Lipids: Structure, Function, and Properties PDF

LIPIDS Greek word ‘lipos’ means fat. The term lipid was first used by a German biochemist ‘Bloor’ in 1943. Lipids are heterogenous group of compounds related to fatty acid, fats, oils, waxes and other related compounds. Lipids are insoluble in water and soluble in orga...

LIPIDS Greek word ‘lipos’ means fat. The term lipid was first used by a German biochemist ‘Bloor’ in 1943. Lipids are heterogenous group of compounds related to fatty acid, fats, oils, waxes and other related compounds. Lipids are insoluble in water and soluble in organic solvent such as benzene, ether, chloroform, acetone etc. They yield fatty acids upon hydrolysis which are utilized by living organisms. Unlike the polysaccharides, proteins and nucleic acids, lipids are not polymers. STRUCTURE OF LIPIDS The structure is typically made of a glycerol backbone, 2 fatty acid tails (hydrophobic), and a phosphate group (hydrophilic). The hydrophilic part faces outward, and the hydrophobic part faces inward. This arrangement helps monitor which molecules can enter and exit the cell. FUNCTIONS OF LIPIDS 1. Lipids protect the internal organs, serve as insulating materials and give shape and smooth appearance to the body. 2. They are the concentrated fuel reserve of the body (triacylglycerols). 3. They serve as a source of fat-soluble vitamins (A, D, E and K). 4. Lipids are the constituents of membrane structure and regulate the membrane permeability (phospholipids and cholesterol). 5. Lipids are important as cellular metabolic regulators (steroid hormones and prostaglandins). PROPERTIES OF LIPIDS SOLUBILITY Lipids are hydrophobic, meaning they are insoluble in water but soluble in organic solvents like chloroform, benzene, and ethanol. Short-chain fatty acids are more soluble than long-chain ones. CONSISTENCY Lipids' consistency depends on fatty acid composition: fats are solid, oils are liquid. Saturation, chain length, and double bonds affect consistency—more saturation and longer chains make lipids more solid while unsaturated and shorter chains make lipids liquid. HYDROLYSIS Hydrolysis breaks lipids into glycerol and fatty acids using water, aided by enzymes (lipases) or acidic/alkaline conditions. It's essential for digestion and metabolism. HYDROGENATION Hydrogenation adds hydrogen to unsaturated fats, increasing saturation to extend shelf life and raise melting point. It can produce trans fats, which have negative health effects. EMULSIFICATION OF LIPIDS Emulsification is the process of breaking down large fat droplets into smaller, more manageable particles, allowing fats to mix with water. This is essential for digestion, as it increases the surface area of fats, making them more accessible to digestive enzymes like lipases. In the body, bile salts act as natural emulsifiers, aiding in the absorption of fats within the small intestine. RANCIDITY OF LIPIDS Rancidity is the process by which fats and oils break down, causing unpleasant flavors, odors, and texture changes. It has TWO TYPES: Hydrolytic Rancidity: Caused by water breaking down triglycerides into free fatty acids and glycerol, often due to enzymes, resulting in off-flavors and smells. Oxidative Rancidity: Occurs when fats react with oxygen, forming free radicals and peroxides, accelerated by heat, light, and metals. Antioxidants like tocopherols (Vitamin E), hydroquinone, and synthetic preservatives (e.g., BHA, BHT) are added to prevent oxidative rancidity, extending shelf life and preserving food quality. LIPID PEROXIDATION IN VIVO Lipid peroxidation is the oxidative degradation of lipids, leading to the formation of lipid peroxides and other reactive aldehydes. Initiated by free radicals, this process damages cell membranes, leading to cell dysfunction and is linked to chronic diseases such as cardiovascular diseases and cancer. The body combats this with antioxidant systems like glutathione