Lipid Structure and Function PDF

LIPID STRUCTURE AND FUNCTION Lipids ❖ They are a heterogeneous group of compounds which includes fats, oils and waxes ❖ They have high energy value and are an important constituent of the diet ❖ The molecule in comprised of the elements carbon (C), hydrogen (H), and oxygen (O) ❖ They are insoluble...

LIPID STRUCTURE AND FUNCTION Lipids ❖ They are a heterogeneous group of compounds which includes fats, oils and waxes ❖ They have high energy value and are an important constituent of the diet ❖ The molecule in comprised of the elements carbon (C), hydrogen (H), and oxygen (O) ❖ They are insoluble in water and are thus hydrophobic ❖ They act as energy stores ❖ They are stored under the skin and help to keep the body warm ❖ They are the structural components of cells forming membranes ❖ They are used as hormones which are important in the regulation of metabolic processes ❖ They are carriers of fat soluble vitamins (Vit. A, D, E, K) ❖ Antibiotic agent (squalamine – steroid found in sharks) ❖ Lipids aid to “waterproof ” us Fatty Acids  Fatty acids are building blocks of lipids  Fatty acids have the general formula R-COOH, where R = H, hydrocarbon chain  The R group is therefore non polar and is hydrophobic  Most fatty acids are straight-chain compounds with an even number of carbon atoms.  Odd-numbered fatty acids are mostly frequent in bacteria and lower plants or animals.  Chain-lengths commonly range from 12 up to 24. With a chain length from 2 to 6 (or 4) they are called short-chain, from 8 (or 6) to 10 they are called medium-chain and 12 up to 24 they are called long-chain fatty acids. Their physical and biological properties are related to the 3 classes mentioned. Fatty Acids  Even number straight chain fatty acids are found abundantly in plants and animals (C16 – palmitic acid; C18 stearic acid ……C10)… reflecting the pathway for their biosynthesis from the two-carbon building-block acetyl CoA  Two groups of fatty acids depending on the structure of the hydrocarbon chain saturated and unsaturated fatty acids Fatty acids ❖ If the hydrocarbon chain contains no double bonds then the fatty acid is saturated Fatty Acids  If the hydrocarbon chain of fatty acids contain double bond(s) they are called unsaturated fatty acids Fatty Acids ❖ Unsaturated fatty acids may be classified based on the degree of unsaturation monoethanoid acids: oleic acid Diethanoid acids: linoleic acid EFA (must be obtained from diet) Triethanoid acid: linolenic acid N.B all foods contain small quantities of bound fats or invisible fats (takes care of half to the daily fat requirements Linoleic Acid (once called vitamin F) Linolenic Acid Saturated fatty acids Unsaturated fatty acids Solids at room temperature Liquid at room temperature They are found in animals They are found in plants Stored mainly in liver and bone Stored in seeds and fruits Fatty Acids ❖ Many naturally occurring fatty acids contain 2-3 double bonds in the cis position ❖ The carbon chains of saturated fatty acid tend to be fully extended because this minimizes repulsion between neighboring methylene groups ❖ The cis conformation of the double bond of an unsaturated fatty acid puts a rigid bend in the carbon chain that interferes with packing causing reduced van der Waals attractions between molecules ❖ Therefore unsaturated fatty acids have lower melting points http://courses.washington.edu/conj/membrane/fat tyacids.htm Fatty Acids ❖ There are enzymes present in the ER that are responsible for desaturating fatty acids (i.e. adding cis double bonds) ❖ The body is able to convert stearic acid to oleic acid by inserting a double bond but incapable of inserting further double bonds ❖ Humans have carbon 4,5,6 and 9 desaturases, but lack the ability to introduce double bonds from carbon 10 to the ω end of the chain ❖ Unsaturated fatty acids from vegetable oil can have their double bonds removed by adding hydrogen (Hydrogenation): hydrogenated fat (saturated fat even though derived from vegetable oil) Properties of Fatty Acids ❖ Fatty acids are important sources of fuel …when metabolized they yield large quantities of ATP ❖ Many cell types can use either glucose or fatty acids for fuel. In particular the heart and skeletal muscle prefer fatty acids Classification of Lipids ❖ Lipids can be classified as – (a) Simple lipids/homolipids – e.g. triglycerides/triacylglycerols, oils and waxes (b) Compound lipids/ heterolipids – e.g. phospholipids and glycolipids (c) Derived Lipids - they are derived from simple and compound lipids e.g. steroids Simple Lipids: Triglycerides/Triacylglycerols ❖ They are the commonest lipids in nature ❖ They are non polar and are therefore insoluble in water ❖ They are less dense than water → floats ❖ It is formed via esterification - reaction between an alcohol (glycerol) and a tricarboxylic acid ❖ Triglycerides can undergo hydrolysis to form free glycerol and fatty acids Functions