Part 1: General Structure & Function of Lipids

Transcription

1 CND/S 412 Biochemistry Lipids Part 1: General Structure & Function of Lipids Jess Keane BSc PG Cert mntoi

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3 Lipid Classification Fatty Acids (FAs) - monomers Triglycerides: 3 FAs esterified with glycerol Found in fats and oils Predominate in food and in the body

4 Lipid Classification Phospholipids: 2 FAs and a phosphate group and amino alcohol (e.g. choline) esterified with glycerol Glycolipids: FAs compounded with glycerol or sphingosine + monosaccharide

5 Lipid Classification Steroids: 3 cyclohexanes + 1 cyclopentane attached to a hydrocarbon chain Sterols: a subgroup of steroids Waxes: fatty acid + a long-chain alcohol

6 Lipid Characteristics Lipids are insoluble in water, but soluble in an organic solvent (e.g. ether, benzene, acetone, chloroform) The main feature, in all lipids, is the large number of carbon-hydrogen bonds which makes them non-polar.

7 Energy Stores Function of Lipids Triglycerides act as energy reserves. The majority of triglycerides are stored in adipose tissue, in adipocytes (fat cells) Sources of energy Fatty acids are released from triglycerides are broken down in the mitochondria and used in the production of energy. Insulation Subcutaneous adipose tissue is important in the maintenance of body temperature

8 Function of Lipids Protection against shock Visceral fat protects organs like the kidneys Absorption Phospholipids help to emulsify fats and increase absorption of fats and fat-soluble nutrients (e.g. sterols, vitamins) Local hormones Membrane phospholipids can be converted into hormone-like substances called eicosanoids which control smooth muscle contraction, blood clotting and immune cell stimulation

9 Metobolic Functions: Function of Lipids Cholesterol is metabolised to: Sex Hormones - reproduction Corticosteroids stress response Mineralcorticoids blood pressure regulation Bile Acids digestive health Vitamin D immune modulation, cellular growth

10 Function of Lipids Structural element of cell membrane Phospholipids Extremely important for brain health Cell Communication Inositol phospholipids translate extracellular signals into intracellular ones Sphingolipids are involved in cell recognition and cell signalling cascades involved in apoptosis, proliferation, inflammation and differentiation

11 CND/S 412 Biochemistry Lipids Part 2: General Structure & Function of Fatty Acids Jess Keane BSc PG Cert mntoi

12 Structure of Fatty Acids Methyl End Common building block for most lipids Long-chain carboxylic acids Made up of carbon, hydrogen, oxygen Mostly carbon & hydrogen atoms Usually have an even number of carbons

13 Structure of Fatty Acids Hydrocarbon chains of varying length: 4-30 carbons (most usual is 12-18) The non-polar hydrocarbon chain is an important counter balance to the polar acid carboxylic group

14 Fatty Acid Characteristics A fatty acid may be characterised as saturated, unsaturated, monounsaturated, polyunsaturated or trans fatty acid. This is determined by its chemical bonds and structure

15 Fatty Acid Characteristics Carboxyl group: can react through the weak acidity of the hydroxyl hydrogen via the difference in electronegativity between carbon and oxygen. In acids with only a few carbons, the acid functional group dominates and gives the whole molecule a polar character.

16 Fatty Acid Nomenclature Nomenclature reflects location of double bonds Also used are common names (e.g., oleic, stearic, palmitic) Linoleic acid is also known as 18:2 n-6 i.e. FA is 18 carbons in length, has 2 double bonds, the first of which is on the 6th carbon, from the omega (ω) or methyl group end so it is known as an omega 6 fatty acid Arachidonic acid = 20:4 n-6

17 Very Short Chain Fatty Acids Contain 2-3 carbons Acetic acid: 2 carbons Found in vinegar esp. cider vinegar Propionic acid 3 carbons Butyric acid (4 carbons) Easily metabolized / ~ water soluble

18 Medium Chain Fatty Acids Contain 6,8,10 or 12 carbons Caproic: 6 carbons Caprylic: 8 carbons Capric: 10 carbons Lauric: 12 carbons Main sources: dairy, palm, coconut & breast milk Easily metabolised Good source of energy

19 Saturated Fatty Acids Mainly found in: dairy, meats, palm and coconut oils SFAs have all bonding positions between carbons occupied by hydrogens. There are no double bonds between carbons in chain Vary in length Number of Carbons Butyric Acid (4:0) Palmitic (16:0) Number of double bonds Stearic (18:0)

