Lipids Definition, Classification Lecture Notes PDF

‫ﻛﻠﯾﮫ اﻻﺳراء اﻟﺟﺎﻣﻌﺔ‬ University of Al-esrra Collage of Pharmacy Lipids Definition, Classification Instructor: Dr. Alaa Alnoori & Dr. Tariq Alhakeem...

‫ﻛﻠﯾﮫ اﻻﺳراء اﻟﺟﺎﻣﻌﺔ‬ University of Al-esrra Collage of Pharmacy Lipids Definition, Classification Instructor: Dr. Alaa Alnoori & Dr. Tariq Alhakeem Lecture 11: Biochemistry 3rd-year class, 1st semester 19/02/2024  Lipids Ø Lipids are an organic substances found in living systems that is insoluble in water but is soluble in organic solvents. Ø Lipids are used for efficient energy storage, as structural components of cell membranes, as chemical messengers and as fat- soluble vitamins with a variety of functions Ø Our cells can also biosynthesize most lipids Ø Lipids are fundamental structural component of cell. Ø In humans, lipids are stored as triglycerides (TG) in adipose tissues and serve as “body’s major fuel storage reserve” in times of starvation. Ø Adipose tissue is specialized connective tissues designed to synthesize , store and hydrolyze TG. Ø Lipids are transported in the blood combined with proteins in lipoprotein particles  Function of lipids 1. Lipids are important component of diet, this is not only because of their high energy value but they also provide “essential fatty acid” and are carrier of fat-soluble vitamins (Vitamins E, D, A, K), which are contained in fats of natural foods. 2. Structural element of cell, subcellular components. Phospholipids and sterols are major structural elements of biological membranes 3. Components of hormones, precursors for prostaglandin synthesis, enzyme cofactors and electron carriers. Ø The building blocks of most lipids are fatty acids. Some lipids such as cholesterol and terpenes lack fatty acids. Ø However, these lipids are potentially related to fatty acids because they are synthesized from the catabolic end product of fatty acid degradation (i.e. acetyl CoA).  Lipid Classification Ø Classified according to their chemical nature, lipids fall into two main groups: 1. One group, which consists of open-chain compounds with polar head groups and long nonpolar tails, includes fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, and glycolipids. 2. The second major group consists of fused ring compounds, the steroids; an important representative of this group is cholesterol.  Classification of Lipids According Their Function Lipids can be divided into five categories, on the basis of lipid function 1. Energy-storage lipids (triacylglycerols) 2. Membrane lipids (e.g. phospholipids) 3. Emulsification lipids (bile acids) 4. Messenger lipids (e.g. steroid hormones) 5. Protective-coating lipids (biological waxes)  Fatty Acids Ø Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (C4 to C36). In some fatty acids, this chain is unbranched and fully saturated (contains no double bonds); in others the chain contains one or more double bonds. Ø A few contain three-carbon rings, hydroxyl groups, or methyl group branches. Ø Fatty acids are amphipathic compounds because the carboxyl group is hydrophilic and the hydrocarbon tail is hydrophobic. The carboxyl group can ionize under the proper conditions. Ø Amphipathic refers to a molecule that has one end with a polar, water-soluble group and another end with a nonpolar hydrocarbon group that is insoluble in water  Fatty Acids and Types of Fatty Acids Ø A fatty acid that occurs in a living system normally contains an even number of carbon atoms, and the hydrocarbon chain is usually unbranched, and may be classified as: Ø Long-chain (C12 to C26) Ø Medium-chain (C8 and C10) Ø Short-chain (C4 and C6)  Types of fatty acids Ø Fatty acids may also be categorized with regards to the presence (and number) of unsaturated units (double bonds). 1. Saturated fatty acids (SFAs) contain no double bonds – all C-C single bonds in the carbon chain component. 2. Monounsaturated fats (MUFAs) contain only one C-C double bond in a monocarboxylic acid structure, and nearly all naturally occurring MUFAs have cis- stereochemistry. 3. Polyunsaturated fatty acids (PUFAs) contain more than one double bond in the carbon chain component of the fatty acid. Up to six double bonds may be found in biochemically important PUFAs.  Nomenclature Ø The nomenclature of fatty acids is based on the following characteristics of the hydrocarbon chain: Chain length Presence of double bonds and their positions. 16:0 (Palmitic acid) Fatty acid with 16 C-atoms and no double bonds 18:1 ∆9 (Oleic acid) Fatty acid with 18 C-atoms and one double bond at C-9 18:3 ∆9,12,15 (Linolenic acid) Fatty acid with 18 C-atoms and three double bonds at C- 9, C-12, and C-15 Ø Fatty acids typically have the following characteristics: An unbranched carbon chain An even number of carbon atoms in the chain When double bonds are present, they have cis-stereochemistry Ø The family of polyunsaturated fatty acids (PUFAs) with a double bond between the third and fourth carbon from the methyl end of the chain are of special importance in human nutrition. Ø Because the physiological role of PUFAs is related more to the position of the first double bond near the methyl end of the chain than to the carboxyl end, an alternative nomenclature is sometimes used for these fatty acids. Ø PUFAs with a double bond between C-3 and C-4 are called omega-3 (ω-3) fatty acids, and those with a double bond between C-6 and C-7 are omega-6 (ω -6) fatty acids.  