Lipids I Fatty acids and simple lipids Complex lipids Medical Chemistry Lecture 11 2007 (J.S.)
Two major biological function of lipids: – Triacylglycerols (fats) are nutrients , the turnover about 100 g per day in adult humans, the only source of essential fatty acids, and a vehicle allowing intake and resorption of lipophilic vitamins. Fat represents an energy reservoir in animal bodies and oily fruits of plants. – Phospholipids and glycolipids form lipid dilayer of biomembranes , the turnover about 2 g / d in adult humans. Lipids are – naturally occurring compounds, of animal or plant origin, – because of their hydrophobicity, soluble in organic solvents (only sparingly, if at all, soluble in water). Structure of lipids from the chemical point of view: Esters (or amides ) of long-chain fatty acids with aliphatic or alicyclic) alcohols or aminoalcohols .
Major classes of lipids Simple lipids Triacylglycerols (fats) Waxes Ceramides Complex lipids Phospholipids Glycolipids Derived " lipids " Eicosanoids (originate by transformation of polyunsaturated fatty acids) Isoprenoid compounds – squalene, dolichol, carotenoids, cholesterol , phytosterols, calciols and other steroids Accompanying compounds – lipophilic vitamins (tocopherols, phylloquinone, retinol) amphipathic molecules (tensides) hydrophobic molecules
Classification of fatty acids ( FA ) : – saturated – monounsaturated (monoenoic) – polyunsaturated ( PUFA , containing two or more double bonds) – short-chain fatty acids ( C 4 – C 10 ) – medium-chain ( C 8 – C 10 ) – long-chain fatty acids ( C 12 ) Fatty acids – general properties Fatty acids – are aliphatic monocarboxylic acids – contain an even number of carbons because they are synthesized from two-carbon units (acetyl-CoA) – may have saturated chains (containing no double bonds) or unsaturated chains (one or more double bonds, nearly all of the c is configuration and not conjugated) – are non-polar due to their aliphatic chains (practically water-insoluble)
The common naturally occurring fatty acids Number of carbons and double bonds Common name Systematic name Saturated fatty acids 4:0 Butyric butanoic 6:0 Caproic hexanoic 8:0 Caprylic octanoic 10:0 Capric decanoic 12:0 Lauric dodecanoic 14:0 Myristic tetradecanoic 16:0 Palmitic hexadecanoic 18:0 Stearic octadecanoic 20:0 Arachidic eicosanoic 22:0 Behenic docosanoic 24:0 Lignoceric tetracosanoic Unsaturated fatty acids 16:1 (9) Palmitoleic cis - hexadec-9-enoic 18:1 (9) ( n –9) Oleic cis -octadec-9-enoic 18:2 (9,12) ( n –6) Linoleic cis , cis - octadeca-9,12-dienoic 18:3 (6,9,12) ( n –6) -Linolenic cis , cis - octadeca-6,9,12-trienoic 18:3 (9,12,15) ( n –3) - Linolenic all - cis - octadeca-9,12,15-trienoic 20:4 (5,8,11,14) ( n –6) Arachidonic all - cis - eicosa-5,8,11,14-tetraenoic 20:4 (5,8,11,14,17) ( n –3) (Timnodonic) EPA all - cis - e i cosa-5,8,11,14,17-pentaenoic ( n –7)
18 : 3 ( n –3 ) C O O – 18 15 12 9 n n-3 n -6 n -9 Linolenic acid (unionized form) Linolenate (ionized form) 18 : 3 ( 9,12,15 ) positions of double bonds (numbering from the carboxyl carbon number 1) number of double bonds total number of carbon atoms Nomenclature of unsaturated fatty acids Example: all- cis -Octadeca-9,12,15-trienoic acid (systematic name) Common trivial name occasionally also 18:3 ( 3 )
18:1 ( n -9 ) 16:0 Saturated long-chain fatty acids are solid
Presence of unsaturated acids
lowering of the temperature
The presence of cis double bonds is the cause of " bent“ molecules:
Polyunsaturated fatty acids ( n -6 and n -3) are essential for animals In higher animals , only the desaturases are known which generate double bonds at carbons 9 , 6 , 5 , and 4 . Fatty acids containing double bonds beyond C-9 (acids n -6 and n -3) are synthesized by plants. They are essential dietary constituents for animals and serve as precursors of eicosanoids (prostanoids and leukotrienes) . Providing the dietary intake is sufficient (vegetable seed oils, resp. fish), linoleate and α-linolenate act as precursors of other essential polyenoic acids such as arachidonate (n-6) and eicosapentaenoate (n -3) , from which eicosanoids are formed. Linoleate 18:2 (9,12) γ-Linolenate 18:3 (6,9,12) Eicosatrienoate 20:3 (8,11,14) Arachidonate 20:4 (5,8,11,14) 6-desaturation elongation 5-desaturation α-Linolenate 18:3 (9,12,15) Octadecatetraenoate 18:4 (6,9,12,15) Eicosatetraenoate 20:4 (8,11,14,17) Eicosapentaenoate 20:5 (5,8,11,14,17) 6-desaturation elongation 5-desaturation
