Biochemistry-Lehinger CH10
INCOMPLETE/UNSORTEDHome Intersection Biochemistry
343-366 Chemically diverse common and defining feature of insolublity in water funcitons: fats and oils, stored forms of energy, phsopholipids and sterols, major strucutral elements, enzyme cofactors, electron carriers, light absorbing pigments, hydrophobic anchors for proteins, chaperones to help membrane proteins fold, emulsifying agents in the digestive tract, hormones and intracellular memssengers Emphasis on chemicla structure and phsycial properties Fats and oils used almost universally as stored forms of energy in living organisms are derivatives of fatty acids Hydrocarbon derivativesat at about the same low oxidation state as hydrocarbons is highly exoergonic Two types: triglycerols and waxes Fatt yacids are carboxylic acids with hydrocarbon chains from 4-36 carbons long Chain is unbranched and fully saturated in others the chain contains one or more double bonds Few contain trhee carbon rings, hydroxyl groups or methyl group branches Nomenclature - chain length, number of double bonds, separated by colon EX 16C saturated palmitic acid is abbreviated 16:0 18C monounsaturated oleic acid is abbreviated 18:1 20C fatty acid with one double bond between C9 and C10 and another between C12 and C13 is abbreviated 20:2(delta^9,12) Positions of any double bonds are specified by super script following delta Most commonly occuring fatty acids have even numbers of carbon atoms in an unbrached chain of 12-24C Even number results mode of synthesis of these compounds condensesation of two carbon acetate units Common pattern in most monosaturated fatty acids double bond is between C9 and C10 (delta^9) Other bonds of polyunsaturated fatty acids are generally (delta^12 and delta^15) Arachidonic acid exception to this generalization Double bonds of polyunsaturated are almost never conjugated but are separated by methylene group Nearly all naturally are in cis configuration Are produced by fermentation in the rumen of dairy animals Produced during hydrogenation of fish or vegetable oils Diets high in trans fatty acids with increased blood levels of LDL decreased Physicla properties length and degree of unsaturation of the hydrocarbon chain Carboxylic acid group is polar and accounts for the slight solubility of short chain fatty acids in water Melting points are also strongly influenced by the length and degree of unsaturation At room temperature 25C saturated fatty acids from 12-24 have a waxy consistency Unsaturated fatty acids are oily liquids Differences in melting points is due to different degrees of packing of the fatty acid molecules Fully saturated free rotation around each carbon-carbon gives hydrocarbon chain great flexibility most stable fully extended form Steric hindrance of neighboring atoms is minimized can pack together in nearly crystalline arrays Atoms all along their lengths in van der Waals contact with the atoms of neighboring molecuels Cis double bond king in the hydrobarbon chain Fatty acids cannot pack otgether as tightly as fully saturated fatty acids Interactions it takes less thermal energy to disorder these poorly ordered arrays of unsaturated fatty acids markedly lower melting points than saturated fatty acids of the same chain legnth Free fatty acid circulate in the blood bound noncovalently to a protein carrier, serum albumin, fatty acids are present in blood plasma mostly as carboxylic acid derivatives esters or amides. Lacking the charged carboxylate group these fatty acid derivatives are generally less soluble in water Simplest lipids constructed from fatty acids are triacylglycerols as triglycerides, fats, or neutral fats Composed of three fatty acids each in ester linkage Same kind of fatty acid in all three spotions named after the fatty acid they contain EX 16:0 tristearin 18:0 tripalmitin 18:1 triolein Name and position of each fatty acid must be specified Polar hydroxyls of glycerol and the polar carboxylates of the fatty acids are bound in ester linkages, triacylglycerols are nonpolar, hydrophobic molecules essentially insoluble in water. Have lower specific gravities than water, mixture of oil and water Oil with lower specific gravity floats