of Triglycerides/Fats ❖ They store long term energy The provide more energy per gram (9 kcal/gram) than proteins or carbohydrates (~4.0 kcal/gram) ❖ They have greater satiety value than carbohydrates ❖ They act as insulators for the body ❖ FACT! The human body has enough fat to make 7 bars of soap ❖ They act as shock absorbers for organs ❖ May make up more than 90% of the cytoplasm ❖ % fat women 25 – 31 % and males 18 – 24 % ❖ The % fat stored is sufficient to allow us to survive 2 -3 months. Glycogen only provides 1 day of energy Functions of Triglycerides/Fats ❖ When the body has used up the calories from carbohydrates… (occurs after the first 20 minutes of exercise), it begins to depend on the calories from fat. ❖ Healthy skin and hair are maintained by fat. Fat helps the body absorb and move the vitamins A, D, E, and K through the bloodstream ❖ About 98% of the lipids in our diet are triglycerides, the remaining 2% consists of complex lipids and cholesterol ❖ Triglycerides cannot pass through cell membranes freely. Enzymes called lipoprotein lipases must break down triglycerides into free fatty acids and glycerol. The hormone glucagon stimulates the process ❖ The lipid is first emulsified by bile salts produced in the liver ❖ The glycerol component of triglycerides can be converted into glucose for brain fuel Functions of Triglycerides ❖ The amount of fat in the diet, especially saturated fat is a health concern …High levels of triglycerides in the bloodstream may result in atherosclerosis (hardening of the arteries) resulting in the risk of heart disease and stroke Compound Lipids: Phospholipids ❖ They are the most abundant membrane lipids ❖ They are the structural components of the membrane ❖ They differ from triglycerides having one polar head and two hydrophobic tails Compound Lipids: Phospholipids ❖ The bipolar nature of phospholipids allows for the formation of bimolecular sheets (bilayers) http://www.bioteach.ubc.ca/Bio-industry/Inex/graphics/lipidbilayer.gif Compound Lipids: Glycolipids ❖ These comprise of lipids linked to carbohydrates ❖ The carbohydrate portion forms a polar head to the molecule ❖ They are an important constituent of cell membranes, particularly in the myelin sheath and outer surface of the nerve cells and the chloroplast membrane ❖ Both phospholipids and glycolipids form self- sealing lipid bilayers that are the basis of all cellular membranes Derived Lipids: Cholesterol ❖ Found in the blood of all animals ❖ Blood plasma levels range from 15 – 250mg/100mls ❖ Cholesterol is acquired by the body in two ways (a) Production in the cells of the body (b) Dietary sources e.g. meat and dairy products ❖ Sites of production includes – liver, gonads, adrenal glands, nervous tissues and intestines Derived Lipids: Cholesterol ❖ Cholestrol is absent from plant cells…vegetable oil is considered as cholesterol free ❖ Important component of cell membrane ❖ Important precursor of many biological compounds e.g. bile acids and steroid hormones ❖ The end product of cholesterol metabolism → bile salts ❖ These salts are formed in the liver and secreted into the small intestines to absorb lipids ❖ When the bile salts are attached to the lipid the complex is called a chylomicron ❖ Bile salts are largely reabsorbed during lipid absorption Lipoprotein ❖ Cholesterol is insoluble in blood ❖ It travels in the blood bound to lipoproteins. These particles are classified based on their composition and density ❖ Lipoproteins contain lipids surrounded by hydrophilic molecules such as proteins and phospholipids. There exist four kinds of lipoproteins, these include ❖ High density lipoprotein (HDL) - removes excess cholesterol from the cell and returns it to the liver where it is metabolized to bile acids and salts that are eventually eliminated through the intestine ❖ Low Density lipoproteins (LDL) – These molecules contain a large amount of cholesterol and cholesteryl esters. Its main function is to delivers cholesterol to the cell to be used mostly as a membrane component Lipoprotein ❖ Intermediate Density Lipoproteins (IDL) have density in between the HDL and LDL and are responsible for the transport of cholesterol and triacylglycerol ❖ Very Low Density Lipoprotein (VLDL) – It is synthesized by the liver. Its main purpose is to deliver triglycerides, cholesteryl esters and cholesterol to the peripheral tissues ❖ Chylomicrons – They have the lowest lipid to protein ratio and hence the lowest density. They are synthesized by the absorptive cells of the intestinal lining and are secreted by these cells into the lymphatic system which joins the blood circulation. They carry dietary lipids synthesized in the intestine to the heart muscle, skeletal muscle, adipose tissue and lactating mammary tissue HDL vs LDL ❖ LDL sticks to the artery walls and can lead to plaque build up (atheriosclerosis) ❖ Higher proportion of LDL compared to HDL → associated with cardiovascular