Lecture 18 Membranes 1: Lipids and Lipid Bilayers

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1 Lecture 18 Membranes 1: Lipids and Lipid Bilayers Subsequent 3 lectures: Membrane Proteins 2 lectures on Membrane Transport Reading: Berg, Tymoczko & Stryer, 6th ed., Chapter 12, pp Problems: Chapter 12, p. 150, #9 Key Concepts Major functions of lipids: energy storage, major membrane components Other functions: signals, electron carriers, emulsifying agents... Membrane lipids (amphipathic) -- responsible for spontaneous formation of lipid bilayers Glycerophospholipids: glycerol backbone + 2 fatty acyl "tails" in ester linkage + a polar "head group (a phosphate ester of another alcohol like choline, ethanolamine, serine, inositol, etc.) Sphingolipids: sphingosine backbone (1 "tail") + fatty acid chain in amide linkage (another "tail") + either carbohydrate (glycosidic bond to sphingosine) or phosphate ester of another alcohol like choline or ethanolamine (ester bond to sphingosine) glycosphingolipids (cerebrosides, gangliosides) phosphosphingolipids (sphingomyelins) Cholesterol Membrane fluidity (vital to membrane function) depends on lipid composition of bilayer. fatty acid chainlength (more C atoms more packing of tails, less fluidity) fatty acid numbers of double bonds (fewer double bonds more packing of tails, less fluidity) cholesterol content ("buffers" fluidity) Bilayers 1

2 Learning Objectives Terminology: micelle, lipid bilayer, amphipathic List the biological roles and the molecular components of membranes. With the structure of a lipid as an example, point out the features that make a molecule amphipathic. Explain why amphipathic membrane lipids form self-sealing bilayers in aqueous environments, including the types of interactions stabilizing the bilayer structure. Write out the structure of a 16-carbon saturated fatty acid (i.e., no double bonds), and describe the general properties of the fatty acyl components of membrane lipids. Be able to recognize the structures of phosphoglycerides, phosphosphingolipids, glycosphingolipids, and cholesterol. What type of lipids are cerebrosides and gangliosides? Briefly explain the consequences if an individual has a genetic deficiency in any one specific enzyme involved in glycosphingolipid degradation. What bond in a glycerophospholipid is cleaved (hydrolyzed) by phospholipase A 1? A 2? C? D? Learning Objectives, continued Discuss how living organisms regulate the fluidity of their membranes, including in your discussion the effects on fluidity of temperature, fatty acyl chainlength, and number of double bonds. Discuss the concepts of lateral and transverse ( flip-flop ) diffusion of membrane lipids and proteins, and the asymmetric distribution of membrane components (especially carbohydrate portions) on the extracellular and intracellular sides of the bilayer. Describe the permeability properties of lipid bilayers. Bilayers 2

3 Biological Membranes sheet-like structures, a few molecules thick, forming closed boundaries (self-sealing) amphipathic lipids: polar "head" groups and nonpolar "tails With 2 hydrophobic "tails", amphipathic lipids form bilayers instead of micelles. Proteins carry out most of the specific functions. carbohydrates (covalently attached to lipids = glycolipids, or to proteins = glycoproteins) - important in communication/recognition noncovalent assembly (interactions between components) into a fluid 2-dimensional solution Proteins and lipids can diffuse rapidly in plane of membrane, but Proteins and lipids do not rotate across the membrane (no "flipflop" in orientation across membrane). asymmetric arrangement 2 sides (faces) different biosynthesized that way Components don t "flip-flop" their orientation. Membranes always synthesized by growth of preexisting membranes Amphipathic nature of membrane lipids hydrophilic portion and hydrophobic portion hydrophilic portion = "head"; hydrophobic chain(s) = "tails" Consequence: Amphipathic lipids form micelles or bilayers, to bury their hydrophobic tails so they're NOT exposed to H 2 O, but keep the hydrophilic head groups in contact with H 2 O. Lipids with single hydrophobic tails can form micelles, but Membrane lipids almost all have 2 tails, and thus form bilayers. Bilayers curve around and seal edges closed vesicles (liposomes). The hydrophobic effect provides the major driving force for the formation of lipid bilayers. slice through a micelle slice through a bilayer Berg et al., Fig Berg et al., Fig Bilayers 3