Triacylglycerols Provide Stored Energy and Insulation Triacylglyceros form a separate phase of microscopic Adipocyetes/fat cells: specilized cells Store large amounts of tryacylglycerols as fat droplets that nearly fil lthe cell Storled as oils in the seeds Adipocytes and germinating seeds contain lipases Lipases: enzymes that catalyze the hydrolysis of stored triacylgrlycerols Two significant advantages to using triacylglycerols as stored fuels Carbon atoms of fatty acids are much more reduced than those of sugars, oxidation more than twice as much energy Oxidation of carbohydrates Second hydrophobic and unhydrated does not carry the extra weight of water of hydration that is associated with stored polysaccharaides Human body can store less than a day's energy supply I nthe form of glycogen Do offer certain advantages as quick sources of metabolic energy Tryacylglycerols stored under the skin serve not only as energy stores but as insulation Low density triacylglycerols Most natural fats are complex mixtures of simplified and mixed triacylglycerols Variety in chain length, degree of saturation Composed largely of triacylglycerols with unsaturated fatty acids and thus are liquids at room temp Lipid rich foods exposed to oxygen in air may spoil and become rancid Oxidative cleavage of double bonds in unsaturated fatty acids which produces aldehydes and carboxylic acids of shorter chain length and therefore higher volatility Esters of long chian saturated and unsaturated fatty acids Melting points are generally higher than those of triacylglycerols Also serve a diversity of funcitons Certain skin glands of vertebreate waxes to protect hair and skin and keep it pliable lubriacted and waterproof Variety of aplicaitons
Central achritectural double layer of lipids acts as a barrier to passage of polar molecules and ions Membrane lipids are ampiphatic - one end hydrophobic and other hydrophillic 5 general types: Glycerophospholipids - hydrophilic regions are composed of two fatty acids joined to clycerol Galactolipids - two fatty acids esterfied to glycerol lack the phosphate Sulfolipids - two fatt yacids esterfied to glycerol lack the phosphate Archeabacterial tetraether - two very long alkyl chains are ether linekd to glycerol at both ends Sphingolipids: single fatty acid joined to a fatty amine (sphingosine) Sterols: rigid system of four fused hydrocarbon rings Hydrophobilc moieties OH gorup or much more complex Glycerophospholipids and some sphingolipids, a polar head is joined to hydrophobic moiety by a phosphodister linkage these are phospholipids Sphingolipids lack phosphate at their polar ends glycolipids Enormous diversity various combiniatons of fatty acid tails and polar heads
Glycerophospholipids - phosphoglycerides and two fatty acids in ester linkage Ester linkage to the first and second carbons of glycerol and a highly polar or charged group is a attached phosphodiester linkage It has no asymmeric carbons but attachement of phosphate at one end converts it into to a chiral compound L glycerol 3 phosphate D glycerol 1 phosphate/synglycerol 3 phosphate Named accoridng to the polar alcohol in the head group Phosphatidylcholine and phosphatidylethanolamine have choline and ethanolamine in their polar head groups Joined to glycerol through a phosphodiester bond Negative charge at netural pH Polar alcohol may be negatviely charged Neutral or positively charged These charges contribute grealty to the surface properties of membranes Fatty acids can be a wide variety May consist of number of molecular species Distribution is specific for different organisms, different tissues Glycerophospholipids C16-18 saturated fatty acid at C1 and C18-20 unsaturated fatty acid at C2
Phospholipids have ether liked fatty acids Ether lipids - two acyl chains is attached to glycerol in ether Ether linked chain may be saturated or may contain a double bond between C1 and 2 as in plasmalogens Half of the heart phospholipids are plasmalogens The functional siginificance of ether lipids is unknown perhas their resistance to phospholipases Platelet activating factor is a potent molecular signal From leukocytes called basophils and stimulates platelet aggregation and release of serotonin from platelets Variety of effects on liver smooth muscle, heart, uterine, and lung tissues and plays an important role in inflammation and the allergic response
Chloroplasts contain Galactolipids and Sulfolipids - predominate plant cells Galactolipids one or two galactose residues are connected by a glycosidic linkage to C3 of 1,2diacylglycerol are localized I nthe thakyloid membranes of chloroplasts Most abundant membrane lipids in the biosphere Also contain sulfolipids which a sulfonated glucose residue is joined to a diacylglycerol in glycosidic linkage the sulfonate on the head group bears a fixed negative chage like that of the phosphate group in phospholipids
