Lipids Overview and Classification

Lipids: Overview

• Lipids are a heterogeneous group of molecules related to fatty acids.
• They are insoluble in water but soluble in non-polar solvents.
• Their hydrophobic nature allows them to self-assemble in water, forming structures like bilayers and micelles.

Lipids: Role in Organisms

• Serve as a dietary source of high energy value, vitamins, and essential fatty acids (EFAs).
• Major energy reserve for organisms.
• Act as insulating materials (e.g., adipose tissue provides thermal and electrical insulation for nerves).
• Play a role in cell membrane segregation, dynamic interface formation, transport, and signaling (e.g., lipoproteins).

Lipids: Classification

• Simple Lipids: Esters of fatty acids with various alcohols.
  • Fats: Esters of fatty acids with glycerol. Liquid fats are called oils; their melting point depends on the percentage of saturated fatty acids and molecular weight.
  • Waxes: Esters of fatty acids with high molecular weight alcohols.
• Complex Lipids: Esters of fatty acids containing additional groups besides an alcohol and a fatty acid.
  • Phospholipids: Contain fatty acids, an alcohol (glycerol or sphingosine), and a phosphate group.
    • Glycerophospholipids: Alcohol is glycerol.
    • Sphingophospholipids: Alcohol is sphingosine.
  • Glycolipids: Contain a fatty acid, sphingosine, and a carbohydrate.
  • Lipoproteins: Complex lipids with protein components.
• Precursor and Derived Lipids: These include free fatty acids, steroids, terpenes, and glycerol (neutral lipids).

Fatty Acids (FAs)

• Long-chain carboxylic acids, typically containing an even number of carbon atoms due to their biosynthesis from C2 fragments (AcCoA).
• Classified as saturated or unsaturated, which significantly affects their shape and physicochemical properties.
• Essential Fatty Acids: Cannot be synthesized by the body and must be obtained through the diet. Examples include linolenic acid (C18, three unsaturated bonds) and linoleic acid (C18, two unsaturated bonds).

Unsaturated Fatty Acids

• Contain one or more double bonds.
• Nomenclature: Chemists count from the carboxyl carbon, while physiologists count from the omega (ω) carbon.
• Eicosanoic fatty acids (EFAs), particularly arachidonic acid, are precursors of prostaglandins, leukotrienes, thromboxanes, and prostacyclins. These lipids are associated with inflammation and pain.
• Due to the presence of double bonds, unsaturated fatty acids have lower melting points, contributing to membrane fluidity.
• Susceptible to oxidation, leading to rancidity and potential tissue damage. May be implicated in cancer and inflammation.
• Trans Fatty Acids: Rare in nature, but produced during lipid metabolism and the manufacture of liquid vegetable oils and margarine. Can increase plasma cholesterol levels.

Triacylglycerols (Triglycerides, Fats)

• Triesters of glycerol with fatty acids.
• Fatty acid chains can be identical or different.
• Non-polar and water-insoluble.
• Lower density than water, making them excellent thermal and electrical insulators.
• Serve as energy storage and can be hydrolyzed by lipases.

Waxes

• Esters of fatty acids with long-chain alcohols.
• Function in energy storage, water repellency, and have various pharmaceutical and cosmetic applications.

Glycerophospholipids

• Major components of cellular membranes.
• General structure: glycerol backbone with two fatty acids esterified to the first and second carbons, and a phosphate group linked to the third carbon.
• Lecithins: Important surfactants, with examples like dipalmitoyl phosphatidyl choline (DPPC).

Lipids with Ether Linkages

• Found in vertebrate heart tissue (e.g., plasmalogens).
• Released from leukocytes (basophils).
• Stimulate platelet aggregation and serotonin release, playing a role in inflammation and allergic responses.

Sphingolipids

• Contain sphingosine (an amino alcohol) instead of glycerol.
• Important components of cell membranes, particularly in nerve tissue.
• Glycosphingolipids: Contain carbohydrate groups, play a role in cell recognition, and determine ABO blood types.

Enzymatic Degradation of Phospholipids: Phospholipase Specificity

• Phospholipases are enzymes that hydrolyze phospholipids, with different types targeting specific bonds in the molecule. This specificity plays a role in signaling and other cellular processes.

Sterols

• Structural lipids that are components of eukaryotic cell membranes.
• Characterized by the steroid nucleus (planar and rigid structure).
• Cholesterol: A key sterol, synthesized in the liver and used for membrane structure, hormone production, and bile acid formation.

Lipids: Signaling Role

• Many lipids act as signaling molecules, playing a crucial role in intracellular communication.
• Phosphatidylinositol-4,5-bisphosphate (PIP2): A key signaling lipid in cell membranes that can be hydrolyzed by phospholipase C to generate inositol triphosphate (IP3) and diacylglycerol (DAG).
  • Inositol Triphosphate (IP3): Water-soluble, releases calcium from intracellular stores, influencing a range of cellular processes.
  • Diacylglycerol (DAG): Remains attached to the membrane, activates protein kinase C, which initiates signaling cascades.

Eicosanoids: Paracrine Hormones

• Locally acting signaling molecules derived from arachidonic acid.
• Influence the environment around their site of synthesis.
• Examples include prostaglandins, leukotrienes, thromboxanes, and prostacyclins.

Steroid Hormones

• Oxidized derivatives of sterols, more polar than sterols.
• Travel from their production site to target tissues via the bloodstream, where they activate specific nuclear receptors.
• Examples include sex hormones (male and female) and hormones produced by the adrenal cortex (involved in glucose metabolism).
• Synthetic steroids are used as anti-inflammatory agents.

Vitamin D3 Production and Metabolism

• Vitamin D3 is a steroid hormone produced in the skin upon exposure to sunlight.
• Essential for calcium absorption and bone health.
• Deficiency leads to rickets (characterized by bone deformities and other health problems).

Isoprenoids: Vitamin A1, Precursors, and Derivatives

• Vitamin A1 is an isoprenoid compound found in milk, fish liver oil, and other sources.
• Plays a role in vision, epithelial tissue development, and skin health.

Other Biologically-Active Isoprenoid Compounds

• Vitamin E (tocopherol): An antioxidant, protecting cells from oxidative damage.
• Coenzyme Q: A vital component of the electron transport chain, involved in energy production.

Drugs and Diseases

• Diseases Linked to Lipid Metabolism: Respiratory Distress Syndrome (RDS) in neonates, thrombosis, inflammation, asthma, rickets.
• Drugs and Vitamins: Synthetic DPPC (surfactant), linoleic and linolenic acids (EFAs), prednisone and prednisolone (synthetic steroids), vitamins A, D3 (Calcitriol), E, K1, coenzyme Q, warfarin (anticoagulant).

This quiz covers the fundamental aspects of lipids, including their characteristics, roles in organisms, and classification. Participants will explore lipids' significance as energy sources and their functional purposes in biological systems. Test your knowledge on this essential topic in biochemistry!