Lipids and Membranes Lecture Note

Lipids & Membranes 1 Lipids anything “greasy” Lipids: Roles • Membrane structure – main structure of cell membranes • Storage of energy – reduced compounds: lots of available energy – hydrophobic nature: good packing • Signaling molecules – – – – paracrine signals (act locally) steroid hormones (act body-wide) growth factors vitamins A and D (hormone precursors) • Vitamins, Cofactors, and secondary products – – – – – Vitamins E & K: antioxidant & blood clot formation, resp. coenzyme Q: ATP synthesis in mitochondria Pigments, e.g., tomatoes, carrots, pumpkins, some birds Water repellant in feathers and hides Insulation & bouyancy control in marine mammals (blubber) 2 Lipids: Roles Functions of Membranes • Define the boundaries of the cell • Allow import and export – Selective import of nutrients (e.g. lactose) – Selective export of waste and toxins (e.g. antibiotics) • Retain metabolites and ions within the cell • Sense external signals and transmit information into the cell • Provide compartmentalization within the cell – separate energy-producing reactions from energy-consuming ones – keep proteolytic enzymes away from important cellular proteins • Produce and transmit nerve signals • Store energy as a proton gradient • Support synthesis of ATP Lipids: Roles Electron Micrograph of Biological Membranes Pancreas cell 3 Lipids: Classes Biological molecules that are characterized by low solubility in water, that is, are relatively hydrophobic. They have a high hydrocarbon content TABLE 10-2 Eight Major Categories of Biological Lipids Category Category code Examples Fatty acids ① FA Oleate, stearoyl-CoA, palmitoylcarnitine Glycerolipids ② GL Di- and triacylglycerols GP Phosphatidylcholine, phosphatidylserine, phosphatidyethanoloamine Glycerophospholipids ④ Sphingolipids SP Sphingomyelin, ganglioside GM2 ST Cholesterol, progesterone, bile acids Prenol lipids PR Farnesol, geraniol, retinol, ubiquinone Saccharolipids SL Lipopolysaccharide Polyketides PK Tetracycline, erythromycin, aflatoxin B1 Sterol lipids Waxes ⑤ ③ Lipids: Classes Biological molecules that are characterized by low solubility in water, that is, are relatively hydrophobic. Classes of Lipids They have a high hydrocarbon content 1. Fatty acids 2. Fats (triglycerides) 3. Waxes 4. Membrane Lipids 5. Isoprenes 2 & 3 are sometimes classified together as ”simple lipids” 4 Lipids: Fatty Acids • Carboxylic acids with hydrocarbon chains containing between 4 to 36 carbons – Almost all natural fatty acids have an even number of carbons. – Most natural fatty acids are unbranched. • Biologically, most are found in ester linkages as the pKa is ~3.0, and would otherwise be very acidic. • TWO CLASSES – Saturated: no double bonds between carbons in the chain ⇌ – pKa≈3 + H+ m.p. > 37 °C – Unsaturated: ≥1 cis-double bonds between carbons in the chain m.p. < 20 °C • Monounsaturated: one double bond between carbons in the alkyl chain • Polyunsaturated: more than one double bond in the alkyl chain – never conjugated Lipids: Fatty Acids Conformation of Fatty Acids • The saturated chain tends to adopt extended conformations. • The double bonds in natural unsaturated fatty acids are in a cis configuration, which kinks the chain. C18:0 = stearate C18:1 = oleate 5 Lipids: Fatty Acids Turkey 1 18-20 12-14 Saturated FA 18-20 25-30 Unsaturated FA Lipids: Fatty Acids (D9) (D9) (D9,12) (D9,12,15) (D6,9,12) (D5,8,11,14) (D15) Need to Know: Common names, structure, symbol 6 Lipids: Fatty Acids Nomenclature • Fatty acids can be described by: – systematic name: cis-9-octadecanoic acid – common name: oleic acid 18:1w 9 – delta numbering of carbon skeleton: 18:1Δ9 • describes location of the first carbon of the alkene in relationship to the carbonyl carbon – omega numbering of carbon skeleton: 18:1ω9 • describes location of the first carbon of the alkene in relationship to the terminal methyl • Omega-3 fatty acids are essential nutrients. – Humans need them; cannot synthesize them. – They include a-linolenic