Lipids and Membranes

1. Lipids and Membranes

2. Energy reserves (particularly fatty acids, lipids with long hydrocarbon chains) - There is a large energy yield upon oxidation of these highly reduced hydrocarbons. As lipid bilayers, main components of biological membranes. Intra- and intercellular signaling. Functions of Lipids

3. Structures of Ionized Form of Some Representative Fatty Acids pKa ≈ 4.5 Rigid bend (~30º) in hydrocarbon chain of oleic acid/oleate due to presence of cis double bond.

4. Some Fatty Acids • Saturated: no double bonds • Unsaturated: one (monounsaturated) or more (polyunsaturated) double bonds (almost always cis configuration) 18:0 18:2c∆9,12 18:1c∆9 18:3c∆9,12,15

5. Some Biologically Important Fatty Acids Most fatty acids have an even number of carbons because they are synthesized by concatenation of activated two-carbon units (acetyl-CoA).

6. Triacylglycerols: Fats • Major energy reservoir • Very efficient way to store metabolic energy (less oxidized than carbohydrates or proteins)

7. Soaps and Detergents • Hydrolysis of fats with alkali such as NaOH or KOH yields soaps (saponification), salts of ionized fatty acids. • Synthetic detergents: Sodium dodecyl sulfate Triton X-100

8. Waxes • Formed through esterification of fatty acid and long-chain alcohol • Completely water-insoluble • Water-repellent protective coating in some animals and plants • Energy storage in some microorganisms

9. Glycerophospholipids (Phosphoglycerides):Main Lipid Components of Biological Membranes Naturally occuring glycerophospholipids have L stereochemistry.

10. Glycerophospholipid Structure Kink or bend in one fatty acyl chain in this phospholipid because of cis double bond Hydrophilic “head” group Hydrophobic hydrocarbon “tails” = fatty acid-derived side chains = acyl chains

11. The Hydrophilic Head Groups of Major Glycerophospholipids

12. Phospholipids and Membrane Structure Bilayer Micelle

13. Phospholipid Hydrolysis by Phospholipases

15. Phospholipase A2 Bound to a Phospholipid

16. Phosphatidylinositol-4,5-Bisphosphate (PIP2) Hydrolysis and Signal Transduction Along with Ca2+ (released from ER as described below), DG activates protein kinase C. IP3 binds to Ca2+ channels on ER membrane, causing them to open and release of Ca2+ into cytoplasm. DG, IP3, Ca2+ are examples of “second messengers” that transmit signals inside the cell, leading to cellular response.

17. The PIP2 Hydrolysis Pathway

18. Lipid Composition of Some Biological Membranes

19. Sphingolipids: Another Major Class of Lipids Found in Biological Membranes Black = sphingosine Black + red = a ceramide Black + red + blue = a sphingomyelin Ceramide Sphingosine = amino alcohol with a long hydrocarbon chain. Ceramide = sphingosine with a fatty acid linked by an amide bond to the amine to form an N-acyl chain. Sphingomyelin = ceramides with a phosphocholine head group. The myelin sheath that surrounds and electrically insulates many nerve cell axons is rich in sphingomyelin.

20. Glycosphingolipids Cerebrosides = ceramides with a single sugar residue as head group. Gangliosides = ceramides with attached oligosaccharide as head group containing at least one sialic acid residue.

21. Gangliosides Gangliosides constitute a significant fraction (~6%) of brain lipids. The ABO blood group antigens are also examples of gangliosides.

22. Cholesterol: The Third Major Class of Lipid in Biological Membranes

23. Cholesterol Biosynthesis Cholesterol is just one of many isoprenoids (or terpenes), lipids derived from isoprene units, which includes other steroids and non-steroidal lipids, such as bile acids, lipid-soluble vitamins, certain coenzymes, etc. Activated, five-carbon “isoprene units”