20 Types and Sources of Saturated Fatty Acids Name of Acid Occurence in Foods Name of Acid Occurence in Foods Acetic Priopronic vinegar Myristic milk and some vegetable fat, palm oil Butyric Valeric Caproic % in cow butter 1-2% in cow butter, trace in palm oil Caprylic 1-2% in cow butter, 6-8% in cocoa butter Capric milk fat, palm oil Palmitic Stearic almost all natural fats fats of land animals Arachidic grount nut oil (3%), traceds widely distributed Behenic Lignoceric 2% in ground nut and rapeseed oil ground nut and rapeseed oil-under 3% Lauric cow butter and palm oil Cerotic traces in most vegetable fats 20

21 Saturated Fatty Acids

22 Characteristics of SFA Are solid at room temperature and have a high melting point The reason for this is the geometry of the molecule; the tetrahedral bond angles on carbon results in a molecular geometry for saturated fatty acids that is relatively linear although with zigzags.

23 Characteristics of SFA This structure allows SFAs molecules to aligned closely (aggregate) and interact by Van der Waals forces, resulting in relatively high melting points. They are solid at room temperature and exhibit less fluid natures => important for cell membrane consistency

24 Fatty Acid Structure

25 Saturated Fats

26 Unsaturated Fatty Acids Unsaturated Fatty Acids: have one or more double bonds in the chain Monounsaturated Fats One double bond. Most common MUFA - Oleic Acid - 18:1 n-9 (Omega 9) Main fat present in: olive oil, nuts, avocado Polyunsaturated: two or more double bonds Most double bonds are cis- bonds that cause a bend in the chain

27 Unsaturated Fatty Acids Linoleic acid (LA) 18:2 n-6 (Omega 6) Main food sources: abundant in most seeds & their oils Carboxyl End Δ-end Methyl End ω- end α-linolenic acid (ALA) 18:3 n-3 (Omega 3) Main food sources: flaxseed, hemp, rapeseed, soybean, and walnut oils and in dark green leaves

28 Unsaturated Fatty Acids Gamma linolenic acid (GLA) 18:3 n-6 (Omega 6) Main food sources: evening primrose oil (EPO), blackcurrant seed and borage oils Note omega the end is the opposite to the carboxyl group

29 Polyunsaturated Fats Eicosapentaenoic acid (EPA) 20:5 n-3 (Omega 3) Main food sources: fish, human breast milk Docosahexaenoic acid (DHA) 22:6 n-3 (Omega 3) Main food sources: fish, algae, human breast milk 29

30 Characteristics of USFAs Unsaturated fatty acids are liquid at room temperature and have a low melting point One or more double bonds in the hydrocarbon chain results in one or more "bends" in the molecule. This is due to steric hindrance between the hydrogen atoms either side of the double bond.

31 Characteristics of USFAs These molecules do not "stack" very well. The intermolecular interactions are much weaker than saturated molecules. As a result, the melting points are much lower for unsaturated fatty acids and they exhibit the fluid nature (important in membrane structure) and are mostly found to be liquid at room temperature.

32 Unsaturated Fatty Acids

33 Characteristics of Unsaturated Easily oxidised When exposed to oxygen, the carboxyl group or double bonds oxidise» Saturated fats are more resistant to oxidation Fatty Acids

34 Oxidation of Fatty Acids Radical formation is accelerated by light, oxygen and heat Keep polyunsaturated fats in dark glass bottle in the fridge Avoid high temperature cooking 34

35 Characteristics of Unsaturated Fatty Acids Unsaturated fatty acids undergo a chemical change known as auto-oxidation. The process requires oxygen (air). Vegetable oils resist this process because they contain antioxidants, such as tocopherol.

36 Cis & Trans Fatty Acid Structure Double bonds in fatty acid prevents the free rotation around the bond, creating two configurations; cis and trans These are geometrical isomers.

37 Cis & Trans Fatty Acid Structure In cis-form the hydrogen atoms of double bonded carbon atom oriented on same side In trans-form they oriented in opposite direction. The differences in geometry play an important role in biological processes.

38 Cis & Trans Fatty Acid Characteristics Cis-fatty acids are generally found naturally while trans-fatty acids are rare Trans-fatty acids are found in manufactured fats. They are created at high temperatures such as those reached during frying, deep frying and hydrogenation. Hydrogenation of polyunsaturated fatty acids stabilizes them and prevents them from becoming rancid and keeps them solid at room temperature.