Archaebacteria - Unique membrane lipids Live in extreme coditions Linkages are through ether bonds - much more stable to hydrolysis at low pH and high temp than are the ester bonds found in bact and euks These archaebacterial lipids are twice the length of phospholipids and sphinglipids At each end of is a polar head consisiting of glycerol linked to either phospahte or sugar residues Glycerol dialkyl glycerol tetraethers (GDGTs) Glycerol moiety is not the same stereoisomer as that in the lipids of bact and euks Central carbon is R in archaebacteria and S in other kingdoms
Sphingolipids - Derivatives of Sphingosine Fourth large class of membrane lipids Have polar head grop and two nonpolar tails, unlike glycerophospholipids and galactolipids they contain no glycerol Composed of one molecule of the long chain amino alcohol sphingosine (4-sphingenine) or one of its derivatives One molecule of a long chained fatty acid and a polar head group that is joined by aglycosidic linkage in some cases by a phosphodiester in the others Structurally analogous to the three carbons of glycerol in glycerophospohlipids Resulting compound is a ceramide - strucutrally similar to diaclylglycerol Structural arent of all sphingolipids - ceramide Three subclasses - all derivatives of ceramide but differing in their head gorups Shingomyelins - neutral uncharged Phospohcholine or phosphoethanolamine as their polar head group Classified along with glycerophospholipids as phospholipids Resemble phosphatidylcholines in their general properties and three dimensional structure No net charge on their head groups Present in the plasma membranes of animals cells escpecially in myelin - insulates the axons of some neurons sphingomeylins Glycolipids - glycosphingolipids Occur largely I nthe outer face of plasma membranes Head groups with one or more sugars connected directly to the OH at C1 of the ceramide moiety They do not contain phosphate Cerebrosides - single sugar linked to ceramide with galactose are characteristically found in the plasma membrane of cells in neural tissue Globosides - neutral two or more sugars Glycolipids - no charge at pH 7 Gangliosides - most complex shingolipids, polar head and one or more residues of N acetylneuraminic acid (sialic acid) Sialic acid gives negative charge at pH 7 Gangliosides with one sialic acid residue are the GM seires (mono), GD (di) so one GT (tri) GQ (four) Were discovered carboxylate moieties of certain sphyngolipids define the human blood groups
Gangliosides are concntrated in the outer surface of cells points of recognition Kinds and amounts of gangliosides chagne dramatically during embryonic development Tumor formation induces the sytnheis of new complement of gangliosides Found to induce differentiation of cultured neuronal tumor cells
Phospholipids and Sphingolipids are Degraded in Lysosomes Most cell continually degreate and replace their membrane lipids There is a specific hydrolitic enzyme in the lysosome A type remove one of the two fatty acids producing a lysophospholipid Esterases attack the ether link of plasmalogens Lysophospholipases remove the remaining fatty acid Gangliosides are degraded by a set of lysosomal nxymes that catalyze stepwise removal of sugar units, finally yielding a ceramide Leads to the accumulation of ganglisides in the cell Sterols are structural lipids present in the membranes of most eukaryotic cells Characteristic stru cutre of 5th group of membrane lipids is the steroid nucleus consisting of four fused rings Three with six carbons and one with five Cholesterol the major sterol in animal tissues is amphipathic with a polar head group and a nonpolar hydrocarbon body As long as 16 carbon fatty acid in the extended form Stagmasterol in plants, ergosterol in fungi Bact cannot styntheisze sterols - however can incorporate exogenous sterols into their membranes Synthesized from simple five carbon isoprene subunits are as fat soluble vitamins quinones and dolichols Membrane constituents, precursors Steroid hormones are potent biological signals that regulate gene expression Bile acids are polar derivaties of cholesterol that act as detergents in the intestine Dietary fats to make them more readily accessible to digestive lipases
