Lecture on Lipid Structure and Function

Chapter 15: Lipid Structure and Function

Chapter Outline

  • 15.1 Many Lipids Are Made from Fatty Acids
  • 15.2 Triacylglycerols Are Energy Storage Lipids
  • 15.3 Cell Membranes Contain Three Major Types of Lipids
  • 15.4 Lipids Function in Cell Signaling

15.1 Many Lipids Are Made from Fatty Acids

  • Definition of Lipids:
    • Water Insolubility: Lipids are characterized as water insoluble and amphipathic molecules.
    • Roles of Lipids:
    • Energy storage
    • Components of cell membranes
    • Endocrine signaling
    • Provide protection and insulation;
  • Fatty Acids (FAs):
    • Composed of:
    • Carboxylic Group: Structure includes COOH
    • Hydrocarbon Tail: Typically comprises 4–36 carbons
    • Characteristics of Fatty Acids:
    • The most common are unbranched chains with 12–20 carbons
    • Fatty acids typically contain even-numbered carbon atoms.

15.1.1 Types of Fatty Acids

  • Saturated Fatty Acids:
    • Fully reduced methylene groups; no double bonds.
  • Unsaturated Fatty Acids:
    • Contain one or more C ═ C double bond(s).
    • Types:
    • Monounsaturated Fatty Acids: One C ═ C double bond.
    • Polyunsaturated Fatty Acids: Two or more C ═ C double bonds.
  • Classification by Chain Length:
    • Short chain: up to 8 carbons
    • Medium chain: up to 20 carbons
    • Long chain: more than 20 carbons
  • Physiological Significance:
    • Non-essential: Not required in diet
    • Essential: Required in diet; examples include omega fatty acids.

15.1.2 Physical Properties of Fatty Acids

  • Melting Point (Tm) Factors:
    • Chain length
    • Number of double bonds
    • Configuration (cis or trans)
    • Generally, cis fatty acids possess lower melting points.
  • Impact of Unsaturation:
    • Unsaturation introduces kinks, impacting packing and rigidity.
    • Example structures include:
    • Stearic acid (18:Δ0)
    • Oleic acid (trans 18:Δ9)
    • Oleic acid (cis 18:Δ9)

15.1.3 Omega Fatty Acids (PUFAs)

  • Definition: First double bond from the terminal methyl group determines omega classification.
  • Examples:
    • Omega-3 (linolenic acid)
    • Omega-6 (linoleic acid)
  • Iodine Number:
    • A biochemical method to measure unsaturation; expressed in terms of mg of iodine per 100g of fat.
  • Health Implications:
    • Omega-3 fatty acids correlate with reduced cardiovascular disease risks, particularly in the Mediterranean diet.

15.1.4 Hydrogenation and Trans Fats

  • Hydrogenation Process:
    • Reduce C ═ C (convert unsaturated fatty acids to saturated) using H2, affecting rancidity and melting points.
    • Partial hydrogenation can produce trans fats associated with cardiovascular diseases.
  • **Trans Fat Content in Fast Foods:
    • French fries: 4.7-6.1%
    • Pizza: 1.1%
    • Doughnut: 2.7%

15.2 Triacylglycerols Are Energy Storage Lipids

  • Chemical Definition:
    • Composed of three fatty acids esterified with glycerol.
    • Both highly reduced and hydrophobic.
    • Common examples: animal fats, plant oils.
  • Functions of TG:
    • Energy storage ~ 9.3 kcal/g
    • Insulation and thermogenesis (e.g., in grizzly bears).

15.2.1 Metabolism of Dietary Triacylglycerols

  • Process Overview:
    1. In the Intestine:
    • Emulsification and cleavage by lipases to release free fatty acids and glycerol.
    1. Within Epithelial Cells:
    • Reassembly of fatty acids and glycerol to form triacylglycerols.
    • Packaged into chylomicrons for transport.
    1. In the Lymphatic System:
    • Chylomicrons enter circulatory system.

15.2.2 Chylomicrons

  • Function:
    • Largest lipoprotein particles responsible for carrying triglycerides and cholesterol esters.
  • Structure:
    • Monolayer phospholipid membrane derived from intestinal epithelial cells.
    • Contains apolipoproteins for enzyme binding.

15.2.3 VLDL (Very Low-Density Lipoprotein)

  • Function:
    • Carrier of triglycerides from the liver to peripheral tissues.
  • Synthesis Process:
    1. Fatty acids synthesized in liver cytosol.
    2. Esterification with glycerol in the ER to create triglycerides.
    3. Packaging of triglycerides into VLDL particles.

15.2.4 Lipid Droplets

  • Definition:
    • Storage sites for triacylglycerols in adipocytes.
  • Phospholipid Monolayer:
    • Contains perilipin and assists in lipid management.
  • Types of Adipocytes:
    • White adipocytes (white fats)
    • Brown adipocytes (rich in mitochondria).

15.2.5 Glucagon and Fat Mobilization

  • Mechanism:
    • Glucagon stimulates the mobilization of fatty acids from lipid droplets in adipocytes, transported via albumin.

15.3 Cell Membranes

  • Function:
    • Act as protective hydrophobic barriers that