39 Hydrogenation Hydrogenation (adding H2) - Converts a double bond to a single bond Hydrogenated fatty acids more saturated than natural vegetable oils able to pack together more tightly more solid at room temperature behave more like saturated fatty acids

40 Cis & Trans Fatty Acid Characteristics Cis and trans forms of fatty acids show different physical and chemical properties just like other organic geometrical isomers. Trans isomers show higher melting points due to closely packed structure compare to cis isomers. Trans-fatty acid resemble saturated fatty acids in their structure which can interrupt many biochemical reactions.

41 Trans Fatty Acid Characteristics Harmful effects include blood platelets aggregation (more sticky), increased cell membranes rigidity, decreased permeability of the cell membranes and decreased liver detoxification capacity. See link above for factsheet on trans fats

42 CND/S 412 Biochemistry Lipids Part 3: Essential Fatty Acid Metabolism Jess Keane BSc PG Cert mntoi

43 Fats are synthesized from Acetyl-CoA in mitochondria Acetyl-CoA is created in the breakdown of carbohydrates and proteins Therefore fats can be synthesized from proteins and carbohydrates Fatty Acid Synthesis

44 Fatty Acid Synthesis Plants and animals can produce unsaturated FAs from saturated FAs. A double bond can be formed by removing 2 hydrogen atoms in the process of Desaturation In most mammals, double bonds can be introduced at the 4, 5, 6, and 9 positions but never beyond the 9 position.

45 Fatty Acid Synthesis Humans do not have the enzyme to insert a double bond (desaturate) at C3 and C6 (3 and 6 carbon atoms from the methyl end) therefore these fats are essential

46 Fatty Acid Synthesis Plants have the ability to and therefore make linoleic acid and α- linolenic acid. These 2 FAs are called Essential Fatty Acids; they are vital to human health but cannot be made by our cells

47 Fatty Acid Food Sources (Lord and Bralley 2008)

48 Fatty Acid Food Sources Foods do not consist of just 1 type of fat (Lord and Bralley 2008)

49 Fatty Acid Metabolism Conversion of Linoleic acid (LA) and alpha Linolenic Acid (ALA) into their longer-chain derivates occurs in the process of elongation Elongase enzymes add carbon atoms to preformed fatty acids (usually acetyl CoA, two-carbon units are added) E.g. EPA and DHA are created from ALA as a result of elongation and desaturation

50 Fatty Acid Metabolism

51 Fatty Acid Metabolism Delta-6 Desaturase: This enzyme is inhibited by Mg, Zn deficiency Vitamin B6, B3, C deficiency Insulin resistance Age Alcohol Adrenaline and glucocorticoids

52 Fatty Acid Metabolism Delta-5 Desaturase: Is inhibited by : Mg, Biotin deficiency Insulin resistance Glucagon Adrenaline and glucocorticoids

53 Omega 6 pathway Omega 3 pathway Sunflower, vegetable, safflower, soya oil and processed foods LA Linoleic acid Delta 6 Desaturase enzyme Mg, Zn, B3, B6, C ALA Alpha-linolenic Acid Flax, pumpkin seed, walnuts, hemp, dark green leafy vegetables Evening primrose oil, borage ( (starflower) oil GLA Gamma-Linolenic Acid Stearidonic acid elongase enzyme DGLA Dihomo-Gamma-Linolenic Acid Eicosatetraenoic acid Meat, dairy, eggs AA Arachidonic Acid Delta 5 Desaturase enzyme Mg, Bio n EPA Eicosapentaenoic acid Seafood especially oily fish: herring, mackerel, sardines, salmon COX-1 & COX-2 enzyme DHA Docosahexaenoic acid Prostaglandin Series 1 An - inflammatory Leukotrienes Proinflammatory Prostaglandin Series 2 Proinflammatory Prostaglandin Series 3 An - inflammatory Elspeth Stewart Key Nutri on

54 Fatty Acids Metabolism Fatty acid molecules are broken down into two-carbon fragments (Acetyl CoA) in mitochondria in a reaction called beta-oxidation Inside mitochondria they enter the Krebs Cycle

55 Essential Fatty Acids Functions EFA help maintain membrane fluidity and thus all the aspects of cell-to-cell communication (activity of most membrane enzymes increases with the degree of membrane lipid unsaturation) EFA (ALA, DHA, AA) are essential for foetal brain development EFA are the precursors of eicosanoids Refer to Part 7: Eicosanoids

56 CND/S 412 Biochemistry Lipids Part 4: General Structure & Function of Triglycerides Jess Keane BSc PG Cert mntoi

57 3 fatty acids + glycerol The form of lipid most found in food Also adipocytes contain triglycerides (storage molecule) Triglycerides

