AIU Medical Biochemistry 2024-2025 Lecture Notes - Lipids PDF

Medical Biochemistry 2024-2025 MACRONUTRIENTS (LIPIDS 1) Definition Lipids are heterogeneous group of organic compounds, contain C, H, and O may also contain P, N, and S related, either actually or potentially to fatty acid. They have the c...

Medical Biochemistry 2024-2025 MACRONUTRIENTS (LIPIDS 1) Definition Lipids are heterogeneous group of organic compounds, contain C, H, and O may also contain P, N, and S related, either actually or potentially to fatty acid. They have the common property of being: 1. Relatively insoluble in water. 2. Soluble in nonpolar solvents (fat solvents) as ether, chloroform, benzene and acetone. Functions of Lipids 1. Important dietary constituents, because of their high energy value (provide more energy per gram than carbohydrates and proteins.) 2. Storage form of energy (triglycerides) 3. Structural components of biomembranes (phospholipids and cholesterol) 4. Metabolic regulators (steroid hormones and prostaglandins) 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) 1  Medical Biochemistry 2024-2025 CLASSIFICATION OF LIPIDS I. SIMPLE LIPIDS: They are esters of fatty acids with various alcohols. 1. True fats: esters of fatty acids with glycerol. 2. Waxes: esters of fatty acids with higher molecular weight monohydric alcohol. II. COMPLEX LIPIDS (COMPOUND LIPIDS): They are esters of fatty acids with alcohol and in addition they contain another group. 1.Phospholipids: These lipids contain fatty acid, alcohol, the additional group phosphoric acid 2.Glycolipids: These lipids contain fatty acid, alcohol, and the additional group carbohydrate. 3.Lipoproteins: lipids and proteins. III. DERIVED LIPIDS: 1. Fatty acids 2. Alcohols, as glycerol, sphingosine 3. Steroids 4. Carotenoids 5. Fat soluble vitamins: A, D, E and K. 2  Medical Biochemistry 2024-2025 About 95% of fats in the diet are in the form of triglycerides. Sterols (like cholesterol) make up about 3% of dietary fat intake and phospholipids make up roughly 2% of dietary fat intake. *** Clinical Applications 1. Excessive fat deposits cause obesity. Truncal obesity is a risk factor for heart attack. 2. Abnormality in cholesterol and lipoprotein metabolism leads to atherosclerosis and cardiovascular diseases. 3. In diabetes mellitus, the metabolisms of fatty acids and lipoproteins are deranged, leading to ketosis. 3  Medical Biochemistry 2024-2025 FATTY ACIDS: They are aliphatic (having an open-chain structure) monocarboxylic organic acids. They are obtained mostly from the hydrolysis of natural fats and oils. Fatty acids that occur in natural fats usually contain an even number of carbon atoms, ranging from 2-24 carbon atoms. All of them possess a long hydrocarbon chain and a terminal carboxyl group. Methods of numbering of fatty acid carbons: 4  Medical Biochemistry 2024-2025 Classification of fatty acids: A. According to chain length: 1. Low fatty acids (Short chain fatty acids): i.e. Containing 10 carbon atoms or less (from 2-10). Such as: Acetic acid (2 C): CH3-COOH Butyric (4 C): CH3-CH2-CH2-COOH 2. High fatty acids (Long chain fatty acids): Contain more than 10 carbon atoms. Such as: Palmitic (16 C): CH3-(CH2)14-COOH Stearic (18 C): CH3-(CH2)16-COOH *** CLINICAL SIGNIFICANCE The ability of short chain length fatty acids to be taken up by enterocytes without aid has made them important in dietary therapy for individuals with malabsorption disorders and gall bladder diseases. B. According to saturation and unsaturation: 1. Saturated fatty acids such as: Butyric fatty acid Palmitic Stearic 5  Medical Biochemistry 2024-2025 2. Unsaturated fatty acids: contain one or more double bonds. They can be classified into: a. Monounsaturated fatty acids contain one double bond e.g. Oleic acid 18: 1; 9 (ω 9) CH3-(CH2)7-HC = CH-(CH2)7-COOH *** Eating foods rich in monounsaturated fatty acids as olive oil instead of saturated fats improves blood cholesterol levels, which can decrease risk of heart disease. b. Polyunsaturated fatty acids: Contain more than one double bond. Linoleic 18: 2; 9, 12: (ω 6) CH3-(CH2)4-CH = CH-CH2-CH = CH-(CH2)7-COOH Linolenic 18: 3; 9 12, 15: ( ω 3) Trienoic acid CH3-CH2-CH=CH-CH2-CH=CH-CH2-CH = CH(CH2)7-COOH Arachidonic 20: 4; 5, 8, 11, 14: (ω 6) CH3-(CH2)3-(CH2-CH = CH)4 – (CH2)3-COOH 6  Medical Biochemistry 2024-2025 N.B. Various ways are used for indicating the number and position of the double bonds: e.g.: in oleic acid 18: 1; 9 or Δ 9 18 : 1 or ω 9 ▪ Δ9 this means a double bond between carbon 9 and 10 counting from the carboxylic group. 