diseases such as heart attack and stroke ❖ Diet and exercise help to maintain balance in the LDL and HDL ❖ Two important cholesterol diseases (a) Familial hypercholesterolemia : Over production of cholesterol (b) Zanthomatosis: Build up of cholesterol in the skin Hormones ❖ Hormones are chemical messengers that are produced in any one part of an organism and is transferred to another part where it influences a specific physiological response ❖ The tissue or organ that produce the hormone: effectors Where they exert their influence: targets ❖ Many steroid hormones are formed from cholesterol: Progesterone Cortisol Aldosterone Testosterone Estradiol http://humanphysiology2011.wikispaces.com/file/view/steroid_hormones.jpg/201094460/426x636/steroid_ hormones.jpg ❖ Progesterone…secreted by the corpus luteum 1. Promote manifestation of the uterine mucosa to receive the fertilized ovum 2. Brings mammary glands to full maturity during gestation 3. Inhibits contraction of the uterus during pregnancy 4. Exerts a anti ovulatory effect if given between the 5th and 25th day of normal 5. ❖ 1. 2. 3. 4. 5. 6. 7. menstrual cycle Precursor of cortisol Cortisol (Glucocorticoid)…secreted by adrenal or suprarenal glands Increase the release of glucose from the liver and promote the transformation of amino acids to carbohydrates (gluconeogenesis) Inhibit protein synthesis in muscle tissues Regulate lipogenesis Reduce bone matrix..osteoporosis and heavy loss of calcium from the body Decrease immune response Cause increase secretion of HCl and pepsinogen in the stomach Cause retention of sodium and water, and loss of potassium ❖ 1. 2. ❖ 1. 2. 3. 4. ❖ 1. 2. 3. 4. 5. Aldosterone (Mineralocorticoid)… secreted by adrenal or suprarenal glands Maintain water balance in the body Regulate salt (NaCl) concentration in the blood by stimulating the reabsorption of sodium ion from the kidney tubules Testosterone (Androgens) …secreted mainly by the testes Increase the size and strength of bones Development of secondary sex characters in male Controls libido Stimulates spermatogenesis Estradiol (Estrogen)…secreted by the ovaries Development of secondary sex characters in females Preserves elasticity of the skin Protects women from osteoporosis by slowing the rate at which calcium leaches from the bone In combination with progestin…Hormone replacement therapy Improves memory Eicosanoids ❖ Eicosanoids differ from true hormones in that they are produced in small amounts by all cells rather than by specialized cells ❖ Eicosanoids are not stored, they have an extremely short half life, being rapidly metabolized to inactive products. ❖ The compound constitutes 20 C atoms (eicosa – means 20) e.g. arachidonic acid ❖ Arachidonic acid is formed from linoleic acid and linolenic acid Eicosanoids ❖ Eicosanoids are classified into two main groups 1. Prostaglandins, Thromboxanes 2. Leukotrienes ❖ Eicosanoids are involved in complex control reactions in inflammation, immunity and as CNS messengers ❖ They affect many of the body’s functions, example in heart disease, levels of triglycerides and arthritis ❖ Prostaglandins, leukotrienes and thromboxanes are produced “at the site” are considered “local hormones” Prostaglandins ❖ Prostaglandins can lower blood pressure ❖ Clinical use of prostaglandins is in uterine contractions and to stop postpartum bleeding ❖ Causes vasodilation ❖ Inhibits platelet aggregation ❖ Control inflammation and vascular permeability Leukotrienes ❖ Over production occurs in asthma and allergic rhinitis ❖ Contraction in smooth muscles lining bronchioles ❖ Regulators of allergic reactions Thromboxanes ❖ Aid in blood clotting ❖ Reduce blood flow to the site of a clot Terpenoids ❖ Terpenoids are a large and diverse class of naturally occurring organic chemicals ❖ Most terpenoids are multicyclic structures that differ from one another not only in functional groups but also in their basic carbon skeletons ❖ Essential oils can be obtained from plants by heating or applying steam distillation to certain plant material. These can be used in medicine or in the perfume industry ❖ Hydrocarbons known as terpenes and oxygen containing compounds known as terpenoids are the most important constituent of these oils Biological Importance of Terpenoids ❖ The steriods and sterols in animals are biologically produced from terpenoid precursors ❖ Sometimes terpenoids are added to proteins, e.g., to enhance their attachment to the cell membrane; this is known as isoprenylation Industrial Importance of Terpenoids ❖ Plant terpenoids are used extensively for their aromatic qualities ❖ Terpenoids play a role in traditional herbal remedies and are under investigation for antibacterial, antineoplastic, and other pharmaceutical functions ❖ Terpenoids contribute to the scent of eucalyptus, the flavors of cinnamon, cloves, and ginger ❖ Well-known terpenoids include citral, menthol, camphor and the cannabinoids found in Cannabis