Polyunsaturated fatty acids of lipids exposed to oxygen may be subjects of auto-oxidation initiated by light or by metal ions – the primary cause of deterioration of fat (rancidity). In vivo, similar undesirable lipid peroxidation is initiated by free radicals, mostly by the hydroxyl radicals •OH . Lipid peroxidation is a chain reaction providing a supply of further free radicals (RO•, ROO•). malondialdehyde (•OH) (O 2 ) hydroperoxide further products (ethane, unsaturated hydrocarbons, higher unsaturated aldehydes, etc.) endoperoxide
Trans - fatty acids are present in certain foods. The presence of trans -fatty acids in human nutrition seems to be harmful (e.g., an unfavourable effect on cholesterol metabolism). Most arise as a by-product during the "hardening" of vegetable oils into margarines by means of metal-catalyzed hydrogenation. An additional contribution comes from the ingestion of ruminant fat that contains trans -fatty acids (beef tallow 3-7 %, butter 3 %) arising from the action of microorganisms in the rumen. The shape of trans -unsaturated acids resembles that of saturated fatty acids: cis- unsaturated trans- unsaturated saturated FA
Glycerol ( propane-1,2,3-triol ) – a component of triacylglycerols (fats and oils) and glycerophospholipids Alcohols as constituents of lipids: Glycerol is a symmetrical (non-chiral) compound. Derivatives, in which two of the carbons bind different groups, are chiral. In lipid chemistry, it is required to number the carbon atoms of glycerol unambiguously and the – sn – ( stereospecific numbering ) system is used: 3 1
Sphingosine (4 E )- sphingenine (systematic name (2 S ,3 R ,4 E )- 2-aminooctadec-4-ene-1,3-diol ) contains 18 carbons atoms, trans -double bond in position 4, amino group at position 2, and two hydroxyls at position 1 and 3. CH–CH–CH 2 –OH NH 2 OH The group name sphingosine is used also for dihydrosphingosine (sphinganine) and the C 16 , C 17 , C 19 , and C 20 homologs of sphingenine and sphingosine. Long-chain aliphatic alcohols are constituents of waxes, e.g. hexadecan -1- ol ( cetyl alcohol , palmityl alcohol ) – in spermaceti, octadecan -1- ol (stearyl alcohol), hexacosan -1- ol (ceryl alcohol) - in bees-wax.
Simple lipids Triacylglycerols (fats and vegetable oils) Waxes Ceramides
Triacylglycerols are esters of glycerol and fatty acids. Natural fats and oils (butterfat, lard, olive oil, etc.) consists not of a single triacylglycerol, but of a complex mixture of triacylglycerols . A small portion of diacyl- and monoacylglycerols may be included. For example: 1 - stearoyl - 2 - oleoyl - 3 - palmitoyl - sn - glycerol Triacylglycerols TG Diacylglycerols DG Monoacylglycerols MG, e.g. 2 - oleoyl - glycerol (a 2-monoacylglycerol)
Physical properties of triacylglycerols The melting points of triacylglycerols depend on the degree of unsaturation of bound acyls. A triacylglycerol containing all saturated fatty acids of 12 carbons and more is a solid fat at body temperature . Oils contain a higher percentage of unsaturated fatty acids than do fats. The more double bonds are present in the fatty acid portion of the triacylglycerol, the lower is its melting point. The reason for the effect of saturation or unsaturation on the melting point of triacylglycerols is in the bent structure of unsaturated fatty acyls. In fully saturated triacylglycerols, the long, saturated chains have extended conformations that can pack together fairly regularly (numerous dispersion inter-molecular interactions), as in a crystal. Unsaturated triacylglycerols cannot align in a crystalline array, the substance therefore remains a liquid.