Essential Fatty acid Ø Human body can synthesis all F.As except two (linoleic acid, linolenic acid) which are polyunsaturated F.As that contain 18 C atom, it must be obtained from the diet they are called Essential F.As: A fatty acid needed by the body but not synthesized within the body ,it distributed in plants and fish oils. Ø Linolenic acid called omega-3 F.A which mean the end most double bond is three carbons from the methyl end chain. linolenic acid (fish oil) 18:3 cis ∆9,12,15 Ø Linoleic acid an omega-6 F.A the end most double bond is located six carbons from the methyl end of the chain (vegetable oil) 8:2 cis ∆9,12  Biomedical Significance of Essential Fatty Acids Ø In general essential fatty acids are very important for normal growth, function and optimal health of body: 1. Membrane structure and function. These fatty acids are important constituent of phospholipids in cell membranes and help to maintain the membrane fluidity. 2. ω-6 fatty acids, linoleic acid and arachidonic acid, lower of serum cholesterol level, whereas ω-3 fatty acids, linolenic acid, lowers the serum TG level. Therefore both ω-3 and ω-6 fatty acids are suggested to lower the risk of atherosclerosis and fatty liver. 3. Fatty acid may decrease the risk of heart attack (MI) by inhibiting platelet aggregation. **Consumption of rich in EFAs prolongs the blood clotting time. *** Deficiency of EFAs is characterized by scaly skin (or toad skin), eczema (in children), loss of hairs and poor wound healing.  Physical Properties of Saturated Fatty Acids Ø Saturated fatty acids have: Molecules that fit closely together in a regular pattern Strong attractions (dispersion forces) between fatty acid chains High melting points that makes them solids at room temperature. Physical Properties of Unsaturated Fatty Acids Ø Unsaturated fatty acids have: Nonlinear chains that do not allow molecules to pack closely Weak attractions (dispersion forces) between fatty acid chains Low melting points and so are liquids at room temperature  Triacylglycerols Ø The simplest lipids constructed from fatty acids are the triacylglycerols, also referred to as triglycerides, fats, or neutral fats. Ø Glycerol is a simple compound that contains three hydroxyl groups Ø Triacylglycerol 3 fatty acids linked to glycerol Ester linkage = between OH & COOH  Triacylglycerols Ø Triacylglycerols (also called triglycerides) are tri-fatty acid esters of glycerol Ø Triacylglycerols are the major form of fatty acid storage in plants and animals Ø Triacylglycerols can be classified as fats or oils Fats are solid at room temperature and most come from animals oils are usually liquid at room temperature and come from plants (palm and coconut oils are solids at room temperature) Ø Because the polar hydroxyls of glycerol and the polar carboxylates of the fatty acids are bound in ester linkages, triacylglycerols are nonpolar, hydrophobic molecules, essentially insoluble in water. Ø It is usual for three different fatty acids to be esterified to the alcohol groups of the same glycerol molecule. Ø Triacylglycerols do not occur as components of membranes (as do other types of lipids), but they accumulate in adipose tissue (primarily fat cells) and provide a means of storing fatty acids, particularly in animals. They serve as concentrated stores of metabolic energy. Ø Complete oxidation of fats yields about 9 kcal g-1, in contrast with 4 kcal g-1 for carbohydrates and proteins. When an organism uses fatty acids, the ester linkages of triacylglycerols are hydrolyzed by enzymes called lipases. Ø When a base such as sodium hydroxide or potassium hydroxide is used, the products of the reaction, which is called saponification , are glycerol and the sodium or potassium salts of the fatty acids. These salts are soaps. Ø When soaps are used with hard water, the calcium and magnesium ions in the water react with the fatty acids to form a precipitate—the characteristic scum left on the insides of sinks and bathtubs. Ø The other product of saponification, glycerol, is used in creams and lotions as well as in the manufacture of nitroglycerin.  Partial Hydrogenation of Cooking Oils Produces Trans Fatty Acids Ø Most natural fats, such as those in vegetable oils, dairy products, and animal fat, are complex mixtures of simple and mixed triacylglycerols. Ø When lipid-rich foods are exposed too long to the oxygen in air, they may spoil and become rancid. Ø The unpleasant taste and smell associated with rancidity result from the oxidative cleavage of double bonds in unsaturated fatty acids, which produces aldehydes and carboxylic acids of shorter chain length and therefore higher volatility. Ø To improve the shelf life of vegetable oils used in cooking, and to increase their stability at the high temperatures used in deep-frying, commercial vegetable oils are subjected to partial hydrogenation.  