Unit 1 B Lipids I PDF

WHAT ARE LIPIDS? Lipids are a diverse group of organic compounds that are essential for several biological functions, ranging from energy storage to cell signalling. They are regarded as organic substances relatively insoluble in water, soluble in organic solvents (such as alcohol, ether), actually...

WHAT ARE LIPIDS? Lipids are a diverse group of organic compounds that are essential for several biological functions, ranging from energy storage to cell signalling. They are regarded as organic substances relatively insoluble in water, soluble in organic solvents (such as alcohol, ether), actually or potentially related to fatty acids and utilised by living cells. In the human body, these molecules can be synthesized in the liver and are found in oil, butter, whole milk, cheese, fried foods, and also in some red meats. Properties of Lipids 1. Lipids are a family of organic compounds, composed of fats and oils. 2. Lipids are oily or greasy nonpolar molecules, stored in the adipose tissue of the body. 3. Lipids are a heterogeneous group of compounds, mainly composed of hydrocarbon chains. 4. Lipids are energy-rich organic molecules, which provide energy for different life processes. 5. Lipids are a class of compounds characterised by their solubility in nonpolar solvents and insolubility in water. 6. Lipids are significant in biological systems as they form for a mechanical barrier dividing a cell from the external environment known as the cell membrane. 7. Lipid molecules have no ionic charges 8. pure fats and oils are colorless, odorless, and tasteless 9. Triglycerides on hydrolysis with alkali (NaOH or KOH) or lipase enzymes (termed alkaline hydrolysis) lead to the formation of two products: soap or fatty acid salts of sodium or potassium, and glycerol. This reaction is called as saponification. 10. Hydrogenation: The breakage of double bonds occurs after the reaction of unsaturated fatty acids with hydrogen. This turns the molecules into saturated fatty acids. 11. Rancidity: Oxidation and hydrolysis of fats and oil to generate a disagreeable odor – this is known as rancidity. CLASSIFICATION OF LIPIDS There are numerous specific types of lipids important to live, including fatty acids, triglycerides, glycerophospholipids, sphingolipids, and steroids. Lipids can be classified in five ways, depending on: chemical reaction it undergoes (saponification), chemical composition, fatty acids type, dietary requirements, and functions.  Overview of lipids classification Classification based on saponification: According to their ability to undergo saponification reaction, lipids can be classified into two categories: saponifiable lipids (such as acyl glycerol, waxes, sphingolipids and phospholipids) and nonsaponifiable lipids (such as steroids and prostaglandins). A saponification reaction is an organic chemical reaction that utilizes an alkali to cleave an ester into a carboxylic acid salt and alcohol. The primary use for this reaction is during the production of soap products, hence the reaction is named as saponification. Potassium hydroxide (KOH) and sodium hydroxide (NaOH) are the two most common alkalis used with saponification to produce soap products. A saponifiable lipid is a compound that undergoes saponification reaction in to yield smaller product molecules. Fig 1: saponification reaction Based on chemical composition these are broadly classified as simple lipids, complex lipids, derived and miscellaneous lipids which are further divided as follows. Classification based on chemical composition: A. Simple Lipids Esters of fatty acids with various alcohols. 1. Fats and oils (triacylglycerols): Esters of fatty acids with glycerol. The difference between Oils are fats is only physical. Oil is in the liquid while fats are solid at room temperature. 2. Waxes: Esters of fatty acids (usually long chain) with higher molecular weight monohydric alcohols other than glycerol. These alcohols maybe aliphatic/alicyclic. Cetyl alcohol is most commonly found in waxes. B. Complex Lipids (compound lipids) The complex or compound lipids contain some other organic molecules in addition to fatty acids and glycerols. The other group can be phosphate, nitrogenous base, carbohydrate, protein etc. Complex lipids include phospholipids, glycolipids, and lipoproteins. They are further divided as: 1. Phospholipids: These are lipids containing, in addition to fatty acids and alcohol, a phosphoric acid residue. They frequently have nitrogen-containing bases and other substituents. There are two classes of phospholipids. The first, glycerophospholipids the alcohol is glycerol (examples of glycerophospholipids: lecithin, cephalin) the second is sphingophospholipids the alcohol is sphingosine (example of spingophspholipid: spingmyelin). 2. Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate and nitrogenous base. Since the alcohol is sphingosine, they are also called as glycosphingolipids. Glycerol and phosphate are absent. Eg., cerebrosides, gangliosides. 3. Lipoproteins: Macromolecular complexes of lipids with proteins. They can be classified according to their density and in descending order, they are HDL (high-density lipoprotein), IDLs (intermediate-density lipoproteins) LDL (low-density lipoprotein), VLDL (very low-density lipoprotein). Lipoproteins play a role in metabolism. They are used to store and transport lipids and cholesterol. 4. Other complex lipids: Lipids such as sulfolipids and amino lipids, Lipopolysaccharides. Lipoproteins may also be placed in this category C. Precursor and Derived Lipids: These compounds are produced by the hydrolysis of simple and complex lipids (hence termed derived). These include fatty acids, glycerol, other alcohols, mono and diacylglycerols, hydrocarbons and ketone bodies, lipid-soluble vitamins, and steroid, sterols, hormones. Derived lipids also act as Precursor lipids, since they are like the building blocks of other lipids (esp. fatty acids, glycerol and other molecules). D. Miscellaneous lipids: These include a large number of compounds possessing the characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane (in bees wax), terpenes etc. E. Neutral lipids: The lipids which are uncharged are termed neutral lipids. These are acylglycerols (glycerides: mono,di and tri), cholesterol, and cholesteryl esters. Classification based on fatty acid type: saturated and unsaturated fatty acids. FATTY ACIDS Fatty acids are carboxylic acids with hydrocarbon side chain. They are the simplest form of lipids. Fatty acids are comprised of a polar head (a carboxyl group) and a nonpolar aliphatic tail. The exhibition of both polar and non-polar properties is described as amphipathy They span a length of between 4 and 36 carbons in length. Most of the fatty acids that occur in natural lipids are of even carbons (usually 14C-2OC). This is due to the fact that biosynthesis of fatty acids mainly occurs with the sequential addition of 2 carbon units. Palmitic acid (16C) and stearic acid (18C) are the most common. Among the odd chain fatty acids, propionic acid (3C) and valeric acid (5C) are well known. Within a cell, they are associated with other biological molecules. In the body, fatty acids are released from triacylglycerols during fasting to provide a source of energy. They circulate in the blood by binding to a protein carrier, serum albumin where they travel to the tissue for use in metabolism or biosynthetic pathways. Fatty acids can be broadly classified as saturated or unsaturated. The physical properties of fatty acids depend on length and degree of unsaturation of their aliphatic chains. Saturated Fatty acids In their fully saturated forms, the most stable conformation is the fully extended form, in which steric hindrance of neighbouring atoms is minimized. This allows ordering into crystalline arrays with the aliphatic tails associating through van der waals forces. Unsaturated Fatty acids If there is one double bond in the molecule, then it is known as a monounsaturated fatty acids –MUFA (e.g., olive oil), and if there is more than one double bond, then it is known as a polyunsaturated fatty acids -PUFA (e.g., canola oil). Most unsaturated fats are liquid at room temperature and are called oils. These are usually of plant origin and contain cis unsaturated fatty acids. Cis and Trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is referred to as a cis fat; if the hydrogen atoms are on two different planes, it is referred to as a trans-fat. The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature. Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels whereas saturated fats contribute to plaque formation in the arteries. Role of Fats Fats play several major roles in our body. Some of the important roles of fats are mentioned below: Fats in the correct amounts are necessary for the proper functioning of our body. Many fat-soluble vitamins need to be associated with fats in order to be effectively absorbed by the body. They also provide insulation to the body. They are an efficient way to store energy for longer periods. FIG 2: Lipid Structure – Saturated and Unsaturated Fatty Acids Classification Based on the requirement: Essential and non essential. 