Lipids Lecture 13 PDF

10| Lipids © 2013 W. H. Freeman and Company  An introductory slide Unsaturated vs Saturated vs Trans Fats, An imation - YouTube Lipids: Structurally Diverse Class Organic molecules that are characterized by low solubility in water, that is, are relatively Biological Functions of L...

10| Lipids © 2013 W. H. Freeman and Company  An introductory slide Unsaturated vs Saturated vs Trans Fats, An imation - YouTube Lipids: Structurally Diverse Class Organic molecules that are characterized by low solubility in water, that is, are relatively Biological Functions of Lipids Storage of energy – Reduced compounds: lots of available energy – Hydrophobic nature: good packing Insulation from environment – Low thermal conductivity – High heat capacity (can “absorb” heat) – Mechanical protection (can absorb shocks) Water repellant – Hydrophobic nature: keeps surface of the organism dry Prevents excessive wetting (birds) Prevents loss of water via evaporation Buoyancy control in marine mammals – Increased density while diving deep helps sinking (just a hypothesis)  More Functions Membrane structure – Main structure of cell membranes Cofactors for enzymes – Vitamin K: blood clot formation – Coenzyme Q: ATP synthesis in mitochondria Signaling molecules – Paracrine hormones (act locally) – Steroid hormones (act body-wide) – Growth factors – Vitamins A and D (hormone precursors) Pigments – Color of tomatoes, carrots, pumpkins, some birds Antioxidants – Vitamin E  Classification of Lipids Based on the structure and function Lipids that do not contain fatty acids: cholesterol, terpenes, … Lipids that contain fatty acids (complex lipids) – can be further separated into:either glycerol or sphingosine as the backbone – storage lipids and membrane lipids two very long, fatty acids in ester linkage branched alkyl chains In phospholipids the polar head group is joined through a phosphodiester, whereas glycolipids have a direct glycosidic linkage between the head-group sugar and the backbone glycerol https://byjus.com/chemistry/difference-between-ester-and-ether/  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 Saturated: no double bonds between carbons in the chain Monounsaturated: one double bond between carbons in the alkyl chain Polyunsaturated: more than one double bond in the alkyl chain  Fatty Acid Nomenclature Omega-3 fatty acids are essential nutrients – Humans need them but cannot synthesize them Docosahexaenoic Acid (DHA) – Including ALA, Alpha-Linolenic DHA, andAcid EPA(ALA) Although DHA and EPA can be synthesized from ALA Fatty Acid Nomenclature 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  Conformation of Fatty Acids The saturated chain tends to adopt extended conformations The double bonds in natural unsaturated fatty acids are commonly in cis configuration, which kinks the chain 18:0 (stearate at pH 7) 18:1(Δ9) (oleate)  Melting Point and Double Bonds Saturated fatty acids pack in a fairly orderly way – extensive favorable interactions Unsaturated cis fatty acid pack less orderly due to the kink – less-extensive favorable interactions It takes less thermal energy to disrupt disordered packing of unsaturated fatty acids: – Fully unsaturated cis fatty acids have a lower melting saturated point fatty acids less stable in the aggregates extended form  Trans Fatty Acids Trans fatty acids form by partial hydrogenation of unsaturated fatty acids Change from liquid form to solid form – Done to increase shelf life or stability at high temperature of oils used in cooking (especially deep frying) 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  Triacylglycerols (Nonpolar) 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 lack of charged carboxylate group Less dense than water: fats and oils float  Triacylglycerols different fatty acids at C-1 and C-3, C-2 is a chiral center  Fats Provide Efficient Fuel Storage The advantage of fats over polysaccharides: – Fatty acids carry more energy per carbon because they are more reduced – Fatty acids carry less water per gram because they are nonpolar Glucose and glycogen are for short-term energy needs, quick delivery Fats are for long-term (months) energy needs, good storage, slow delivery  Waxes Waxes are esters of long-chain saturated and unsaturated fatty acids with long- chain alcohols Insoluble and have high melting points Variety of functions: – Storage of metabolic fuel in plankton – Protection and pliability for hair and skin in vertebrates – Waterproofing of feathers in birds – Protection from evaporation in tropical plants alcohol triacontanol and ivy – Used by people in lotions, ointments, and polishes https://byjus.com/chemistry/difference-between-ester-and-ether/  Wax: The Material of the Honeycomb Beeswax is a mixture of a large number of lipids, including esters of triacontanol, and a long-chain alkane hentiacontane  Structural Lipids in Membranes Contain (Polar) polar head groups and nonpolar tails (usually attached fatty acids) Diversification can come from: modifying a different backbone changing the fatty acids modifying the head groups The properties of head groups determine the surface properties of membranes Different organisms have different membrane lipid head group compositions Different tissues have different membrane lipid head group compositions  Glycerophospholipids Primary constituents of cell membranes Two