4 Liposomes lipid vesicles, aqueous compartments enclosed by a lipid bilayer experimental tools for studying membrane permeability vehicles for delivery to cells of chemicals/drugs/dna for gene therapy slice through a liposome Berg et al., Fig Membrane Functions 1) HIGHLY SELECTIVE PERMEABILITY BARRIERS regulate molecular & ionic compositions of cells and intracellular organelles a) channels and pumps (proteins that act as selective transport systems) b) electrical polarization of membrane (inside of plasma membrane negative, typically - 60 millivolts) (maintain different ionic concentrations on opposite sides of membrane) 2) INFORMATION PROCESSING - biological communication a) signal reception by specific protein receptors (BINDING) b) transmission/transduction of signals (via protein conformational changes) sometimes generation of signals, chemical or electrical, e.g.,nerve impulses 3) ENERGY CONVERSION - ordered arrays of enzymes and other proteins, organization of reaction sequences a) photosynthesis (light energy chemical bond energy): inner membranes of chloroplasts, and plasma membranes of some prokaryotes b) oxidative phosphorylation (oxidation of fuel molecules chemical bond energy "stored" in ATP): inner membranes of mitochondria, and plasma membranes of prokaryotes Bilayers 4

5 Lipid Components of Animal Cell Membranes LIPIDS (definition): water-insoluble biomolecules that are highly soluble in organic solvents Biological functions: fuels (highly concentrated energy stores) signaling molecules membrane components Membrane lipid functions: bilayer structure compartments/permeability barriers provide environment for proteins to work electrical insulation (e.g., myelin sheath on myelinated nerve fibers, but also maintenance of electrical potential in other cells) Membrane lipid distribution: functional significance of all the differences not really understood proportions of different lipids vary by type of membrane (plasma membrane vs. mitochondrial membrane vs. nuclear membrane, etc.) type of cell Membrane Assymmetry inner vs. outer "leaflets" [layers of bilayer] -- different lipid compositions, different proteins or protein domains asymmetry maintained because of extremely slow rate of rotation of components across membrane "flip-flop" essentially doesn t occur except when catalyzed by "flippases" (proteins involved in creating/maintaining lipid asymmetries across membrane) Carbohydrate components: on outer surface of membrane Glycolipids (have carbohydrate components) found only in outer leaflet of plasma membranes. Glycoproteins: Carbohydrate components found only on outsides of cells, even when protein itself spans membrane. Overall lipid composition related to environment (esp. temperature) - lipid composition regulates fluidity) Berg et al., Fig Bilayers 5

6 Fatty Acids Fatty acyl groups are components of membrane lipids, in ester or amide linkages. longchain carboxylic acids, typically C atoms C16 & C18 most common (amphipathic) RCOO with 0-4 double bonds, usually cis palmitate (16-C saturated F.A.) oleate (18-C unsaturated F.A., with 1 cis double bond. NOTE "kink" in structure) F.A.s are amphipathic molecules Berg et al., Fig Main classes of membrane lipids (all amphipathic) 3 types of BACKBONE in membrane lipids Glycerol (glycerophospholipids), a 3-carbon tri-alcohol: CH 2 OH-CHOH-CH 2 OH Sphingosine (sphingolipids, both sphingophospholipids and sphingoglycolipids) Cholesterol (a steroid compound) + Cholesterol (a steroid compound) Bilayers 6

