1. COMPLEX LIPID METABOLISM

2. 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

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

4. 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, and “dissolve” intrinsic membrane proteins

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

6. Types of phospholipids
Other glycero-phospholipids derived from PA include:

7. Cardiolipin is found in mitochondrial membranes

8. Phospholipids are distributed asymmetrically in the plasma membrane
Outside
Inside

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

10. 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)

11. 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

12. 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

13. 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

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

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

16. 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

17. 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

18. Roles of phosphatidylinositol - I
PI can provide arachidonate for eicosanoid synthesis

19. 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

20. Roles of phosphatidylinositol - III
PI anchors some enzymes to the plasma membrane through a glycan chain
Examples include alkaline phosphatase and acetylcholine esterase

21. Synthesis 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)

22. 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 glycero-phospholipids: phospholipases A1, A2, C, and D

23. 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

24. 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

25. 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

26. 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;
and undergo major changes during cell transformation

27. 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

28. 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

29. 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)

30. 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

31. 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

32. Some sphingolipidoses

33. Fabrazyme® = α-galactosidase A

34. 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

35. 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

36. 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

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

39. 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

40. 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

41. 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)

42. 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

43. 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

44. 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.

45. 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

46. Next 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®