Biochemistry of Lipids PDF

Learning outcome Learning outcome: the students will be able to: ❑ Classify lipids and their function in the body ❑ Classify fatty acids FA and enumerate the properties of SFA, MUFA, and PUFA ❑ Describe the key chemical properties of fatty acids. ❑ Describe the structure, properties and function...

Learning outcome Learning outcome: the students will be able to: ❑ Classify lipids and their function in the body ❑ Classify fatty acids FA and enumerate the properties of SFA, MUFA, and PUFA ❑ Describe the key chemical properties of fatty acids. ❑ Describe the structure, properties and function of triacylglycerol ❑ Classify phospholipids and enumerate their function ❑ Identify the major classes lipids and describe their biochemical functions ❑ Outline the general features of the fluid mosaic model of the structure of biological membranes  DEFINITION, CLASSIFICATION ANDFUNCTIONS OF LIPIDS ❑ DEFINITION Lipids are a heterogenous group of water insoluble (hydrophobic) organic molecules. ▪ Lipids include fats, oils, steroids, waxes and related compounds. ▪ Insoluble in water but soluble in organic solvents like chloroform, ether and benzene. ❑ CLASSIFICATION OF LIPIDS Based on the chemical nature, lipids are classified as: 1. Simple lipids: they are esters of fatty acids with glycerol or other higher alcohols a. Triacylglycerol or Triglycerides or neutral fat b. Waxes 2. Compound lipids: These are esters of fatty acids, with alcohol containing additional (prosthetic) groups. These are subclassified according to the type of prosthetic group present in the lipid as follows: a) Phospholipids b) Glycolipids c) Lipoproteins 3. Derived lipids: they include the products obtained after the hydrolysis of simple and compound lipids which possess the characteristics of lipids, e.g. a) Fatty acids b) Steroids c) Cholesterol d) Lipid soluble vitamins and hormones e) Ketone bodies. 4. Lipids complexed to other compound ❑ FUNCTIONS OF LIPIDS 1. Storage form of energy (triacylglycerol) 2. Structural components of biomembranes (phospholipids and cholesterol) 3. Metabolic regulators (steroid hormones and prostaglandins) 4. Act as surfactants, detergents and emulsifying agents (amphipathic lipids) 5. Act as electric insulators in neurons 6. Provide insulation against changes in external temperature (subcutaneous fat) 7. Give shape and contour to the body 8. Protect internal organs by providing a cushioning effect (pads of fat) 9. Help in absorption of fat soluble vitamins (A, D, E and K) 10. Improve taste and palatability of food. FATTY ACIDS Fatty acids, are included in the group of derived lipids. It is the most common component of lipids in the body. They are generally found in ester linkage in different classes of lipids. In the human body, free fatty acids are formed only during metabolism. ✓ Fatty acids are aliphatic carboxylic acids and have the general formula, R—CO—OH, where COOH (carboxylic group) represents the functional group. For example, Acetic acid CH3—COOH Butyric acid CH3(CH2)2—COOH Palmitic acid CH3—(CH2)14—COOH Stearic acid CH3—(CH2)16—COOH ✓ Depending on the R group (the hydrocarbon chain), the physical properties of fatty acids may vary. ✓ In humans, most fatty acids have an even number of carbon atoms, with a chain length of 16 to 20 carbon atoms FATTY ACIDS ❑ Classification of fatty acid is in the following Table: 1. Depending on total number of carbon atoms: a. Even chain: They have carbon atoms 2,4,6 and similar series. Most of the naturally occurring lipids contain even chain fatty acids. b. Odd chain: They have carbon atoms 3, 5, 7, etc. Odd numbered fatty acids are seen in microbial cell walls. They are also present in milk. 2. Depending on length of hydrocarbon chain: a. Short chain with 2 to 6 carbon atoms b. Medium chain with 8 to 14 carbon atoms c. Long chain with 16 and above, usually up to 24 carbon atoms d. Very long chain fatty acids (more than 24 carbon). 