7 1. Glycerophospholipids (phosphoglycerides, glycerophosphatides) start with glycerol backbone (3 carbon tri-alcohol, CH 2 OH-CHOH-CH 2 OH) diacylglycerol (fatty acids esterified to the C1 and C2 OH groups on glycerol; R 1 usually saturated, R 2 usually unsaturated; F.A.s usually C's) C3 esterified to phosphate That gives parent compound = phosphatidic acid (phosphatidate at ph 7) + another substituent also esterified to phosphate (any of several alcohols): ethanolamine, choline, serine, glycerol, inositol, phosphatidyl glycerol Berg et al., <-- Fig Berg et al.,fig Glycerophospholipids, continued Results of esterifying different alcohols to the phosphate on C3: phosphatidyl serine phosphatidyl choline (lecithin) phosphatidyl ethanolamine phosphatidyl inositol phosphatidyl glycerol diphosphatidyl glycerol (cardiolipin) Berg et al., Fig Bilayers 7

8 Phospholipase (PL) cleavage sites Phospholipases catalyze hydrolysis of ester bonds in phospholipids. PLA 1 cleaves ester bond to C1 OH PLA 2 cleaves ester bond to C2 OH PLC cleaves phosphate ester bond to C3 OH PLD cleaves phosphate ester bond to other alcohol on C3 phosphate (choline, ethanolamine, etc.) Phospholipase specificities > activity of phospholipases important in signaling pathways PLC generates 2 intracellular signaling molecules: diacylglycerol (DAG) and inositol phosphate (IP) PLA 2 removes arachidonic acid from membrane lipids for COX enzymes to make prostaglandins. Corticosteroid drugs like prednisone inhibit PLA 2. What effect would steroids have on inflammation? Nelson & Cox, Lehninger Principles of Biochemistry, 4th ed., Fig Sphingolipids backbone = sphingosine Similarity/differences with glycerol-based lipids (easier to see in figure on next slide): C1 has an OH group (can be esterified to phosphate, or in a glycosidic bond to carbohydrate) C2 has amino group (-NH 3+ ) instead of -OH on glycerol fatty acyl group in amide linkage (not ester) C3 has -OH group that does NOT get derivatized, and instead of one H atom on glycerol C3 has a long hydrocarbon chain, with 1 double bond, Ceramides have fatty acid in amide linkage to amino group of C2 in ALL sphingolipids. Nelson & Cox, Lehninger Principles of Biochemistry, 4th ed., Fig Bilayers 8

9 Structure comparison: glycerophospholipid and sphingophospholipid Note polar head groups and 2 nonpolar tails -- one of the tails on sphingolipid is the long chain of the sphingosine backbone continuing from C4-C18 Berg et al., Fig Sphingolipids Phosphosphingolipids: phosphate esterified to C1 OH. Sphingomyelins: choline or ethanolamine esterified to C1 phosphate Glycosphingolipids: especially abundant in nerve cell membranes; carbohydrate(s) on C1-OH instead of phosphate group Phosphosphingolipids Niemann-Pick types A&B: lack of enzyme to hydrolyze this bond Glycosphingolipids Tay-Sachs: lack of enzyme to hydrolyze this bond Bilayers 9

10 2. Sphingolipids, continued gangliosides (complex oligosaccharides, branched sugar chains on C1 OH) Degradation of lipids: specific enzymes required for each different bond hydrolyzed Membrane lipids undergo constant metabolic turnover, rate of synthesis and rate of breakdown being balanced. Genetic defects (deficiencies in specific enzymes) in glycosphingolipid breakdown abnormal accumulation of partially degraded lipids, with toxic results (genetic diseases). example: Tay-Sachs disease -- lack of hexosaminidase A, needed to hydrolyze glycosidic bond attaching terminal N-acetylgalactosamine residue in ganglioside GM2 (previous slide); causes mental retardation, blindness, muscular weakness, death by age 3-4 Electron micrograph of portion of a brain cell from infant with Tay-Sachs disease, showing abnormal ganglioside GM2 deposits in the lysosomes Niemann-Pick disease types A and B -- lack of sphingomyelinase, enzyme needed to hydrolyze phosphate ester linkage of phosphocholine to ceramide; symptoms include enlarged liver and spleen, mental retardation, early death Nelson & Cox, Lehninger Principles of Biochemistry, 4th ed., Box 10-2, Fig Cholesterol structure: 4 fused hydrocarbon rings, 3 with 6 C's, 1 with 5 C's ( steroid nucleus ) planar, rigid, electrically neutral amphipathic ("head" group = OH) mainly in plasma membranes of animal cells; organelle membranes generally have less; rarely found in bacteria functions: important membrane constituent (influences fluidity) precursor of bile acids (emulsifiers) precursor of hormones (steroid hormones) Bilayers 10

