Lecture 28 - Chemistry and Function of Lipids PDF

LECTURE-28 Chemistry and functions of lipid 1  Objectives The objectives of this lecture are to Discuss the classification of lipids, fatty acids, phopholipids, triacylglycerols and cholesterol; prostaglandins and their biochemical action....

LECTURE-28 Chemistry and functions of lipid 1  Objectives The objectives of this lecture are to Discuss the classification of lipids, fatty acids, phopholipids, triacylglycerols and cholesterol; prostaglandins and their biochemical action. 2  Introduction The lipids are a heterogeneous group of compounds, including fats, oils, steroids, waxes, and related compounds, that are related more by their physical than by their chemical properties. They have the common property of being relatively insoluble in water and soluble in nonpolar solvents such as ether and chloroform. 3 Functions of lipid 4  Biochemical Functions of lipid 1.Storage form of energy (stored as triglycerides in adipose tissue). 2.Structural components of bio membranes (phospholipids and cholesterol).  Biochemical Functions of lipid 3.Metabolic regulators (steroid hormones and prostaglandins). 4.Act as surfactants, detergents and emulsifying agents (amphipathic lipids). 6 7  Biochemical Functions of lipid 5. Nonpolar lipids act as electrical insulators- Myelin is a lipid-rich substance that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. 8  Biochemical Functions of lipid 6. Provide insulation against changes in external temperature (subcutaneous fat). Protect internal organs by providing a cushioning effect. 7. Give shape and contour to the body. 8. Help in absorption of fat soluble vitamins. (A, D, E and K).  Classification of lipids Fats are classified into 4 categories as follows: 1. On the basis of chemical composition 2. On the basis of fatty acids 3. On the basis of requirement 4. On the basis of sources 1. On the basis of chemical composition Sterols 11  1. On the basis of chemical composition 1. Simple lipids Fats and Oils: These are esters of fatty acids and glycerol. Also called as neutral fats or triglycerides. These neutral fats make up to 98 -99% of dietary lipid and body fats.  1. On the basis of chemical composition Waxes: are esters formed from long-chain fatty acids and long-chain alcohols. Plant waxes on the surfaces of leaves, stems, flowers, and fruits protect the plant from dehydration and invasion by harmful microorganisms. Carnauba wax- myricyl cerotate, obtained from the leaves of certain Brazilian palm trees are used extensively in floor waxes, automobile waxes, and furniture polish. Animals and birds produces waxes that serve as protective coatings; keeping the surfaces of feathers, skin, and hair pliable and water repellent. 13  1. On the basis of chemical composition 2. Compound lipids The compound lipids contain, lipid and non-lipid compounds. 1.Phospholipids: These contain phosphoric acid and a nitrogenous base in addition to fatty acids and glycerol (e.g.)Lecithin and cephalin. 14  1. On the basis of chemical composition 2.Glycolipids: Complex lipids containing carbohydrates in combination with fatty acids and glycerol (e.g) Cerebrosides. 3.Sulphalipids: which possess a sulfur- containing functional group and found in many photosynthetic organism. 4.Lipoproteins: Lipoproteins are the carriers of lipids in the blood and involved in the formation cell membranes (e.g) LDL and HDL 15 1. On the basis of chemical composition Phospholipid 16  1. On the basis of chemical composition 3. Derived lipids Derived from simple and compound lipids i. Sterols : Sterols are solid alcohols and form esters with fatty acids. Eg: Cholesterol is a waxy, fat-like substance found in all cells of the body and has several important functions in the body. It is synthesized by the liver. Precursor for steroid hormones, Vitamin D and bile. High blood cholesterol is a risk factor for heart disease. Rich sources of dietary cholesterol include meat, poultry and full fat dairy products. ii. Fatty acids: They are the key, refined fuel form of fat that the cell burns for energy. They are the basic structural unit of fats. Classified as saturated or unsaturated. Eg: Oleic acid, linoleic  2. On the basis of fatty acids Fats can be classified based on the fatty acids present in them as follows 2. Example of MUFA and PUFA 19 2. Example of Omega-3,6 and 9 20 21  3. On the basis of requirement 1. Essential fatty acids Fatty acids which are essential to be taken in our diet because they cannot be synthesized in our body are known as essential fatty acids.