Lipids Overview

Lipids

• Lipids are a heterogeneous group of organic compounds
• Defined by solubility in nonpolar solvents (chloroform, ether, benzene) and poor solubility in water
• Unlike other polymers (polysaccharides, proteins, nucleic acids), lipids are not polymers
• Mostly small molecules
• Can be polar or nonpolar (amphipathic)
• Major polar lipids include fatty acids, cholesterol, glycerophosphatides, and glycosphingolipids
• Very short chain fatty acids and ketone bodies are readily soluble in water
• Present in humans, animals, plants, and microorganisms to some extent
  • Animal fat, egg yolk, butter, and cheese are animal-origin lipids
  • Vegetable/cooking oils are plant-origin lipids

Functions of Lipids

• Thermal insulator against cold (under skin)
• Padding against injury (fat around kidneys)
• Energy source for cells, comparable to carbohydrates
• Ideal for energy storage in humans because of higher energy content compared to carbohydrates and proteins
• Structural components of cell membranes and nervous tissue
• Precursors for complex molecules (e.g., acetyl-CoA for cholesterol synthesis)
• Involved in lipid transport in blood as lipoproteins
• Hormones and fat-soluble vitamins are lipids
• Essential for fat-soluble vitamin absorption

Classification of Lipids

• Simple lipids: Esters of fatty acids with different alcohols
  • Fats and oils (TAGs): Esters of fatty acids with glycerol
  • Physical difference: Oil is liquid at room temperature; fat is solid.
• Complex (compound) lipids: Esters of fatty acids with alcohols containing additional groups (phosphate, nitrogenous base, carbohydrate, protein, etc.)
  • Phospholipids:
  • Glycerophospholipids (e.g., lecithin, cephalin): Contains glycerol
  • Sphingophospholipids (e.g., sphingomyelin): Contains sphingosine
  • Glycolipids: Contain fatty acid, carbohydrate, and nitrogenous base.
  • Examples: Cerebrosides, gangliosides.
  • The alcohol is typically sphingosine, hence glycosphingolipids.
  • Glycerol and phosphate are missing.
  • Lipoproteins: Macromolecular complexes of lipids with proteins involved in transporting lipids in blood.
  • Other complex lipids: Sulfolipids, amino lipids, and lipopolysaccharides
• Derived lipids: Derivatives obtained after hydrolysis of simple and complex lipids
  • Examples: Fatty acids, monoglycerides, diglycerides, steroid hormones, fat-soluble vitamins, ketone bodies.
• Miscellaneous lipids: Large number of compounds with lipid properties
  • Examples: Carotenoids, squalene, hydrocarbons (e.g., pentacosane in beeswax), terpenes.

Fatty Acids

• Carboxylic acids with hydrocarbon side chains
• Simplest form of lipids
• Anionic group has an affinity for water, giving amphipathic nature (hydrophilic and hydrophobic regions)
• Fatty acids with less than 12 and more than 24 carbon atoms are uncommon
• Palmitic acid (16C) and stearic acid (18C) are the most common
• Most have an even number of carbon atoms due to biosynthesis of fatty acids with sequential addition of 2 carbon units
• Long-chain fatty acids (LCFAs) are hydrophobic and need protein for transport in circulation. LCFA comprise more than 90% of plasma fatty acids in ester forms

Saturated vs. Unsaturated Fatty Acids

• Saturated: No double bonds, long straight chains, solid at room temperature, contribute to cardiovascular disease (atherosclerosis).
• Unsaturated: Contain one or more double bonds, create kinks, liquid at room temperature, are more prominent in plant and fish fats and vegetable oils.

Trans Fats

• Formed when hydrogen is added to unsaturated fats (hydrogenation)
• More solid than oils
• Increase shelf life of foods

Fatty Acid Structure

• Double bonds create kinks, prevent close packing, thus less Van der Waals interactions making the substance more fluid and decreasing melting points in room temperature.

The Peanut Butter Puzzle

• Fats are typically found in animals, and Oils in plants.
• Peanut butter is solid due to hydrogenation.

Essential Fatty Acids

• Not synthesized in the body
• linoleic acid (precursor to arachidonic acid, used in prostaglandin synthesis) crucial for growth and development
• a-linolenic acid important for growth and development
• Arachidonic acid becomes essential if linoleic acid is deficient in the diet

Storage Lipids

• Fats and oils are used for energy storage
• Two main types: Triacylglycerols and Waxes
• Triacylglycerols are esters of fatty acids and glycerol in living organisms
• Waxes are esters of long-chain fatty acids and long-chain alcohols

Fats (Triacylglycerols)

• Esters of fatty acids with glycerol
• Also called triglycerides or triacylglycerols
• All three hydroxyl groups of glycerol are esterified
• Commonly called neutral fats

Steroids

• Complex molecules containing four fused rings
• Include cyclopentanoperhydrophenanthrene (CPPP) or sterane ring

Compound Lipids

• Lipids that contain additional elements (i.e. additional to fatty acids and alcohols) in addition to fatty acids and alcohols.
• Examples: Phospholipids, Glycolipids, and lipoproteins

Cardiolipin

• Component of the inner mitochondrial membrane
• Only human glycerophospholipid that is antigenic
• Used in serological diagnosis of syphilis and autoimmune diseases.

Plasmalogens

• Ether linkage. Acyl group
• Glycerol backbone.

Platelet-Activating Factor (PAF)

• Synthesized by variety of cell types
• Binds to surface receptors, triggering thrombotic and inflammatory events
• Activates inflammatory cells, mediating hypersensitivity, acute inflammatory, and anaphylactic reactions
• Causes platelets to aggregate.

Sphingophospholipids (Sphingomyelin)

• Important component of nerve tissue (myelin sheath)
• Insulate and protect central nervous system fibers.

Glycolipids

• Contains carbohydrate residues, and a very long chain fatty acid (sphingosine),
• Found in cerebral tissues
• Classified based on carbohydrate presence (e.g., cerebrosides, gangliosides, sulfatides)

Gangliosides

• Compound lipids in the gray matter of the brain, ganglionic cells, and RBCs
• Composed of ceramide and oligosaccharides with one or more sialic acids linked to a sugar chain
• Classified based on their oligosaccharide attached to the ceramide.

Membrane Lipids

• Located in cellular and subcellular membranes.
  • e.g. Glycerophospholipids, Sphingolipids, and Glycolipids

Lipid Rafts

• Have higher concentrations of sphingolipids, cholesterol, and glycolipids.

Lipoproteins

• Lipids combined with proteins
• Two types: Structural and Transport
• Structural: Widely distributed in tissues (e.g., lungs and eye).

Lipoprotein Classification

• Classified based on density, which reflects size and lipid/protein ratio.
  • Chylomicrons, largest and least dense, transport dietary triglycerides.
  • Very low-density lipoproteins (VLDL): Transport endogenous triglycerides form the liver to cells
  • Intermediate-density lipoproteins (IDL): Formed during VLDL conversion into LDL, usually undetectable in normal plasma.
  • Low-density lipoproteins (LDL): Carry cholesterol from VLDL or IDL to cells.
  • High-density lipoproteins (HDL): Transport cholesterol from cells back to the liver.

LDL and HDL

• LDL cholesterol is 'bad' because high levels contribute to buildup of cholesterol in arteries, thus increasing the risk of heart disease.
• HDL cholesterol is 'good' because it transports cholesterol from cells back to the liver for removal.

This quiz covers the essential aspects of lipids, including their classification, structure, and functions in biological systems. Learn about their properties, types, and roles as energy sources and structural components in living organisms.