11 Other Lipids (not structural components of membranes, but biologically important) eicosanoids paracrine hormones (locally acting) all synthesized starting from arachidonic acid (20-carbon fatty acid with 4 double bonds, removed by phospholipase A 2 from position 2 of membrane glycerophospholipids) prostaglandins: mediate fever, inflammation and pain, among other functions thromboxanes (involved in blood clotting) leukotrienes (smooth muscle contraction, e.g., muscle lining airways to lungs -- overproduction causes asthmatic attacks and is involved in anaphylactic shock, potentially fatal allergic reaction) isoprenoid lipids (all synthesized by condensation of isoprene units (5 C unsaturated branched units) steroid hormones fat-soluble vitamins (A, D, E, and K) mobile electron carriers in membranes ubiquinone in mitochondrial membranes plastoquinone in chloroplast membranes sugar carriers (dolichols) MEMBRANE FLUIDITY -- controlled by lipid composition hydrocarbon chains: close packing, maximum interaction between chains at low temperatures rigid "gel"; the longer the chains and the more saturated (fewer double bonds), the more ordered/rigid the state of the lipid bilayer Above transition temperature, lipid bilayer undergoes phase change ("melting") to more disorderly, FLUID state (chains not so closely packed). Transition temperature is lowered (so relative fluidity increases) by fatty acid structures that reduce favorable packing interactions: a) shorter hydrocarbon chainlength, and/or b) more double bonds (which make "bends" in the chain) Highly ordered packing of fatty acid side chains (stabilized by lots of close van der Waals interactions) is disrupted by cis double bonds (kinks). With more double bonds, membrane remains fluid at lower temperatures (transition temp. is lowered). Berg et al., Fig Bilayers 11

12 Regulation of Membrane Fluidity Membranes of living cells must be fluid -- must have transition temperatures below body temperature of the organism. Regulation of fluidity (especially in organisms that don t rigorously control their body temperature) by lipid composition: 1. fatty acid chainlength (shorter more fluid) 2. number of double bonds (more d.b. more fluid) 3. Cholesterol (animal cells) "stiffens" membrane by packing between unsaturated HC tails, but also disrupts close packing between saturated tails, so broadens the transition sort of like a fluidity "buffer", when temperature or fatty acid composition changes. Rigid bilayer ( gel ) Fluid bilayer Berg et al., Fig Lipid Bilayers -- formed spontaneously by phospholipids Single-tailed amphipathic lipids form micelles in H 2 O (spheres with polar head groups out, exposed to H 2 O; nonpolar tails buried in center) "2-tailed" amphipathic lipids spontaneously form bilayers, burying the tails between the 2 layers; 2 tails (e.g., phosphoglycerides and sphingolipids) don t fit in middle of a micelle -- surface with head groups not large enough to bury double tails self-assembling and self-sealing -- form and grow spontaneously, and close in on themselves spontanously, because a "hole" would expose the lipid tails to the H 2 O. Bilayer structure stabilized by hydrophobic effect (the driving force for their formation) hydration of polar/charged head groups van der Waals interactions (packing between atoms in hydrophobic core) Hydrophobic core of the membrane is like a nonpolar solvent. Permeability coefficients correlated with solubility in nonpolar solvent relative to solubility in H 2 O. highly impermeable to ions and most polar molecules more permeable to nonpolar species Bilayers 12