Chemical transformations of fats and oils Converting of vegetable oils into fats which are solid or semi-solid at room temperature – " fat hardening " Interesterification (intermolecular exchange of acyl residues in triacylglycerols) between vegetable oils and solid fats (coconut fat, hog lard, beef tallow) is a process that provides fats or oils with new properties without the accumulation of trans -FA. The spreadable fats obtained from either vegetable oils or solid animal fat are known as margarines . The fat in margarine, which is 80 – 40 % by weight, contains emulsified water and the emulsion is stabilized by emulsifiers ( 0.5 % mono- and diacylglycerols, crude lecithin, etc.). The product may be also stirred and shaken with skim milk to mimic butter's appearance. An old process (developed in 1902) is based on the hydrogenation of unsaturated triacylglycerols using nickel as a catalyst. A drawback of the hydrogenation is the extent of undesirable stereoisomerization -production of trans -monounsaturated fatty acids.
Hydrolysis of triacylglycerols Soap (alkali salts of long-chain fatty acids) is an anionic tenside . Ester bonds in triacylglycerols can be hydrolyzed by boiling with mineral acids or alkaline hydroxides. Hydrolysis by acids will give glycerol and free fatty acids . Alkaline hydrolysis is called saponification , because triacylglycerols are converted into glycerol and the alkaline salts of fatty acids – soaps . In the small intestine, hydrolysis of ingested fats is catalyzed by pancreatic lipase (at pH value about 7.5 – 8.8). The hydrolysis is not complete, two natural tensides are produced: two molecules of soap (an anionic tenside) and 2-monoacylglycerol (a non-ionic tenside).
The processes responsible for the development of rancidity include
– oxidations (see Lipid peroxidation) that produce hydroperoxides and
many secondary products as hydrocarbons, aldehydes,
carboxylic acids, and
– hydrolysis of ester bonds (namely due to bacterial action) that releases
free short-chain fatty acids with an unpleasant odour.
Rancidity of fats and oils Lipid peroxidation of unsaturated lipids can be retarded by – exclusion of oxygen – storage at low temperature in the dark – addition of antioxidants to food. After certain time, fats containing unsaturated fatty acyls turn rancid. The process is accelerated, if fats are exposed to oxygen, heat, light, contain microorganisms, mould, trace amount of metals, or high proportion of water.
– Fats should not cover more than 30 % energy intake.
– Restriction of fats that contain large amount of saturated fatty acids (often accompanied by cholesterol).
– Enhanced intake of olive or rapeseed oil as well as of emulsified margarines (instead of butter).
– Avoid intake of trans -fatty acids, burnt fat, reasonable amount of fried food.
Fats and oils in nourishment – recommendations:
E.g., small amount of waxes is secreted by sebaceous glands of mammals . Lanolin is the fatty substance extracted from sheep's wool; it is a complex mixture of waxes, long-chain alcohols and free and esterified sterols used as basis of ointments Spermaceti (hexadecyl palmitate) – a liquid wax contained in heads of sperm-whale (cachalot). Beeswax – one of the major components is hexacosyl hexacosanoate. are natural mixtures of esters of saturated long-chain fatty acids and aliphatic long-chain primary alcohols . Waxes (cerides) Cer amides ( N -acyl sphinghosines) Although free ceramides are not found in animal tissues, they represent the basal lipidic structure of all types of sphingolipids .
Schematic structures of complex lipids Glycolipids Sphingo phospholipids Plasmalogens Glycero phospholipids Sphingolipids: The " head“ group The " head“ group The " head“ group
The major glycerophospholipids The " head“ group The simplest glycerophospholipid is phosphatidic acid ( phosphatidate, 1,2- diacyl - sn - glycerol 3 - phosphate ). Only very small amounts of phosphatidate are present in membranes. However, the molecule is a key intermediate in the biosynthesis of the glycerophospholipids . Glycerophospholipids
Ph os ph atidyl is the name of a remainder obtained by taking off –OH group. 1,2- diacyl - sn - glycerol 3- phosphate Phosphatidic acid
Alcohols – the hydrophilic head groups attached to phosphatidate through ester bonds in various types of glycerophospholipids
All phospholipid types are natural tensides – the main lipid constituents of membranes Dipolar ( amphoteric ) glycerophospholipids – phosphatidyl choline – phosphatidyl ethanolamine - each of them has one negative and one positive electric charge Acidic glycerophospholipids – phosphatidyl serine - two negative and one positive electric charge – phosphatidyl inositol - one negative electric charge – bis phosphatidyl glycerol - two negative electric charges
Phosphatidyl inositol (up to 20 % membrane phospholipids) Phosphorylation of PI generates phosphatidyl inositol 3,4- bis phosphate ( PIP 2 ) which is an intermediate of the phosphatidyl inositol cycle that generates by hydrolytic splitting two intracellular messengers IP 3 and diacylglycerol .