and superoxide dismutase, while dietary antioxidants from fruits, vegetables, and nuts also help protect against oxidative stress. SAPONIFICATION The hydrolysis of triacylglycerols by alkali to produce glycerol and soaps is known as saponification. WHAT ARE FATTY ACIDS? Fatty acids are carboxylic acids with hydrocarbon side chain. Fatty acids are essential for energy storage, cell membrane structure, and signaling pathways. Found in animal fats, plant oils, marine organisms, and microorganisms, fatty acids play crucial roles in hormone regulation, inflammation, and brain function. OCCURENCE Fatty acids mainly occur in the esterified form as major constituents of various lipids. They are also present as free (unesterified) fatty acids. Fatty acids of animal origin are much simpler in structure in contrast to those of plant origin. Why most fatty acids are of even carbons? Most of the fatty acids that occur in natural lipids are of even carbons (usually 14- 20 carbon units). This is due to the fact that biosynthesis of fatty acids mainly occurs with the sequential addition of 2 carbon units. SATURATED FATTY ACIDS do not contain double bonds and is solid Nomenclature of Saturated Fatty Acids 1. Identify the longest continuous chain 2. Number the chain from the carboxyl end 3. Use the suffix -anoic acid 4. Prefix indicates chain length UNSATURATED FATTY ACIDS Unsaturated fatty acids contain one(monounsaturated fatty acids) or 2 or more double bonds (polyunsaturated fatty acids (PUFA)) and is liquid. Nomenclature of Unsaturated Fatty Acids 1. Identify the longest continuous chain 2. Number the chain from the carboxyl end 3. Use the suffix -enoic acid (for monounsaturated) or -a(di/tri/tetra)enoic acid (for polyunsaturated) 4. Prefix indicates chain length 5. Indicate double bond position 6. Specify cis or trans configuration Geometric Isomerism in Unsaturated Fatty Acids (Cis configuration vs. Trans configuration) CIS CONFIGURATION 1. Hydrogen atoms on same side of double bond 2. Bent or "kinked" chain shape 3. Increased membrane fluidity 4. More common in natural fatty acids TRANS CONFIGURATION 1. Hydrogen atoms on opposite sides of double bond 2. Linear or "extended" chain shape 3. Decreased membrane fluidity 4. Less common in natural fatty acids OMEGA-3 FATTY ACIDS - Family: Polyunsaturated fatty acids (PUFAs) 1. Alpha-linolenic acid (ALA): -mostly found in chia seeds, flaxseeds, walnuts, canola oil 2. Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA): -mostly found in Fatty fish (salmon, sardines, mackerel), Shellfish (mussels, oysters), Algal oil supplements Biological effects: 1. Anti-inflammatory properties 2. Cardiovascular health support 3. Brain function and development 4. Immune system modulation Health benefits: 1. Reduced inflammation 2. Improved heart health 3. Supported brain function and development 4. Enhanced immune response Omega-6 Fatty Acids - Family: Polyunsaturated fatty acids (PUFAs) 1. Linoleic acid (LA): Mostly found in Vegetable oils (corn, soybean, sunflower), Nuts and seeds (sunflower, pumpkin, sesame), Meat and poultry 2. Arachidonic acid (AA): - Meat and poultry, Fish and seafood, Eggs and dairy products Biological effects: 1. Linoleic acid (LA): - Essential for skin and hair growth, Supports heart health, Anti-inflammatory effects 2. Arachidonic acid (AA): - Involved in brain function and development, Supports immune system and Pro-inflammatory effects (in excess) Health benefits: 1. Reduced inflammation 2. Improved heart health 3. Supported brain function and development 4. Enhanced immune response Based on length of Hydrocarbon Chains Depending on the length of carbon chains, fatty acids are categorized into 3 groups: Short chain with less than 6 carbons; Propionic Acid (Propanoic Acid) Butyric Acid (Butanoic Acid) Valeric Acid (Pentanoic Acid) Caproic Acid (Hexanoic Acid) Medium chain with 8 to 14 carbons ; and Caprylic Acid (Octanoic Acid) Capric Acid (Decanoic Acid) Lauric Acid (Dodecanoic Acid) Myristic Acid (Tetradecanoic Acid) Long chain with 16 to 24 carbons. Palmitic Acid (Hexadecanoic Acid) Lignoceric Acid (Tetracosanoic Acid) Palmitoleic Acid (cis-9-Hexadecenoic Acid) Shorthand Representation of Fatty Acids Determine or count the number of Carbon atoms present in the carbon chain. Identify the number of double bonds found in the chain and its position. The general rule is that the total number of carbon atoms are written first, followed by the number of double bonds and finally the (first carbon) position of double bonds, starting from the carboxyl end. Based on Requirements Nonessential Fatty Acids Fatty acids that can be synthesized by the body. Saturated fatty acids (SFAs) and oleic acid, the primary monounsaturated fatty acid in our diet. examples of nonessential fatty acids. Lauric, myristic, and palmitic acids make up most saturated fatty acids. Essential Fatty Acids Fatty acids that the body cannot synthesize and must be obtained from food. They fall into two categories—omega-3 and omega-6. The 3 and 6 refer to the position of the first carbon double bond and the omega refers to the methyl end of the chain. Omega-3 and omega-6 fatty acids are precursors to important compounds called eicosanoids. Eicosanoids are powerful hormones that control many other hormones and important body functions, such as the central nervous system and the immune system Deficiency of Essential Fatty Acid Essential fatty acid (EFA) deficiency can lead to dry, scaly skin, hair loss, poor wound healing, growth failure, and increased metabolic rate. It may contribute to cardiovascular disease due to chronic inflammation. CLASSIFICATIONS OF LIPIDS (Chemical Structure and Composition) SIMPLE LIPIDS- Esters of fatty acids with alcohols COMPLEX (COMPOUND) LIPIDS- esters of fatty acids with alcohols containing additional groups such as phosphate, nitrogenous base, carbohydrate, protein etc.  DERIVED LIPIDS- substances that come from breaking down other types of lipids (fats and oils). When lipids are hydrolyzed (a process that involves breaking chemical bonds by adding water), they can be converted into simpler compounds. MISCELLANEOUS LIPIDS- are a diverse group of compounds with lipid-like properties but do not belong to the main lipid classes (such as triglycerides or phospholipids). Examples includes Carotenoids, Squalene, Terpenes. − Characteristics: o Hydrophobic: They are non-polar and water-insoluble, like other lipids. o Diverse Structures: Include varied structures, from linear chains to complex ringed compounds. − Functions: o Help protect cells from oxidative damage. o Used in waxes and protective coatings in plants and animals. o Serve as building blocks for vitamins, hormones, and other molecules. Polar Non polar Different electronegativities Similar or identical electronegativities Soluble in polar solvents Soluble in nonpolar solvents Stronger (dipole-dipole, hydrogen bonding) Weaker (London dispersion forces. NEUTRAL LIPIDS Neutral lipids, such as mono-, di-, and triacylglycerols, cholesterol, and cholesteryl esters, are uncharged, non-polar, and hydrophobic. They serve for energy storage and protection. POLAR (CHARGED) LIPIDS have hydrophilic regions and are essential for cellular structure and signaling. TRIACYLGLYCEROLS Triacylglycerols (formerly triglycerides) commonly known as neutral fats. Esters of glycerol with fatty acids, found in fats and oils of plants and animals. Insoluble in water and nonpolar. Storage: Primarily stored in adipose tissue within adipocytes in subcutaneous tissue and around certain organs which serves as thermal insulator. Human Body: Fat reserve percentage (20% in men, 25% in women), enough to sustain energy for 2–3 months. Main Function: Energy reserve in animals; insulation and protection.  Note: Triacylglycerols are not the structural components of biological membranes. 