fatty acids form ester linkages with the first and second hydroxyl groups of L-glycerol-3- phosphate Head group is charged at physiological pH  General Structure of Glycerophospholipids Unsaturated fatty acids are commonly found connected to C2 The highly polar phosphate group may be further esterified by an alcohol; such substituent groups are called the head groups through a phosphodiester bond  Examples of PhosphatidicGlycerophospholipids acid, a phosphomonoester, is the parent compound. Each derivative is named for the head-group alcohol (X), with the prefix “phosphatidyl-.” In cardiolipi two phosphatidic acids share a single glycerol (R1 and R2 are fatty acyl groups). Phosphatidylcholine (glycerophospholipid) Phosphatidylcholine is the major component of most eukaryotic cell membranes Many prokaryotes, including E. coli, cannot synthesize this lipid; their membranes do not contain phosphatidylcholine  Ether Lipids: Plasmalogen Vinyl ether analog of phosphatidylethanolamine Common in vertebrate heart tissue Also found in some protozoa and anaerobic bacteria Function is not well understood – Resistant to cleavage by common lipases but cleaved by few specific lipases – Increase membrane rigidity? – Sources of signaling lipids? – May be antioxidants? Ether Lipids: Plasmalogen https://byjus.com/chemistry/difference-between-ester-and-ether/  Ether Lipids: Platelets- AliphaticActivating ether analog of Factor phosphatidylcholine Acetic acid has esterified position C2 First signaling lipid to be identified Stimulates aggregation of blood platelets Plays role in mediation of inflammation Alkane: single bond Alkene: one or more double bond  Sphingolipids The backbone of sphingolipids is NOT glycerol The backbone of sphingolipids is a long- chain amino alcohol sphingosine A fatty acid is joined to sphingosine via an amide linkage rather than an ester linkage as usually seen in lipids A polar head group is connected to sphingosine by a glycosidic or phosphodiester linkage The sugar-containing glycosphingolipids are found largely in the outer face of plasma Sphingolipids Ceramide is the parent compound for this group GM2 is a type of ganglioside  Sphingomyelin Ceramide (sphingosine + amide-linked fatty acid) + phosphocholine attached to the alcohol Sphingomyelin is abundant in myelin sheath that surrounds some nerve cells in animals What is myelin sheath? Myelin Sheath: What It Is, Purpose & Function (clevelandclinic.org) Sphingomyelin is structurally similar to phosphatidylcholine  Glycosphingolipids and Blood Groups The blood groups are determined in part by the type of sugars located on the head groups in glycosphingolipids. The structure of sugar is determined by an expression of specific glycosyltransferases – Individuals with no active glycosyltransferase will have the O antigen – Individuals with a glycosyltransferase that transfers an N-acetylgalactosamine group have A blood group – Individuals with a glycosyltransferase that transfers a galactose group have B blood group  Sterols and Cholesterol Sterol – Steroid nucleus: four fused rings – Hydroxyl group (polar head) in the A-ring – Various nonpolar side chains The steroid nucleus is almost planar Physiological Role of Sterols Cholesterol and related sterols are present in the membranes of most eukaryotic cells – Modulate fluidity and permeability – Thicken the plasma membrane – Most bacteria lack sterols Mammals obtain cholesterol from food or synthesize it de novo in the liver Cholesterol, bound to proteins, is transported to tissues via blood vessels – Cholesterol in low-density lipoproteins tends to deposit and clog arteries Many hormones are derivatives of sterols  Steroid Hormones Steroids are oxidized derivatives of sterols Steroids have the sterol nucleus, but lack the alkyl chain found in cholesterol More polar than cholesterol Steroid hormones are synthesized from cholesterol in gonads and adrenal glands They are carried through the body in the bloodstream, usually attached to carrier proteins Many of the steroid hormones are male and female sex hormones  Steroid Hormones testes Estradiol, one of the female sex hormones, is produced in the ovaries and placenta Cortisol and aldosterone are hormones synthesized in the cortex of the adrenal gland; they regulate glucose metabolism and salt excretion, respectively  Biologically Active Lipids Are present in much smaller amounts than storage or structural lipids Play vital roles as signaling molecules between nearby cells Lipid soluble vitamins (A, D, E, and K) Arachidonic Acid Derivatives as Signaling Lipids Enzymatic oxidation of arachidonic acid yields Prostaglandins (inflammation and fever) Thromboxanes (formation of blood clots) Leukotrienes (smooth muscle contraction in lungs) Nonsteroidal antiinflammatory drugs (NSAIDs) such as aspirin and ibuprofen block the formation of prostaglandins and thromboxanes Vitamin D regulates calcium uptake breaks the bond shaded light red In the liver, a hydroxyl group is added at C- 25; in the kidney, a second hydroxylation at C-1 produces the active hormone  Vitamin A (Retinol) Involved in visual pigment Precursor for other hormones involved in signaling Golden rice and Vitamin A Golden Rice - Golden rice - Wikipedia Vitamin E, K, and other lipid quinones are antioxidants Polyketides are biologically active lipids with medicinal uses lower cholesterol levels