Two functional storage lipids and strutural lipids Lipids make up 5% to 10% of dry mass of most cells Lipids play a passive role in the cell Fuels are stored until oxidized by enzymes and membrane lipids from impermeable barriers around cells and cellular compartments Active roles in the metabolic traffic as metabolites and messengers Potent signals as hormones Enzyme cofactors in electron transfer reactions of sugar moeties in a variety of glycosylation Lipids with a system of conjugated double bonds: pigment molecules Phosphatidylinositol and its phospholylated derivates act as several levels t o regulate cell strucutre and metabolism Cytoplasmic face of plasma membrane serves as a specific binding site for certain cytoskeletal proteins and form some soluble protiens involved in membrane fusion during exocytosis Serves as a reservioir that are released inside the cell in response to extracellular signals of the plasma membrane Enzymatic removal of a phospholipid head group and ends with activation of an enzyme (protein kinase C) Hormone vasopressin binds to plasma membrane receptors Phospholipase C hydrolyzes the bond between glycerol and phosphate in phosphatydilinositol 4,5 bisphosphate reelaseing two products: inositol 1,4,5trisphosphate (IP3) which is water soluble and diacylglycerol Which remains accosited with the plasma membranes and triffers release of Ca Combination of the diacylglycerol and elevated cytosolic Ca activates the enzyem protein kinease C Enzyme catalyzes the transfer of a phosphoryl grop from ATP to a specific residue in one or more target proteins thereby alternating their actiivty and consequently the cell's metabolism Inositol phospholipids also serve as points of nucleatio for certain supramolecular complexes involved in signaling or exocytosis Contain certain structural motifs called PH and PX domains Bind phosphatidylinositols I nthe membrane with high specificity and affinity, formation of multienzyme complexes at the membrane's cytosolic surface Formation of this phospholipid in response to extracellular signals brings the proteins togetehr at the surface of plasma membrane Serve as sources of intracellular messengers Ceramide and sphingomyelin are potent regulators of protein kinses and ceramide Known to be involved in the regulation of cell division, differentiation, migration, programmed cell death
Eicosanoids are paracrine hormones that act only on cells near the point of hormone synthesis instead of being transported in the blood to act on cells in other tissues or organs Dramatic effects on vertebreate tissues Involved in productive funcition: inflammation, fever, and pain associated with injury or disease Blood clotts and regualtion of blood pressure in gastric acid secretion and in a variety of other processes important in human health or disease All eicosanoids are derived from arachidonic acid Eicosanoids: prostaglandins, thromboxanes, leukotirenes Prostaglandins (PG) PGE - ether soluble PGF - phosphate buffer soluble With numerous subtypes Act in many tissues by regualting the synthesis of intracellular messenger 3',5'-cyclic AMP - cAMP Mediates the action of diverse hormones and prostaglandins affect a wide range of cellular and tissue functions Stimulate contraction of the smooth muslce of the uterus during menstruation and labor Others affect blood flow to specific organs Wake sleep cycle Elevate body temperature and cause inflammation and pain
Thromboxanes are six membered containing an ether Produced by platelets and act in the formation of blood clots and the reduction of blood flow to the site of the clot Nonsteroidal antiinlammatory drugs Inhibit the prostaglindin H2 synthase An early step in the pathway from arachidonate to prostaglandings and thromboxanes Leukotrienes first found in leukocytes contian three conjugated double bonds They are powerful biological siganls Leukotirene D4, derived from leukotriene A4 induses contraction of muslce lining the airways to the lung Overproduction of leukotrienes cuases asthmatic attacks and leukotriene syntheiss is sone taret of antiasthmatic drugs such as prednisone Storng contractio nof the smooth muscle occurs during anaphylactic shock
Steroids are oxidized derivatives of sterols They have the sterol nucleus but lack the alkyl chain atached to the D ring of cholesterol and are more powerful than cholesterol Trigger changes in gene expression and metabolism Very high affinity for their receptors, very low concetrations of hormones are sufficient to produce responses in target tissues Major groups of sex horones produced in adrenal cortex, cortisol and aldosterone Predisone and prednisolone are steroid drugs with potent antiinflammatory activities Inhibition of arachidonate release by phospholipase A2 Consequent inhibition for the sytnehsis of leukotrienes, prostaglandins and thromboxanes Variety of medical applitations Vascular plants contain phosphotadyllinositol4,5-bisphospahte as well as phospohlipase that releases IP3 and they use IP3 regulate the intracellular concentration of Ca Potent growth regulators