1. Essential Fatty Acids Fatty acids that cannot be produced or synthesized in our bodies are called essential fatty acids. These fatty acids need to be taken through a diet to fulfil the body’s requirement for different metabolic functions. It includes linoleic acid, linolenic acid, and arachidonic acid. 2. Non-essential Fatty Acids Non-essential fatty acids include those lipids that are synthesized by our body. They are not needed to be taken through any outside food source. It includes palmitic acid, oleic acid, and butyric acid. ESSENTIAL FATTY ACIDS Essential fatty acids (EFA) are fatty acids required but not synthesized by the human body. Consequently, they have to be supplemented through ingestion via the diet. Omega-3 fatty and omega-6 acids fall into this category. These are polyunsaturated fatty acids. They become saturated fatty acids on hydrogenation and the oils become solid fats. "Omega-3" refers to the position of the final double bond in the chemical structure, which is three carbon atoms from the "omega" or tail end of the molecular chain. Nutritionally important because the body does not make them. Omega-3 fatty acids include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). EPA and DHA are derived from ALA. The most common omega-6 fat is linoleic acid, which can be converted into longer omega-6 fats such as arachidonic acid Thus Essential fatty acids include the following: 1. Linolenic acid (C18:3 n-3) 2. Linoleic acid (C18:2 n-6) 3. Arachidonic acid (C20:4 n-6; in true carnivores, e.g., cats / in other animals if its precursor linoleic acid is not provided in sufficient amounts). This essentiality is due to the inability due to lack of enzymes, to insert double bonds beyond carbon 9 and 12. Functions of EFA: Essential fatty acids are required for the membrane structure and function and are the structural elements of the tissue formation. transport of cholesterol Formation of lipoproteins, prevention of fatty liver etc. They are also needed for the synthesis of another important group of compounds, namely eicosanoid. Essential fatty acids play an important role in the life and death of cardiac cells, immune system function, and blood pressure regulation. Docosahexaenoic acid (DHA) is an omega-3 essential fatty acid shown to play important roles in synaptic transmission in the brain during foetal development. Some excellent sources of omega-3 and omega-6 essential fatty acids are fish, flaxseed oil, hemp, walnuts, and leafy vegetables. Because these essential fatty acids are easily accessible, essential fatty acid deficiency is extremely rare. Deficiency of EFA: The deficiency of EFA results in phrynoderma or toad skin, characterized by the presence of horny eruptions on the posterior and lateral parts of limbs, on the back and buttocks, loss of hair and poor wound healing. The deficiency of these acids in the diet of babies causes eczema. Classification Based on function of lipids : storage and structural lipids STORAGE LIPIDS: Lipids are stored in the body in different forms such as, triglycerides, fat cells, cell membranes and lipoproteins STORAGE LIPIDS: Triacylglycerols Triacylglycerols are the primary storage form of long-chain fatty acids, which are broken down for energy and used in the structural formation of cells. Triacylglycerols (formerly triglycerides) are the esters of glycerol with fatty acids. Simple triacylglycerols contain identical fatty acids, however, most naturally occurring fatty acids are mixed. The fats and oils that are widely distributed in both plants and animals are chemically triacylglycerols. They are insoluble in water and non-polar in character and commonly known as neutral fats. Triacylglycerols are stored in adipocytes in vertebrates or as oils in the seed of plants. Both adipocytes and seeds contain lipase enzymes to liberate fatty acids for export when they are required for fuel or biosynthetic purposes. In some animals, triacylglycerols provide a means of insulation; this is particularly notable in arctic-dwelling mammals such as walruses, polar bears, and penguins. Structures of acylglycerols: Monoacylglycerols, diacylglycerols and triacylglycerols, respectively consisting of one, two and three molecules of fatty acids esterified to a molecule of glycerol, are known. Among these, triacylglycerols are the most important biochemically. A schematic representation of a triacylglycerol structure with three fatty acids on a glycerol backbone is shown below. FIG:3 Structure of Triacylglycerol