生物化学原理课件（英文）：Chapter16 Lipids and biological membranes

1、Chapter16 Lipids highly reduced forms of carbon; yield large amount of energy upon oxidation in metabolism; Lipid Classification Simple lipids: Fatty Acids and their simple esters triacylglycerols and waxes Compound lipids: contain more than simple alcohols and fatty acids glycerophosphatides, sphin

2、golipids etc. Isoprenoid lipids: derived from isoprene steroids, fat soluble vitamins, terpenes etc. 5 fatty acids triacylglycerols phospholipids * Fat soluble vitamins steroids glycolipids waxes * * fatty acids absent eicosanoids 6 Functions Energy source storage compounds (triacyl glycerols) Hydro

3、phobic barriers membrane constituents (phospholipids Double bonds in fatty acids are usually have the cis configuration. Saturated fatty acids are single bonds in all carbon-carbon bonds; Most naturally occurring fatty acids have an even number of carbon atoms. Common Fatty Acids Saturated fatty aci

4、ds: Lauric acid12:0 Myristic acid14:0 Palmitic acid16:0 Stearic acid18:0 Unsaturated fatty acids: Palmitoleic acid16:1 Oleic acid18:1 Linoleic acid18:2 -linolenic acid18:3 (9,12,15) -linolenic acid18:3 (6,9,12) Arachidonic acid 20:4 EPA 20:5 (5,8,11,14,17) DHA 22:6(4,7,10,13,16,19) Essential Fatty A

5、cids (EFA) Linoleic acid - Linolenic Acid Structural Consequences of Unsaturation ESaturated chains pack tightly and form more rigid, organized aggregates (i.e., membranes); EUnsaturated chains bend and pack in a less ordered way, with greater potential for motion. Triglycerols (triglycerides) uTrig

6、lycerols consist of a glycerol esterified with three fatty acids uIf all fatty acid chains are the same, the molecule is called triacylglycerol Waxes Esters of long-chain alcohols with long-chain fatty acids Highly insoluble Animal skin and fur are wax- coated Leaves of many plants Bird feathers Gly

7、cerophospholipids Glycerophospholipids=1, 2-diacylglycerols that has a phosphate group esterified at carbon 3; Glycerophospholipids are a subclass of phospholipids; Glycerophospholipids are essential component of biological membranes; Phosphatidic Acid and Phosphatides Phosphatidic acid is the paren

8、t compound for the glycerol-based phospholipids; Phosphatidic acid consists of sn-glycerol-3-phosphate with fatty acids esterified at the 1- and 2-positions; Phosphatides are phosphatidic acid esterified with a hydroxyl-containing organic functional group. OPO O O H2C CH H2C OCR1 OOC O R2 phosphatid

9、ate OPO O O H2C CH H2C OCR1 OOC O R2 X glycerophospholipid polar non-polar kink due to double bond OPO O O H2C CH H2C OCR1 OOC O R2 X glycerophospholipid Each glycerophospholipid includes w a polar region: glycerol, carbonyl of fatty acids, Pi, Sphingosines are important components of biological mem

10、branes; Ceramide = sphingosine + fatty acid (via an amide linkage); Sphingomyelins = ceramide + phospholipids (via 1-hydroxyl group) Glycosphingolipids = ceramide + -linked sugar at the 1-hydroxyl moiety. H2C H C OH CH N+CH C CH2 CH3 H H3 OH ( )12 sphingosine H2C H C OH CH NHCH C CH2 CH3 H OH ( )12

11、C R O ceramide H2C H C O CH NHCH C CH2 CH3 H OH ( )12 C R O POO O H2 C H2 C N+ CH3 H3C CH3 Sphingomyelin phosphocholine sphingosine fatty acid Examples of Sphingolipids Sphingolipid Polar head group sphingomyelin phosphocholine or phosphoethanolamine cerebroside a monosaccharide such as glucose or g

12、alactose ganglioside a complex oligosaccharide, including the acidic sugar sialic acid Examples of Glycosphingolipids Terpenes Terpenes are a class of lipids from two or more molecules of 2-methyl-1,2-butadiene, or isoprene; All sterols (including cholesterol) and steroid hormones are terpene-based