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. Simple triacylglycerols contain the same type of fatty acid residue at all the three carbons. e.g., tristearoyl glycerol or tristearin. Mixed triacylglycerols more common They contain 2 or 3 different types of fatty acid residues. In general, fatty acid attached to C1 is saturated, that attached to C2 is unsaturated while that on C3 can be either. Triacylglycerols of plants, in general, have higher content of unsaturated fatty acids compared to that of animals. PHOSPHOLIPIDS These are complex or compound lipids containing phosphoric acid, in addition to fatty acids, nitrogenous base and alcohol. Major lipid constituents of cell membranes Amphipathic in nature like fatty acids. FUNCTIONS Phospholipids form membranes with proteins, regulating permeability and controlling substance movement. In mitochondria, phospholipids maintain electron transport chain structure for cellular respiration. Phospholipids aid in the absorption of dietary fats in the intestine. They are essential for synthesizing lipoproteins, which transport lipids in the body. Phospholipids help remove excess cholesterol from the body. Phospholipids act as surfactants in the lungs, reducing surface tension; lack of surfactant can cause respiratory distress in infants. Cephalins are involved in blood clotting processes. Phosphatidylinositol generates second messengers that aid in hormone signaling. Classified on the basis of the type of alcohol: (1) Glycerophospholipids (or phosphoglycerides) (2) Sphingophospholipids (or sphingomyelins) The alcohol present is glycerol. The alcohol present is sphingosine. Ex. Phosphatidic acid Ex. Sphingomyelins GLYCEROPHOSPHOLIPIDS Glycerophospholipids are the major lipids that occur in biological membranes. They consist of glycerol 3-phosphate esterified at its C1 and C2 with fatty acids. Usually, C1 contains a saturated fatty acid while C2 contains an unsaturated fatty acid. PHOSPHATIDIC ACID The simplest form of phospholipid; not found in high concentrations in tissues. an intermediate in the synthesis of both triacylglycerols and phospholipids. Other glycerophospholipids, which contain various nitrogenous bases or other groups, are considered derivatives of phosphatidic acid. LECITHIN Phosphatidylcholine, also known as lecithin (from the Greek - lecithos, meaning egg yolk) is the most abundant group of phospholipids in cell membranes. It is a type of phosphatidic acid with choline as its base. Phosphatidylcholines serve as the storage form of choline in the body. CEPHALIN Phosphatidylethanolamine are phospholipids that contain ethanolamine as their nitrogenous base. The main difference between lecithin and cephalin lies in the base used: Lecithin contains choline, while cephalin contains ethanolamine. SIGNIFICANT LIPIDS TYPES OF LIPOPROTEINS 1. Chylomicrons − Structure: Largest lipoproteins, with high triglyceride content and low protein. − Occurrence: Formed in the intestines after a meal to transport dietary fats. 2. VLDL (Very Low-Density Lipoproteins) − Structure: Contains high triglyceride levels with moderate protein. − Occurrence: Produced by the liver to transport triglycerides to tissues. 3. IDL (Intermediate-Density Lipoprotein) − Structure: Intermediate density with reduced triglycerides, formed from VLDL. − Occurrence: Occurs in the bloodstream as VLDL loses triglycerides. 4. LDL (Low-Density Lipoprotein) − Structure: High cholesterol and low protein, smaller than VLDL. − Occurrence: Formed from IDL in the blood; delivers cholesterol to cells. 