in plants Jasmonate 18:3(delta^9,12,15) chemically similar to t eicosanoids also serves a powerful signal triggering the plant's defenses in response to insect inflicted damage Methyl ester gives characteristic fragrance Vitamins - compounds that are essential to the health of humans and other vertebrates to the health of humans and other vertebrates but cannot be synthesizesd by these animals and must therefore be obtained in the diet Identified two general classes Soluble in nonpolar organic solvents Those extracted from foods with aq solvents (water soluble) A, D, E, K are isoprenoid compouds synthesized by the condensation of multiple isoprene units D and A serve as hormone precursors Vitamin D3/cholecalciferol - skin from 7-dehydrocholesterol in a photochemical reaction by the UV component of sunlight D3 is not iteslf biologiclaly active but it is converted by enzymes in the liver and kidney to 1,25dihydroxycholecalciferol - hormone that regulates calcium levels in kidney and bone Dificiency of vitamin D leads to defective bone formation and disease rickets Vitamin D2 (ergocalciferol) is commercial product formed by UV irradiation of ergosterol of yeast Vitamin D2 is structurally similar to D3 with slight modification to the side chain attached to he sterol D ring Both have same biological effects Like steroid hormones, the porduct of vitamin D metabolism 1,25dihydroxycholecalciferol regulates gene expression Vitamin A (retinol) visual pigment of vertebreate eye Vitamin A derivative retinoic acid regulates gene expression in the development of epithelial tissue including skin active ingredient in the drug tretinoin used in the severe acne and wrinkled skin Pigment that initiates the response of rod and cone cells of the retina to light producing a neuronal signal to the brain Deficiency of vitamin A leads to a variety of symptoms in humans, including dryness of the skin, eyes, mucous membranes, retarded development and growth, night blindness, early symptom commonly used in diagnosing vitamin A deficinecy Vitamin E is the collective name for a group of closely related lipids called tocopherols all of which contain a substituted aromatic ring and a long isoprenoid side chain Hydrophobic, tocopherols associate with cell membranes, lipid depoisits, lipoproteins in the blood Biological antioxidants Aromatic ring reacts and destroys the most reactive forms of oxygen radicals and other free radicals, protecting unsaturated fatty acids from oxidation and preventing oxidative damage to membrane lipids, which can cause cell fragility Eggs and vegtable oils and are especially abundnat in what germ Vitamin E deficiency in humans is very rare Aromatic ring of Vitamin K underoges a cycle of oxidation and reduction during the formation of active prothrombin, a blood plasma protein essential in blood clot formation Prothrombin is a proteolytic enzyme that splits peptide bonds in the blood protein firbrinogen to confert it to fibrin Insoluble fibrous protein that holds blood clots together Vitamin K deficiency slows blood clotting Very uncommon in humans Warfaranin is synthetic compound that inhibits the formation of active prothrombin Poisnous to rats Ubiquinone and plastiquinone are isoprenoids that funciton as lipophilic electron carriers in the oxidation reudciton reactions that drive ATP syntehsis in mitochondria and chloriplasts Accept either one or two electrons and either one or two protons Dilichols activate sugar Sugar units to be added chemcially activated by attachement to isoprenoid alcohols called dolichols Compounds have strong hydrophobic interactions with membrane lipids
10.4 Working With Lipids Biological role of lipids in cells and tissues essential to kow which lipids are present and in what proportions Complex mixtures are separated by differences in polarity/solubility of components in nonpolar solvents Lipids with ester or amide liked fatty acids can be hydrolyzed by treatment with acid or alkali or with highly specific hydrolytic enzymes (phospohlipases, glycosidases) Lipid Extraction Requires Organic Solvents Neutral lipids are readily extracted from tissues with ethyl ether, chloroform, benzene solvetns that do not permit lipid cusltering driven by hydrophobic interactions Membrane lipids are more effectively extracted by more polar organic solvents such as ethanol or methanol Reuce the hydrophobic interactions among lipid molecuels while also weakening the hydrogen bonds and electrostatic interactions that bind membrane lipids to membrane proteins Commonly used extractant is (chloroform, methanol, water) miscible producing