18 10 9 1Δ CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH=CH (CH2)7COOH ▪ ω9 means a double bond on carbon atom no. 9 counting from the methyl group. ω1 2 3 4 5 6 7 8 9 10 18 CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH=CH (CH2)7COOH Polyunsaturated fatty acids (PUFA) 7  Medical Biochemistry 2024-2025 CLINICAL SIGNIFICANCE Saturated fatty acids are densely packed (little space between molecules) so are in a solid state (usually present in triglycerides to surround kidneys or under the skin to give body contour) while unsaturated fatty acids are kinked around axis of double bond not densely packed so are in liquid state so are attached to phospholipids to give fluidity to cell membrane. C. Biological classification: From the nutritional point of view fatty acids can be classified into: 1. Essential fatty acids: cannot be synthesized by mammals and must be obtained from dietary sources. These are the polyunsaturated fatty acids. Linoleic (ω6) Linolenic (ω3) 2. Non-essential fatty acids: mammals can synthesize them in the body from other precursors. Those which are saturated fatty acids or FA containing one double bond. 3. Relatively essential: Arachidonic acid (ω6) Found in animal fats and peanut oil and is synthesized in the body from linoleic acid. Arachidonic acid becomes essential if its precursor, linoleic acid is missing in the diet. *** Importance of arachidonic acid Arachidonic acid is an omega 6 fatty acid It has an important role in cell membrane fluidity and transport. 8  Medical Biochemistry 2024-2025 Arachidonic acid generates very important bioactive lipids (prostaglandins that perform important physiological roles in human body). ω -3 Fatty acids: These are long-chain PUFAs, with the first double bond beginning at the third bond position from the methyl (ω) end. *** Dietary ω -3 PUFAs suppress cardiac arrhythmias, reduce serum TAGs, decrease the tendency for thrombosis, lower blood pressure, and substantially reduce risk of cardiovascular mortality. Sources of linolenic acid ( ω 3). Walnuts, flaxseed oil, chia seeds, soybean oil, salmon, sardines and tuna. ω -6 Fatty acids: These are long-chain PUFAs, with the first double bond beginning at the sixth bond position when starting from the methyl (ω) end of the fatty acid molecule. Sources of linoleic acid (ω 6) Sunflower oil, Peanut butter, Corn oil, Soybean oil and sesame oil. *** Eating too much linoleic acid disrupts healthy cellular function and contributes to inflammation. 9  Medical Biochemistry 2024-2025 *** CIS- TRANS fatty acids Naturally occurring unsaturated long chain fatty acids are all of the cis configuration. Trans fatty acid most of them arise as a byproduct of saturation of fatty acids in the process of hydrogenation (hardening) of naturally occurring oils during the manufacture of margarin. Trans fatty acids are chemically classified as unsaturated fatty acids but behave more like saturated fatty acids in the body. Trans fatty acids compete with essential fatty acids and may exacerbate essential fatty acid deficiency. Moreover, they are structurally similar to saturated fatty acids and have comparable effects in the promotion of hypercholesterolemia and atherosclerosis and they increase the risk of CHD. The greatest danger from Trans-fat lies in its capacity to distort the cell membranes. Trans fat from the diet is incorporated into brain cell membranes and alter the ability of neurons to communicate 10  Medical Biochemistry 2024-2025 11