(eg.) Linoleic, linolenic and arachidonic acids. 2. Non-essential fatty acids Non-essential fatty acids are those which can be synthesized by the body and which need not be supplied through the diet. (eg.) Palmitic acid, oleic acid and butyric acid. 23 24  4. On the basis of sources Fats are divided into 2 types based on their source, namely 1. Visible fats Some fats and oils added to food or used for frying are visible fats. These are also called as pure fats. 2. Invisible fats Milk, cream, egg yolk, meat, fish and even cereals and legumes contribute substantial amount of invisible fats (not visible in the food) to the diet. 4. Example of visible and invisible 26  Eicosanoids Eicosanoids are class of molecules derived from 20- carbon (“eicosa” is Greek means 20) polyunsaturated fatty acids most frequently arachidonic acid. Eicosanoids include: prostaglandins (PG), thromboxanes (TX), leukotrienes (LT), and lipoxins (LX). These molecules act on the cells that produce them or on neighboring cells, i.e., over short distances/time periods-therefore can be classified as autocrine/paracrine hormones. They are considered to act as local hormones functioning through G-protein-linked receptors to elicit their biochemical effects. 27 Eicosanoids 28  Prostaglandins Act as local hormones. Have very short life span and are destroyed within seconds or few minutes. Are highly potent. Even in very less concentration (nanogram) biological effects are observed. Induces smooth muscle contraction, lower blood pressure, and contribute to the inflammatory response. Aspirin and other nonsteroidal anti- inflammatory agents, such as ibuprofen, obstruct the synthesis of prostaglandins by inhibiting cyclooxygenase. 29  Clinical significance of lipids Following diseases are associated with abnormal chemistry or metabolism of lipids. 1. Obesity 2. Atherosclerosis 3. Diabetes mellitus 4. Hyperlipoproteinemia 5. Fatty liver 30  Clinical importance of essential fatty acids. 1. Membrane structure and function: They are structural parts of cell membranes, affecting the function of membrane-bound enzymes, transport systems and modulating cellular immune response. 2. Vision Docosahexaenoic acid is found at very high concentrations in the cell membranes of the retina; the retina conserves and recycles DHA even when omega-3 fatty acid intake is low. 3. Nervous system The phospholipids of the brain's gray matter contain high proportions of long-chain PUFA, suggesting they are important to central nervous system function. 31 Clinical importance of essential fatty acids. 32  Clinical importance of essential fatty acids. Essential fatty acid deficiency Clinical signs of essential fatty acid deficiency include a dry scaly rash, decreased growth in infants and children, increased susceptibility to infection, and poor wound healing and impairment of lipid transport. Infants receiving formula diets low in fat and patients maintained for long periods of intravenous nutrition low in essential fatty acids show deficiency symptoms. Abnormal metabolism of essential fatty acids has been noted in cystic fibrosis, Crohn disease, cirrhosis, alcoholism. 33  Clinical importance of essential fatty acids. Long chain ω3 fatty acids such as linolenic, eicosapentaenoic and docosahexaenoic acids have anti-inflammatory effects, due to their effects in promoting the synthesis of less inflammatory prostaglandins and leukotrienes as compared to ω6 fatty acids? 34  Clinical importance of essential fatty acids. Current evidence suggests that diets rich in ω3 fatty acids are beneficial, particularly for cardiovascular disease, chronic degenerative diseases such as cancer, rheumatoid arthritis, and Alzheimer disease. 35  Learning outcomes At the end of the lecture, student should be able to 1. Classify lipids. 2.Outline the structure and biochemical functions of lipids. 3.Explain the clinical importance of essential fatty acids. 36