13、molecules Example of a terpene molecule: Examples of terpene molecule Steroids Based on a core structure consisting of three 6-membered rings and one 5-membered ring, all fused together; Cholesterol is the most common steroid in animals and precursor for all other steroids in animals; Steroid hormon

14、es serve many functions in animals - including salt balance, metabolic function and sexual function. Steroid Examples Biological Membranes * Membrane Types * The Chemical Composition Of Membranes * Membrane Properties * Membrane Structure * Membrane Phase Transition * Membrane Function Types of Memb

15、ranes * Membranes surrounding the cell * Membrane systems inside the cell -The nucleus, endoplasmic reticulum, Golgi bodies, mitochondria, chloroplast, peroxisome -Endosomes and lysosomes form the endomembrane system Membrane Composition * lipids (including cholesterol)-Form bilayer * Proteins * Car

16、bohydrates (covalently attached to either lipids or proteins) Spontaneously Formed Lipid Structures C Hydrophobic interactions are important Head group Tail group Lipid is an amphipathic molecule, but rarely exists as a monomer. monolayer air water Micelle Lipid bilayerInside-out (in nonpolar solven

17、t) Lipid Bilayers * Phospholipids prefer to form bilayer structures in aqueous solution because their two fatty acid chains do not pack well; * A lipid bilayer is a stable, low energy structure * Self sealing structure/eliminate free edge * Phospholipids can form either unilamellar vesicles (liposom

18、es) or multilamellar vesicles; * Liposomes are highly stable and can be used as drug and enzyme delivery system; * Lipid bilayers have a polar surface and a nonpolar core. The hydrophobic core provides a substantial barrier to ions and other polar entities, but a favorable environment for nonpolar m

19、olecules. Proteins associated with membranes Peripheral proteins (extrinsic proteins) - membrane proteins of the cytoplasmic surface of the membrane that are noncovalently bonded to the polar head group of the lipid bilayer or to an integral membrane protein (mostly globular). Integral proteins (int

20、rinsic proteins - Anchored to membrane by a single transmembrane segment or more transmembrane helices. - Globular and largely embedded in the bilayer Lipid-anchored proteins - membrane proteins that are covalently bonded either to a phospholipid or a fatty acid that is embedded in one of the leafle

21、ts of the lipid bilayer Lipid Anchored Membrane Proteins Glycoproteins and glycolipids * Carbohydrates covalently attached * Found on outside of cell * Involved in cell identity (blood group antigens) Fluid Mosaic Model S. J. Singer and G. L. Nicolson, 1972 * The phospholipid bilayer is a fluid matr

22、ix * The bilayer is a two-dimensional solvent * Lipids and proteins can undergo rotational and lateral movement * Three classes of proteins: peripheral proteins integral proteins Lipid anchored proteins Fluid Mosaic Model Biomembrane properties C Fluidity * Need to be about 50% fluid or cell dies-du

23、e to defects in the membrane forming * important for membrane protein movement * membrane fusion C Asymmetry Diffusion of lipids within a bilayer Membrane fluidity C Movement of hydrophobic tails C Depends on temperature and lipid composition Transition Temperature, Tm * Tm is phospholipid specific;

24、 Tm increases with chain length decreases with unsaturation, depends on the nature of the polar group; * Phase transition are always endothermic (heat-absorbing); * Transition occur over a narrow temperature range ( 1C); * Transition temperature for biological membranes depends on lipids and protein

25、s composition; * Cells can adjust lipid composition of their membranes to maintain proper fluidity as environmental conditions change Lipids and membrane fluidity * Interactions between hydrophobic tails decrease fluidity (movement): -Shorter tails have fewer interactions -Unsaturated fatty acids are kinked and decrease interactions * Cholesterol “buffers” fluidity Effect of Cholesterol on Membrane fluidity * Fills in gaps (helps hold lipid together) * Keeps long FA chains apart * Stiffens bilayer Asymmetry of Proteins and Lipids Two Sides