phase More water is added to the resulting extract and the mixture separates into two phases - methanol/water (top) and chloroform (bottom) Lipids remin in the chloroform More polar molecules such as proteins remain in the methanol/water layer Mixtures of tissue lipids can be fractionated by chromatographic procedures based on the different polarities of each class of lipid Insoluble, polar material such as silica gel High performance chromatography is of smaller diameber and solvents are forced through column under high pressure Polar lipids bind tightly to the polar silicic acid Neutral lipids pass directly throug hthe column and emerge in the first chloroform wash Polar lipids are then eluted in order of increasing polarity by washing the column with solvents of progressively higher polarity Uncharged but polar lipids (cerebrosides) are eluded with acetone and very polar or charged lipids (glycerophospohlipids) are eluted with methanol Thin layer chromatograpy Small sample of lipids dissoloved in chloroform Separated lipids can be deteced by sparaying the plate with a tdye (rhodamine) fluoresces when associated with lipids or by exposing the plate to iodine fumes. Iodine reacts reversibly with the double bonds in fatty acids --> yellow/brown color Gas-liquid chromatography separates volatile coponents of a mixture according to their relative tendencies to dissolve in the inert material packed in the chromatography column and to volatilize and move through the column carried by a current of an inert gas Some lipids are naturally volatile but most must first be derivatized to increase the volatiity (lower boiling point) Lipids are first heated in a methanol/HCl or methanol/NaOH mixture which convertes fatty acids esteried to clycerol into methyl esters (transesteficaiton) Fatty acyl methyl esteras are loaded onto the gas liquid chromatography column and the column is heated to volatize the compounds Fattly acyl esters most soluble I nthe column material partition into that mateiral, les sosluble lipids are carired by the steam of inert gas and emerge first in the column Depends on the nature of the solid and on the BP of components of the lipid mixture
Sepicific Hydrolysis Aids in determination of lipid strucutre Certain classes are susceptible to degradation under speicifc condtions All esterliked triacylglycerols, phospholipids and sterol are released by mild acid or alkaline treatment and somewhat harsher hydrolysis release amide bound fatty acids form sphyingolipids Phospholipases A,C, D each split particuarl bonds in phospholipids and yeild products with characteristic solubilities Phospholipase C releases a water soluble phosphoryl alcohol (phosphocholine from phosphatidylcholine) and a chloroform soluble diacylglycerol Combination of specific hydrolysis with characterizaiton of the products by thin layer, gas liquid, high performance liquid chromatography often allwos determination of lipid structure Unambiguously the length of a hdyrocarbon chain or the position of double bonds mass spectral analysis of lipids derivatives is invaluable Similar lipids are very much alike and their positions of elution from the verious chromatographic procedures often do not distinguish between them Simultaneously separated and identified by their unique pattern of fragmentation
Glycerophospohlipids, sphingolipids and sterols are insoluble in water
Lipid clustering reduces amount of hydrophobic surface exposed to water, minimizing molecules in shell of ordered water at the lipid water interface increasing entropy (order) - more disordered water Hydrophobic interactions provide thermodynamic driving force for the formation and maintenenace of these clusters Three Types of Lipid Aggregates (amphipathic lipids&H2O)
-1-Micelles: sphefical structures containing few dozen-thousand amphipathic molecules hydPHO aggregated in the interior hydPHI contact with water Favored when cross sectional area of head group is greater than that of acyl side thains [Free fatty acids, lysophospholipids (lacking one fatty acid) and detergents - sodium dodecyl sulfate (SDS)]
-2-Bilayer: two lipid monolayers form 2d sheet hydPHO interact with ea/other hydPHI interact with H2O Bilayer sheet is relatively unstable and spontaneously forms to liposome hydPHO at its ends of bilayer are transiently in contact with water Favored when cross sectional area of head group and acyl side chain are similar [Glycderophospholipids and sphingolipids]
-3-Liposme: hollow sphere or vesicle Bilayers lose their hydPHO ends achieving maximal stability Enclose water creating a separate compartment