Cardiolipin - constituent of the inner mitochondrial membrane (1,3- bis phosphatidyl glycerol) Phosphatidyl glycerol
Lysophospholipids (2-deacylphospholipids) are intermediates of phospholipid metabolism Plasmalogens (mostly plasmenyl cholines ) are a group of 1- O -( alken -1- yl )-2- O - acyl glycerophospholipids , enol- ether lipids. They represent about 10 % phospholipids in brain and muscles. . alk -1- enyl enol- ether bond acyl
Glycerophospholipids are – essential structural components of all biological membranes , – essential components of all types of lipoproteins in extracellular fluids, – supply polyunsaturated fatty acids for the synthesis of eicosanoids, – act in anchoring of some proteins to membranes, – serve as a component of lung surfactant – phosphatidyl inositols are precursors of second messengers (PIP 2 , DG), etc.
Anchoring of proteins to membrane The linkage between the COOH-terminus of a protein and phosphatidylinositol fixed in the membrane lipidic dilayer exist in several ectoenzymes (alkaline phosphatase, acetylcholinesterase, some antigens).
Lung surfactant The major component of lung surfactant is dipalmitoylphosphatidylcholine . It contributes to a reduction in the surface tension within the alveoli (air spaces) of the lung, preventing their collapse in expiration. Less pressure is needed to re-inflate lung alveoli when surfactant is present. The respiratory distress syndrome (RDS) of premature infants is caused, at least in part, by a deficiency in the synthesis of lung surfactant.
Sphingolipids – schematic structure A glyco lipid A sphingo phospho lipid The " head“ group Ceramide N -Acylsphingosine
Ceramide is the lipidic part of all types of sphingolipids. Sphingosine ( trans - 2 - aminooctadec - 4-ene-1,3-diol ). Ceramides are N -acylated sphingosines . The acyl residue is attached to the amino group of sphingosine by an amide link : The acyl residue has often 24 carbon atoms (lignoceric acid or its derivatives).
Glyco lipids are ceramides to which a saccharide component is attached by glycosidic bond: monoglycosylceramides – cerebrosides , oligoglycosylceramides, acidic sulphoglycosylceramides , and sialoglycosylceramides – gangliosides . Sphingolipids Sphingo phospho lipids are esters of ceramide-1-phosphate and ethanolamine or (mostly) choline. Ceramide phosphocholines are called sphingomyelins . Phosphocholine β- D-Glucopyranosyl Cerebroside:
Sphingophospholipids Sphingomyelin is the most abundant sphingophospholipid and is found in myelin (the fatty substance of the sheath around neurons. phosphoryl group choline
Cerebroside (1- O - glucosylceramide) Glycolipids are important in nerve tissues and in the plasma membrane. They occur particularly in the outer leaflet of the plasma membrane, where they contribute to cell surface saccharides (glycocalyx). β- D -glucopyranosyl The most simple glycolipids are cerebrosides – monoglycosylceramides Glucosyl ceramide is the predominant glycolipid in extraneuronal tissues, while galactosyl ceramide is a major glycolipid of brain and other nervous tissue. Galactosylceramides can be sulfated and the products – sulfoglycolipids (sulfatides) are present in high amounts in myelin. Complex saccharidic component that contain in addition one or more molecules of sialic acid is present in gangliosides
In spite of the difference in the structures of glycerophosphoplipids and sphingophospholipids, the over-all shape of the both types of phospholipid (as well as of glycolipid) molecules is very similar: Glycerophospholipid Sphingophospholipid Simplified icon of a phospholipid or glycolipid molecule Polar head Two hydrophobic chains
A flat micelle, a bimolecular lipid layer Rapid lateral diffusion ( a fluid mosaic ) Very slow transverse diffusion (flip-flop) Membrane proteins are inserted in lipid b ilayer or bound to either surface. Phospholipids (and glycolipids) are the main lipid constituents of membranes :