5. HDL (High-Density Lipoprotein) − Structure: Small, dense, high in protein with lower fat content. − Occurrence: Produced by the liver and intestines, circulates tocollect excess cholesterol. STEROIDS and STEROLS Steroids are lipids with a four-ring structure (three cyclohexane rings and one cyclopentane). They play key roles in maintaining cell membrane integrity and regulating hormonal signaling. Sterols are lipids with a steroid nucleus, a hydroxyl group at carbon 3, and a side chain at carbon 17. They are key membrane components in eukaryotes, helping maintain membrane rigidity, fluidity, and permeability. CHOLESTEROLS Cholesterol is found in all animal tissues and some fungi, produced by nucleated animal cells. It is a structural component of cell membranes, crucial for membrane stability and fluidity, and a precursor for steroid hormones, vitamin D, and bile acids. Cholesterol (C27H46O) has a hydroxyl group at C3, a double bond between C5 and C6, and an 8- carbon side chain at C17, making it weakly amphiphilic. ERGOSTEROL Ergosterol is found in the cell membranes of fungi and protozoa, serving similar functions as cholesterol in animal cells. It plays a crucial structural role, forms lipid rafts, and regulates growth. Ergosterol is a precursor to vitamin D2 and is targeted by antifungal drugs. Its structure consists of ergostane with double bonds at the 5,6-, 7,8-, and 22,23-positions, and a 3β-hydroxy group, classifying it as a 3β-sterol and phytosterol. VITAMIN D Vitamin D2 has a double bond between carbon atoms 22 and 23 and a methyl group (-CH3) at carbon 24. It regulates calcium and phosphorus levels, supporting bone health and immune function. Vitamin D2 is derived from ergosterol in yeast and fungi when exposed to UV light. Vitamin D3 has a single bond between carbon atoms 22 and 23 and lacks the methyl group at carbon 24, distinguishing it from Vitamin D2. It regulates calcium and phosphorus, promotes bone health, and supports the immune system, making it more potent than Vitamin D2. Vitamin D3 is produced naturally in the skin from sunlight and is found in animal products like fatty fish and egg yolks. CORTISTEROIDS Cortisol has a steroid backbone with three six-membered rings (A, B, and C) and one five-membered ring (D). Hydroxyl groups at C11 and C17, a carbonyl group at C3, and a methyl group at C21 are key to its structure and biological activity. Cortisol regulates stress response, metabolism, inflammation, blood pressure, blood sugar, and the sleep-wake cycle. It is produced by the adrenal glands, located on top of the kidneys. SEX HORMONES Sex hormones, like estrogens (e.g., estradiol) and androgens (e.g., testosterone), are steroid hormones produced primarily by the gonads (ovaries in females, testes in males). They share a common steroid backbone with four fused rings, and the presence and position of functional groups like hydroxyl, carbonyl, and methyl groups determine their biological activity. These hormones regulate sexual development, reproduction, and secondary sex characteristics. BILE ACIDS Bile Acid like Cholic acid is primarily found in the liver, gallbladder, and intestines, playing a key role in fat digestion and absorption. It has a four-ring steroid backbone typical of bile acids, with three hydroxyl groups at C3, C7, and C12 for water solubility. A carboxylic acid group at one end aids digestion, while a long hydrocarbon side chain contributes to its amphipathic nature. Bile acids are crucial for fat digestion, cholesterol regulation, digestive health, and act as signaling molecules, supporting overall metabolic health. SPHINGOPHOSPHOLIPIDS- are phospholipids with a sphingosine backbone, playing key roles in cell signaling and membrane structure. sphingomyelin Sphingomyelin is a sphingophospholipid with a sphingosine backbone, fatty acid chain, and phosphocholine headgroup. It is found in cell membranes, the myelin sheath, brain tissue, nerve cells, red blood cells, and blood plasma lipoproteins. Its structural formula is CH3-(CH2)12-CH=CH- CH(OH)-CH(NH-CO-R)-CH2-O-PO3-CH2-CH2-N(CH3)3+. ceramide-1-phosphate Ceramide-1-phosphate has a ceramide backbone and a phosphate group, with the structural formula CH₃-(CH₂)n-CH=CH-(CH₂)m- CH(OH)-CH(NH-CO-R)-CH₂-O-PO₃H₂. It differs