex mixture of lipids (90%) and proteins (10%), with DPPC being the major component for reducing surface tension. Respiratory distress syndrome (RDS) in preterm infants is associated with insufficient surfactant production and/or secretion and is a significant cause of all neonatal deaths in Western countries.

The respiratory distress syndrome (RDS) of a premature infant Note: Lung maturation can be accelerated by giving the mother glucocorticoids shortly before delivery. Why? RSD which is due to an insufficient amount of surfactant can also occur whose surfactant- producing pneumocytes have been damaged (an adverse side effect of immunosuppressive medication or chemotherapeutic drug use.

Roles of phosphatidylinositol PI can provide arachidonate for eicosanoid synthesis 2. Signal transduction across membranes PI anchors some enzymes to the plasma membrane through a glycan chain

2 . Signal transduction across membranes Ach, NE, Serotonin

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.

3. PI anchors some enzymes to the plasma membrane through a glycan chain Examples include alkaline phosphatase and acetylcholine esterase (an enzyme of the postsynaptic membrane that degrades the neurotransmitter acetylcholine)

Degradation of phospholipids: Phospholipases allow degradation & Remodeling of Phosphoglycerols Long-chain saturated fatty acids are found predominantly in the 1 position of phospholipids. Polyunsaturated acids (eg, the precursors of prostaglandins) are incorporated more into the 2 position, which is frequently arachidonic acid. Phospholipase A1 removes the fatty acyl group on carbon 1 of the glycerol moiety, and phospholipase A2 removes the fatty acid on carbon 2. Removal of the fatty acid moiety from C1 or C2 produces a lysophosphoglyceride, which is the substrate of lysophospholipases.

Phospholipases allow degradation & Remodeling of Phosphoglycerols Lysophospholipid can be attacked by lysophospholipase, forming the corresponding glyceryl phosphoryl base, which in turn may be split by a hydrolase liberating glycerol 3- phosphate plus base. Alternatively lysophospholipid, which in turn may be reacylated by acyl-CoA in the presence of an acyltransferase (Remodeling). Lysophosphatidylcholin may be formed by an alternative route that involves lecithin: cholesterol acyltransferase (LCAT). LCAT is found in plasma and catalyzes the transfer of a fatty acid residue from the 2 position of lecithin to cholesterol to form cholesteryl ester and lysolecithin.

1. Synthesis of sphingomyelin Sphingomyelin is made from: palmitoyl CoA + serine sphingosine sphingosine + FA CoA ceramide ceramide + CDP-choline sphingomyelin

Sphingomyelin degradation Sphingomyelin is degraded in lysosomes by sphingomyelinase to give ceramide, and ceramidase to give sphingosine Niemann-Pick disease is due to sphingomyelinase deficiency

2. 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 Outside Inside

Roles 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.

Glycolipid 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

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

Glycolipid 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

Glycolipid 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

Some sphingolipidoses

Fabrazyme® = α-galactosidase A

Eicosanoids 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

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

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 Initially yields PGH2 Subsequent steps lead to thromboxane A2 and various prostaglandins

Synthesis of leukotrienes from arachidonate Leukotrienes are produced from arachidonic acid via a different enzyme: lipoxygenase

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

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

Linoleate (18:2) (ω-6) arachidonate (AA) (20:4) (ω-6) 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)

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

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

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.

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

Next generation NSAIDs Older NSAIDs 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®