24. Cholesterol Is the Metabolic Precursor of Steroid Hormones

25. Vitamins D Are Sterol Derivatives

26. An Example of Other Types of Lipids: The Eiconasoids Prostaglandins

27. Lipid Bilayers and Biological Membranes

28. Structure of Phospholipid Bilayer

29. The Gel-Liquid Crystalline Transition in a Lipid Bilayer and Factors Affecting the Transition • Presence of Cholesterol • Moderate concentrations of cholesterol broaden transition, making membrane appear more fluid at lower temperatures yet less fluid at higher temperatures. • Bulky, rigid sterol ring structure of cholesterol prevents tight packing of phospholipid acyl chains at low temperatures. • However, the rigid ring structure also reduces mobility of phospholipid side chains at higher temperatures. • Degree of Unsaturation of Fatty Acid Side Chains • Presence of phospholipids with unsaturated fatty acyl chains reduces transition temperature, making membrane more “fluid.” • Bend produced by cis double bonds prevents close packing of side chains at lower temperature.

30. The Gel-Liquid Crystalline Transition in a Lipid Bilayer and Temperature

31. A Model of the Effects of Cholesterol on Plasma Membrane Structure

32. Experimental Demonstration of Biological Membrane Fluidity

33. Diffusion of Lipids in Bilayers Translocases or flippases: protein catalysts that facilitate transverse diffusion (flip-flop) of lipids in biological membranes.

34. Phospholipid Asymmetry in Plasma Membranes Erythrocyte membrane

35. New Lipids Inserted into Inner Leaflet of Membrane Orange = newly synthesized, radioactive lipids Flip-flop rate in biological membrane ~100,000 faster than in artificial lipid bilayer, demonstrating efficiency of translocases. TNBS = trinitrobenzenesulfonic acid (TNBS), cell-impermeant reagent that reacts with phosphatidylethanolamine

36. Structure of a Typical Cell Membrane Fluid Mosaic Model (Singer and Nicholson, 1972).

37. Protein, Lipid and Carbohydrate Compositions of Some Membranes

38. Membrane-Bound Proteins • Integral membrane proteins - span lipid bilayer; can only be removed from membrane with strong treatments such as detergents or organic solvents. • Lipid-linked proteins - interact with membrane via post-translationally attached lipid moeity. • Peripheral membrane proteins - weakly associated with membrane; can be dissociated with mild treatments such as high ionic strength salt solutions or pH changes.

39. Example of a Lipid Attachment in a Lipid-Linked Protein Glycophosphosphatidylinositol (GPI) anchor of GPI-linked proteins • Other types of lipid-linked proteins: • Prenylated = lipid attachment (commonly C15 or C20) built from isoprene (C5) units • Fatty acylated = lipid attachment is fatty acid

40. Protein Prenylation

41. Model of the Structure of the Erythrocyte Membrane Skeleton

42. Major Proteins of the Human Erythrocyte Membrane

43. Glycophorin A Polypeptide Has a Single Membrane-Spanning a-Helix Extracellular domain Intracellular domain Transmembrane domain

44. Hydropathy/Hydrophobicity Plots Glycophorin A Erythrocyte glucose transporter Bacteriorhodopsin

45. Bacteriorhodopsin: A Protein with Multiple Membrane-Spanning a-Helices

46. Another Multiple-Pass Transmembrane Protein: The Photosynthetic Reaction Center from a Purple Bacterium

47. E. coli OmpF Porin: Transmembrane b Barrels

48. Vesicle Trafficking and Biosynthesis of Transmembrane and Secreted Proteins in Eukaryotes

49. Transport Across Membranes

50. Free energy change (chemical potential difference) for transporting 1 mole of a substance from region where its concentration is C1 (e.g., Cout) to region where its concentration is C2 (e.g., Cin): ∆G = RT ln(C2/C1) (favorable with ∆G < 0 if C2 < C1) Transport of ions across membrane (must consider electrical potential in addition to concentration difference): ∆G = RT ln(C2/C1)+ ZF ∆ (Z=charge of ion, F=Faraday’s constant, ∆=membrane electrical potential in volts) Coupled transport (active transport): ∆G = RT ln(C2/C1)+ ∆G´ (∆G´ of coupled process, such as ATP hydrolysis, may be negative enough to compensate for unfavorable transport against concentration gradient when RT ln (C2/C1)> 0) Thermodynamics of Transport