in its ceramide backbone, phosphate group, and hydroxyl (OH) and amine (NH₂) groups. Ceramide-1-phosphate is found in cell membranes, brain tissue, nerve cells, and blood plasma, playing roles in cell growth, apoptosis, and immune response. GLYCOLIPIDS Glycolipids have a structure consisting of a mono- or oligosaccharide group attached to a sphingolipid or glycerol backbone, with one or two fatty acids. They form glycosphingolipids and glycoglycerolipids. The lipid tail’s hydrophobic nature anchors glycolipids to the plasma membrane's lipid bilayer. CEREBROSIDES Structure: Ceramide + monosaccharide (galactose or glucose) Types: Galactocerebroside (GalCer), Glucocerebroside (GluCer) Occurrence: Found in cell membranes, especially in the nervous system, brain tissue, and red blood cells. Key Difference: Differ in sugar residue (galactose or glucose) and fatty acid chain length/saturation. GANGLIOSIDES Structure: Ceramide + oligosaccharide + NANA (sialic acid) Types: GM1, GM2, GM3, GD1a, GD1b, GT1b Occurrence: Found in animal cell membranes, brain, and nervous system. Function: Regulate cell signaling, adhesion, growth, and neurodevelopment. Key Difference: Contain sialic acid (NANA), influencing the structure and function. GLOBOSIDES Structure: Ceramide + oligosaccharide Types: Globotriaosylceramide (Gb3), Globotetraosylceramide (Gb4) Occurrence: Found in eukaryotes, particularly in cell membranes and mammals. Function: Regulate cellular signaling, apoptosis, and blood-type determination. Key Difference: Differ in the number of sugar units (Gb3, Gb4). AMPHIPATHIC LIPIDS Overview of Structure, Examples, and Clinical Relevance What are Amphipathic Lipids? Amphipathic lipids are molecules that contain both hydrophobic (water- repelling) and hydrophilic (water-attracting) regions. This unique structure allows them to form bilayers and micelles, making them essential components of cell membranes and aiding in lipid transport and digestion. Example Of Amphipathic Lipids Fatty acids Phospholipids Sphingolipids Bile salts cholesterol Clinical Relevance Membrane Structure: Amphipathic lipids (e.g., phospholipids) form cell membranes; defects can lead to diseases like cystic fibrosis. Drug Delivery: Used in liposomes for targeted drug delivery, especially in cancer treatments (e.g., Doxil). Cholesterol and Cardiovascular Disease: Cholesterol imbalance (HDL/LDL) contributes to atherosclerosis, increasing risk of heart attack and stroke. Bile Acids: Critical for fat digestion; defects cause gallstones and bile acid malabsorption. Neurodegenerative Diseases: Sphingolipids maintain myelin integrity; abnormalities lead to multiple sclerosis and Fabry disease. Inflammation: Amphipathic lipids involved in immune signaling; imbalances contribute to conditions like rheumatoid arthritis and asthma LIPOSOMES They are produced when amphipathic lipids in aqueous medium are subjected to sonification. They have intermittent aqueous phases in the lipid bilayer. Liposomes, in combination with tissue specific antigens, are used as carriers of drugs to target tissues. EMULSION These are produced when nonpolar lipids (e.g. triacylglycerols) are mixed with water. The particles are larger in size and stabilized by emulsifying agents (usually amphipathic lipids), such as bile salts and phospholipids. THE DIFFERENCE BETWEEN SOAPS DETERGENT Soaps are sodium or potassium salts of fatty acids. Detergents are synthetic cleansing agents e.g. They are produced by saponification of fats. sodium lauryl sulfate. Detergents are superior in their Sodium soaps are hard that result in bar soaps. cleansing action compared to soaps, and are used Soaps serve as cleansing agents since they can in washing clothes, and in toothpaste. emulsify oils and remove the dirt. Lipids fuel life, protect cells, and keep us going—just like rest revives us to enjoy life’s best moments. So don't forget to rest after this, ka-SCI-mates!