1 / 87

Lipids and Membranes

Lipids and Membranes. 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.

kstacey
Download Presentation

Lipids and Membranes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

  14. Phospholipase A2 Bound to a Phospholipid

  15. 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.

  16. The PIP2 Hydrolysis Pathway

  17. Lipid Composition of Some Biological Membranes

  18. 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.

  19. 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.

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

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

  22. 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”

  23. Cholesterol Is the Metabolic Precursor of Steroid Hormones

  24. Vitamins D Are Sterol Derivatives

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

  26. Lipid Bilayers and Biological Membranes

  27. Structure of Phospholipid Bilayer

  28. 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.

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

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

  31. Experimental Demonstration of Biological Membrane Fluidity

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

  33. Phospholipid Asymmetry in Plasma Membranes Erythrocyte membrane

  34. 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

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

  36. Protein, Lipid and Carbohydrate Compositions of Some Membranes

  37. 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.

  38. 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

  39. Protein Prenylation

  40. Model of the Structure of the Erythrocyte Membrane Skeleton

  41. Major Proteins of the Human Erythrocyte Membrane

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

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

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

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

  46. E. coli OmpF Porin: Transmembrane b Barrels

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

  48. Transport Across Membranes

  49. 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

More Related