58 Structure of Triglycerides Note: The constituent fatty acids may be of varying structures & therefore chemical properties

59 The -OH (red) bond on the acid is broken and the -H (red) bond on the alcohol is also broken. Both join to make HOH, a water molecule. Esterification mbook/552triglycerides.html

60 Hydrolysis Lipases hydrolyse triglycerides to fatty acids and glycerol.

61 Triglyceride Metabolism Adipocytes acquire fatty acids from circulating lipoproteins by an enzyme lipoprotein lipase, which releases free fatty acids and glycerol from the lipoproteins and allows them to be taken up by the cell When there is a demand for energy from fats, triglycerides are mobilised from adipocytes by an enzyme hormonesensitive lipase and released into the blood

62 Fig 24.20(b) from Principles of Anatomy and Physiology by Tortora and Derrickson 11 th Edition pub John Wiley & Sons 2006

63 Triglyceride Metabolism Lipoprotein (transports triglycerides in the blood)

64 Characteristics of Triglycerides Not a part of biological membranes Triglycerides are storage molecules Most of the body s energy stores are triglycerides Storage is in adipose tissue Source: dietary or anabolism (synthesis) from carbohydrate or amino acid carbon skeletons Breakdown of triglycerides releases fatty acids into the blood stream

65 Characteristics of Triglycerides What makes triglycerides good stores of energy? Anhydrous Number of hydrogens means more reducing power Less water equals more energy weight for weight. When oxidised, fats produce 9kcal/g, glycogen, starch, and proteins yields about 4kcal/g.

66 Triglyceride Metabolism What makes triglycerides good stores of energy? Hypoglycaemia => lipolysis; mobilisation of triglycerides from adipocytes and hydrolysis of fatty acids => beta oxidation in mitochondria => acetyl Co A enters krebs cycle => energy Glycerol undergoes gluconeogenesis to produce glucose => glycolysis => energy production

67 Triglyceride Metabolism Lipolysis is stimulated by: Adrenaline and noradrenaline Adrenocorticotropic hormone (ACTH) Thyroid-stimulating hormone (TSH) Thyroxine Glucagon Growth hormone

68 CND/S 412 Biochemistry Lipids Part 5: Phospholipids & Glycolipids Jess Keane BSc PG Cert mntoi

69 Phospholipid Structure 1. A glycerol esterified to 2 fatty acids (nonpolar). The two fatty acids tend to be non-identical. They may differ in length and/or the presence or absence of double bonds. 2. The third alcohol group on the glycerol is bonded to phosphoric acid through a phosphate ester bond (oxygen-phosphorus double bond oxygen) 3. There is usually a complex amino alcohol also attached to the phosphate through a second phosphate ester bond. The complex amino alcohols include choline, ethanolamine, and the amino acid-serine (polar)

70 Phospholipid Structure

71 Characteristics of Phospholipids The long hydrocarbon chains of the fatty acids are non-polar. The phosphate group has a negatively charged oxygen and a positively charged nitrogen to make this group ionic. The oxygen of the ester groups also make on whole end of the molecule strongly ionic and polar.

72 Phospholipid Structure Minor portion of lipids in diet Phospholipids are major components in the lipid bilayers of cell membranes Their amphipathic nature (having a non-polar end and a polar end) makes them well suited to this function. Amphipathic lipids in an aqueous environment form complexes in which their polar regions are in contact with water and their hydrophobic regions are away from water.

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74 Phospholipid Characteristics Membrane fluidity: The interior of a lipid bilayer is normally highly fluid - the hydrocarbon chains of phospholipids are disordered and in constant motion. The degree of fluidity depends on the proportion of unsaturated fatty acids in the phospholipids Note: Cholesterol can give the membrane a degree of rigidity (which is also important otherwise the cell membrane could be too fluid and therefore leaky)

75 Phospholipid Structure Lecithin (phosphatidylcholine) is most common Found in eggs, liver, soybeans, wheat germ, peanuts, spinach, legumes Lecithin is also made in liver Lecithin is extracted from soy beans for use as an emulsifying agent in foods. Lecithin is an emulsifier because it has both polar and non-polar properties, which enable it to cause the mixing of other fats and oils with water components.

76 Sphingolipid Structure Sphingolipid: sphingosine + fatty acid + phosphate/amino alcohol sphingosine instead of the glycerol very- long-chain (up to C26), saturated fatty acids these interact with cholesterol to help stabilise cell membranes and therefore regulate cell signaling. Polyunsaturated fatty acids are only rarely present, although sphingomyelins of testes are exceptions; they contain polyunsaturated fatty acids, up to 34 carbon atoms, including 28:4(n-6) and 30:5(n-6) Important in the myelin sheath that surrounds most nerve fibers. Therefore found in brain and nervous tissue.