3. Depending on nature of hydrocarbon chain: a. Saturated fatty acids b. Unsaturated fatty acids which may be subclassified into: Monounsaturated (monoenoic) having single double bond or Polyunsaturated (polyenoic) with 2 or more double bonds. SATURATED FATTY ACIDS Depending on length of hydrocarbon chain: a. Short chain with 2 to 6 carbon atoms b. Medium chain with 8 to 14 carbon atoms c. Long chain with 16 and above, usually up to 24 carbon atoms d. Very long chain fatty acids (more than 24 carbon). ❑ Short-chain (2 to 4 carbons) and medium-chain (6 to 12 carbons) fatty acids occur primarily as metabolic intermediates in the body. (1) Dietary short- and medium-chain fatty acids (sources: coconut oil, palm kernel oil) are directly absorbed in the small intestine and transported to the liver through the portal vein. (2) They also diffuse freely without carnitine esterification into the mitochondrial matrix to be oxidized. ❑ Long-chain fatty acids (14 or more carbons) are found in triacylglycerols (fat) and structural lipids. (1) They require the carnitine shuttle to move from the cytosol into the mitochondria. The two carbon acetic acid and 4 carbon butyric acid are important metabolic intermediates. The C16 (palmitic acid) and C18 (stearic acid) are most abundant in body fat Each animal species will have characteristic pattern of fatty acid composition. Thus, human body fat contains 50% oleic acid, 25% palmitic acid, 10% linoleic and 5% stearic acid. UNSATURATED FATTY ACIDS ❑ Unsaturated fatty acids contain one or more double bonds. ❑ Depending on the number of double bonds present unsaturated fatty acid is again classified into: Monounsaturated Fatty Acid: Only one double bond is present Polyunsaturated Fatty Acid: More than one double bonds are present. Monounsaturated Fatty Acid Polyunsaturated Fatty Acid Essential fatty acids are required for optimal health and cannot be synthesized by the body and should be supplied in the diet. ✓ They are polyunsaturated fatty acids, namely linoleic acid and linolenic acid. ✓ Arachidonic acid can be synthesized from linoleic acid. Therefore, in deficiency of linoleic acid, arachidonic acid also becomes essential fatty acids. ✓ Humans lack the enzymes to introduce double bonds at carbon atoms beyond C9 in the fatty acid chain. ✓ Hence, humans cannot synthesize linoleic acid and linolenic acid having double bonds beyond C9. And thus, linoleic and linolenic are the essential fatty acids. Functions of Essential Fatty Acids (EFA) ❑ Synthesis of Eicosanoids: Arachidonic acid, a fatty acid derived from linoleic acid is an essential precursor of eicosanoids, which include: Prostaglandins Thromboxanes Prostacyclin Leukotrienes ❑ Maintenance of Structural Integrity and function are important constituents of phospholipids in cell membrane and help to maintain the structural integrity of the membrane. ❑ Development of Retina and Brain Docosahexaenoic acid (DHA: ω-3), which is synthesized from linolenic acid is particularly needed for development of the brain and retina during the neonatal period. ❑ Antiatherogenic Effect Essential fatty acids increase esterification and excretion of cholesterol, thereby lowering the serum cholesterol level. Thus, essential fatty acids help to prevent the atherosclerosis. Melting temperatures of fatty acids are key elements in the control of the fluidity of cell membranes, and the proper degree of fluidity is essential for membrane function (Chapter 12). Thus, short chain length and cis unsaturation enhance the fluidity of fatty acids and of their derivatives Essential Fatty Acid Deficiency Skin: Acanthosis, Hyperkeratosis, scaly skin, eczema (in children), loss of hair and poor wound healing Fatty liver may be due to Impaired lipid transport Swelling of mitochondrial membrane and reduction in efficiency of oxidative phosphorylation Decrease in fibrinolytic activities. Cis and Trans Fatty Acids (EFA) Biological Significance of Cis isomers All naturally occurring fatty acids are cis isomers. Cis isomers increases the fluidity of biological membranes Biological Significance of Trans isomers Present in dairy products and partially hydrogenated edible oils (e.g. Margarine) They are used in food industry to improve the shelf life Trans fatty acids are present in high amounts in processed foods, fast foods and bakery items and fried foods Trans fatty acid compete with essential fatty acid, hence exacerbate essential fatty acid deficiency Consumption of trans fatty acid for long terms may raise the risk factor for cardiovascular diseases like Atherosclerosis and Coronary Artery disease and Diabetes mellitus ✓ Melting temperatures of fatty acids are key elements in the control of the fluidity of cell membranes, and the proper degree of fluidity is essential for membrane function. ✓ Thus, short chain length and cis unsaturation enhance the fluidity of fatty acids and of their derivatives