Partial Hydrogenation Ø This process converts many of the cis double bonds in the fatty acids to single bonds and increases the melting temperature of the oils so that they are more nearly solid at room temperature. Ø Some cis double bonds are converted to trans double bonds convert fat from cis fat to trans fats. Ø Dietary trans fatty acids raise the level of triacylglycerol and of LDL (“bad”) cholesterol in the blood, and lower the level of HDL (“good”) cholesterol, and these changes alone are enough to increase the risk of coronary heart disease. Ø They seem, for example, to increase the body’s inflammatory response, which is another risk factor for heart disease.  Phospholipids Phosphoacylglycerol Ø Similar in structure to triglycerides except that they only have two esterified fatty acids, the third position on the glycerol backbone instead contains a phospholipid head group and an amino alcohol component. Ø Phosphoglycerides are the major component of cell membranes. In a phosphoacylglycerol, the polar head group is charged, because the phosphate group is ionized at neutral pH.  In general, glycerophospholipids contain a C16 or C18 saturated fatty acid at C-1 and a C18 or C20 unsaturated fatty acid at C-2, with few exceptions. Phosphoglycerides have different amino alcohol groups like: Ø Because phospholipids contain both hydrophobic fatty acid chains and a hydrophilic head group, they are by definition amphipathic lipid molecules. Ø The lipids are organized in a bilayer in which the hydrophobic chains extend toward the inside of bilayer and the hydrophilic groups (the phosphate groups and other polar groups ) are oriented toward the outside, where they come in contact with water, like the micelle Ø Lecithin and cephalin are glycerophospholipids Ø Abundant in brain and nerve tissues. Ø Lipid bilayer: A structure found in membranes, In the lipid bilayer part of the membrane, the polar head groups are in contact with water, and the nonpolar tails lie in the interior of the membrane. Cell membranes contain lipid , protein and carbohydrates. Phosphoglycerides such as (lecithin, cephalin) and sphingomyelin, cholesterol are lipids found in membranes.  Sphingolipids Ø Sphingolipids do not contain glycerol, but they do contain the long- chain amino alcohol sphingosine, from which this class of compounds takes its name. Ø The simplest compounds of this class are the ceramides, which consist of one fatty acid linked to the amino group of sphingosine by an amide bond Ø They found in cell membrane like: sphingomyelin ,glycolipid.  Ø There are three subclasses of sphingolipids, all derivatives of ceramide but differing in their head groups: sphingomyelins, neutral (uncharged) glycolipids, and gangliosides. Ø In sphingomyelins, the primary alcohol group of sphingosine is esterified to phosphoric acid, which, in turn, is esterified to phosphocholine or phosphoethanolamine as their polar head group and are therefore classified along with glycerophospholipids as phospholipids Ø Sphingomyelins are amphipathic; they occur in cell membranes in the nervous system  Glycolipids Ø Glycosphingolipids, which occur largely in the outer face of plasma membranes, have head groups with one or more sugars connected directly to the -OH at C-1 of the ceramide moiety; they do not contain phosphate. Ø Carbohydrate is bound to an alcohol group of a lipid by a glycosidic linkage, the resulting compound is a glycolipid. Ø Glycolipids: A complex lipid contain a siphingosin ,F.A ,a carbohydrate (glucose or galactose) **Ceramides lipids that contain one fatty acid linked to sphingosine by an amide bond Ø Cerebrosides have a single sugar linked to ceramide; those with galactose are characteristically found in the plasma membranes of cells in neural tissue, and those with glucose in the plasma membranes of cells in nonneural tissues. Ø Globosides are glycosphingolipids with two or more sugars, usually D- glucose, D- galactose, or N-acetyl-D-galactosamine. Ø Cerebrosides and globosides are sometimes called neutral glycolipids, as they have no charge at pH 7.  Steroids Ø Many compounds of widely differing functions are classified as steroids because they have the same general structure: a fused-ring system consisting of three six-membered rings (the A, B, and C rings) and one five-membered ring (the D ring). Structure: From Cholesterol ® Sex Hormones 4 fused C rings + ?? Different steroids created by attaching different functional groups to rings Examples: cholesterol, sex hormones  Cholesterol v Cholesterol is largely hydrophobic. But it has one polar group, a hydroxyl, making it amphipathic. v Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase & its hydrophobic ring system adjacent to fatty acid chains of phospholipids. Important cell component Animal cell membranes Precursor of all other steroids and of vitamin D3 Including sex hormones and adrenocorticoid hormones Used to make bile salts high levels in blood may contribute to cardiovascular disease Thank you for your listening