77 Glycolipid Structure Glycolipid: glycerol + 1 fatty acid + sugar always contain a sugar group rather than a phosphate. The sugar group is often galactose but may be glucose. In cell membranes. Large concentrations in brain tissue. Function in cell adhesion & self-identity markers

78 CND/S 412 Biochemistry Lipids Part 6: Sterols & Steroids Jess Keane BSc PG Cert mntoi

79 Steroid Structure Steroids have a 4 ring structure consisting of 3 cyclohexane rings and 1 cyclopentane ring. Sterols are steroid alcohols, the most common is cholesterol.

80 Cholesterol Characteristics Cholesterol, an important constituent of cell membranes, has a rigid ring system and a short branched hydrocarbon tail. Cholesterol is largely hydrophobic. But it has one polar group, a hydroxyl, making it amphipathic.

81 Cholesterol Characteristics Interaction with the relatively rigid cholesterol decreases the mobility of hydrocarbon tails of phospholipids, decreasing the fluidity of the membrane

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83 Bile Acids Structure

84 Bile Acid Characteristics Bile acids are amphipathic. This means that they have both water-soluble and waterinsoluble (or fat-soluble) parts, making them good emulsifying agents. In order for the human digestive system to digest fats, they must be emulsified into the digestive juices, because the enzymes that break them down are water-soluble.

85 Bile Acid Characteristics

86 CND/S 412 Biochemistry Lipids Part 7: Eicosanoids Jess Keane BSc PG Cert mntoi

87 Eicosanoids The eicosanoids consist of the prostaglandins (PG), thromboxanes (TX), leukotrienes (LT) and lipoxins (LX). The principal eicosanoids of biological significance to humans are a group of molecules derived from the C20 fatty acid, arachidonic acid, from dihomo-γ-linolenic acid (DGLA) and from eicosapentaenoic acid (EPA)

88 Omega 6 pathway Omega 3 pathway Sunflower, vegetable, safflower, soya oil and processed foods LA Linoleic acid Delta 6 Desaturase enzyme Mg, Zn, B3, B6, C ALA Alpha-linolenic Acid Flax, pumpkin seed, walnuts, hemp, dark green leafy vegetables Evening primrose oil, borage ( (starflower) oil GLA Gamma-Linolenic Acid Stearidonic acid elongase enzyme DGLA Dihomo-Gamma-Linolenic Acid Eicosatetraenoic acid Meat, dairy, eggs AA Arachidonic Acid Delta 5 Desaturase enzyme Mg, Bio n EPA Eicosapentaenoic acid Seafood especially oily fish: herring, mackerel, sardines, salmon COX-1 & COX-2 enzyme DHA Docosahexaenoic acid Prostaglandin Series 1 An - inflammatory Leukotrienes Proinflammatory Prostaglandin Series 2 Proinflammatory Prostaglandin Series 3 An - inflammatory Elspeth Stewart Key Nutri on

89 Eicosanoids To do with inflammation Consider an injury

90 One of the responses to injury Prostaglandins series 2 vasodilators Leukotrienes series 4 attract immune cells Thromboxane A2 platelet aggregation (eicosanoids have a short half life mins)

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92 Eicosanoid Metabolism Fatty acid membrane composition determines which prostaglandins will predominate Membrane composition is determined by the diet and the desaturation/elongation reactions More AA, LA in the diet - more PGE2; more ALA/EPA in the diet-more PGE3, more GLA in the diet-more PGE1 (Garrow et al 2004)

93 (Kumar 2005)

94 Eicosanoids Functions Potent hormone like action. Prostaglandins Constrict smooth muscle: uterus Mediate inflammation Thromboxanes Encourage clotting and restrict blood flow to wound Leukotrienes Chemotactic agent, bronchoconstrictors-allergy.

95 Eicosanoids Functions Prostaglandins Series 1-PGE1: Reduces platelet aggregation (lower risk of heart attacks, strokes) Help to remove sodium and water excess from the body Vasodilation improving circulation, lowering blood pressure, relieving angina PGE1 and PGE3 prevent the release of Arachidonic Acid (precursor of proinflammatory PGE2 ) from the cell membranes. Prostaglandins Series 2-PGE2 - opposing effects

96 Further Reading ex.html Web/mb1/part2/ CHEM43/CHEM43/Sphingolipids/sphingolipids.ht ml oids.php#intro