acid (ALA) (18:3D 9,12,15)(18:3w 3,6,9), Eicosapentaenoic acid (EPA), and Docosahexaenoic Acid (DHA). • although DHA (22:6) and EPA (20:5) can be synthesized from ALA 22:6(D4,7,10,13,16,19) Docosahexaenoic Acid (DHA) 22:6w 3,6,9,12,15,18 20:5w 3,6,9,12,15 Lipids: Fatty Acids Solubility and Melting Point of Fatty Acids • Solubility – decreases as the chain length increases • Melting Point – decreases as the chain length decreases – decreases as the number of double bonds increases 7 Lipids: Fatty Acids Melting Point and Double Bonds • Saturated fatty acids pack in a fairly orderly way. – extensive favorable interactions What kind of interaction? • Unsaturated cis fatty acids pack less orderly due to the kink. – less-extensive favorable interactions …. van der Waals • It takes less thermal energy to disrupt disordered packing of unsaturated fatty acids. – Explains the lower melting point of unsaturated cis fatty acids. Lipids: Fatty Acids Trans Fatty Acids • Trans fatty acids form by partial hydrogenation (reduction) of unsaturated fatty acids. – done to increase shelf life or stability at high temperature of oils used in cooking (especially deep frying) – Or to convert plant oils to margarine, a solid fat (partially hydrogenated polyunsaturated oils). • • • A trans double bond allows a given fatty acid to adopt an extended conformation. Trans fatty acids can pack more regularly and show higher melting points than cis forms. Consuming trans fats increases risk of cardiovascular disease. – Avoid deep frying partially hydrogenated vegetable oils. – Current trend: reduce trans fats in foods (Wendy’s, KFC). (Eliadic acid) 8 Lipids: Classes Biological molecules that are characterized by low solubility in water, that is, are relatively hydrophobic. Classes of Lipids They have a high hydrocarbon content 1. Fatty acids 2. Fats (triglycerides) 3. Waxes 4. Membrane Lipids 5. Isoprenes 2 & 3 are sometimes classified together as ”simple lipids” Lipids: Fat Triacylglycerols (Nonpolar) • The majority of fatty acids in biological systems are found in the form of triacylglycerols. – Solid ones are called fats. – Liquid ones are called oils. • The primary storage form of lipids (body fat) • Less soluble in water than fatty acids due to the esterification of the carboxylate group • Less dense than water: fats and oils float. 9 Lipids: Fat Triacylglycerols Name? 1-Myristoyl-2-stearoyl-3palmitoleoyl glycerol Tristearoyl glycerol Lipids: Fat Fats Provide Efficient Fuel Storage • The advantage of fats over polysaccharides: – Fats and oils carry more energy per carbon because they are more reduced. – Fats and oils carry less water per gram because they are nonpolar. • Glucose and glycogen are for short-term energy needs and quick delivery. • Fats are for long-term (months) energy needs, good storage, and slow delivery. • Fats can be treated with alkaline (NaOH), which will hydrolyze the ester bonds leading to glycerol and salts of the fatty acids…...soap! Process is called saponification. 10 Lipids: Fat Fats Provide Efficient Fuel Storage Lipids: Classes Biological molecules that are characterized by low solubility in water, that is, are relatively hydrophobic. Classes of Lipids They have a high hydrocarbon content 1. Fatty acids 2. Fats (triglycerides) 3. Waxes 4. Membrane Lipids 5. Isoprenes 2 & 3 are sometimes classified together as ”simple lipids” 11 Lipids: Waxes • Waxes are esters of long-chain saturated fatty acids with and saturated or unsaturated long-chain alcohols. • Insoluble and have high melting points • Variety of functions: – waterproofing of feathers in birds – protection from evaporation in tropical plants and ivy – protection and pliability for hair and skin in vertebrates – storage of metabolic fuel in plankton – used by people in lotions, ointments, and polishes Lipids: Waxes Wax: The Material of the Honeycomb Myristic acid (C14:0) Beeswax is a mixture of a large number of lipids, including esters of triacontanol (C30:0) and cerylanol (C26:0). Ceryl alcohol (C26:0) 12