Lipids 3

BIOCHEMISTRY

Course Details

  • Course: CHEM 2713

  • Instructor: Dr. Amitabh Jha

  • Institution: Acadia University

Lipids Classification

  • Lipids can be divided into the following classes:
      - Fatty acids
      - Triacylglycerols
      - Wax esters
      - Other Fatty Acids: Ecosanoids
      - Phospholipids
      - Sphingolipids
      - Isoprenoids
      - Lipoproteins

Isoprenoids

Characteristics

  • Comprised of repeating five-carbon isoprene structural units

  • Do not contain fatty acids

  • Commonly found in plants

Classification of Isoprenoids

  • Isoprene Unit: Building block of C5

  • Based on the number of isoprene units:
      - 2x isoprene units = Monoterpene (C10)
      - 3x isoprene units = Sesquiterpene (C15)
      - 4x isoprene units = Diterpene (C20)
      - 8x isoprene units = Tetraterpene (C40)

Functions of Common Terpenes

  • Potent signaling molecules (e.g., steroids)

  • Pigments (e.g., carotenoids)

  • Hormone synthesis intermediates (e.g., vitamins)

  • Signaling molecules (e.g., eicosanoids)

  • Membrane constituents in Archea bacteria

Lipids as Vitamins

Overview

  • Lipids such as isoprenoids serve as fat-soluble vitamins:
      - Vitamin A (Retinol)
      - Vitamin D2 (top) and Vitamin D3 (bottom)
      - Vitamin E (Tocopherol)
      - Vitamin K

Beta-Carotene Metabolism and Vision

  • Beta-Carotene metabolism is vital for visual cycles

  • Isomerization and dehydrogenation processes convert beta-carotene into Vitamin A (Retinol) which is crucial in the visual cycle.

Photoisomerization in Vision

  • The process involves the conversion of 11-cis-retinal which is essential for vision, absorbing light at specific wavelengths.

  • Retinal photoisomerization is the only light-dependent event in vision.

Steroids

Structure and Composition

  • Steroidal lipids consist of a steroid nucleus made up of four fused rings (lanosterol) and various functional groups

  • Cholesterol is a triterpene-based molecule (C27; derived from squalene) with a hydroxylic group as a polar head and a non-polar side chain

Functions of Steroids

  • Key components of cellular membranes

  • Impact the fluidity of the membrane

  • Serve as signaling molecules

Cholesterol Specifics

  • Found in the membranes of most eukaryotic cells

  • Modulates fluidity and permeability

  • Can be sourced from diet or synthesized in the liver

  • Excess levels can lead to artery clogging

Derivatives of Cholesterol

  • Various hormones can be derived from cholesterol including:
      - Cortisol (metabolic hormone)
      - Cholic Acid (bile acid)
      - Sex hormones: Estradiol, Testosterone, Progesterone

Anabolic Steroids

Examples of Anabolic Steroids

  • Notable examples include:
      - Stanozolol
      - Methandrostenolone (Dianabol)
      - Nandrolone

Notable Users of Anabolic Steroids

  • Athletes and celebrities known for anabolic steroid use include:
      - Mark McGwire
      - Roger Clemens
      - Barry Bonds
      - Jose Canseco
      - Alex Rodriguez
      - Arnold Schwarzenegger
      - Marion Jones
      - Maurice Greene
      - Ben Johnson
      - Linford Christie

Lipoproteins

Definition and Functions

  • Lipoproteins are molecular complexes found in the blood plasma that transport lipid molecules throughout the body

  • Comprised of protein components known as apolipoproteins, synthesized in the liver or intestine

Classes of Lipoproteins

  • Classifications include:
      - HDL (high-density lipoprotein)
      - LDL (low-density lipoprotein)
      - IDL (intermediate-density lipoprotein)
      - VLDL (very low-density lipoprotein)
      - Chylomicrons

Density Classification

  • Lipoproteins are classified according to density:
      - Chylomicrons: Transport triacylglycerol, synthesized in the intestines
      - VLDL: Synthesized in the liver; transport lipids to tissues
      - LDL: Principal transporters of cholesterol to tissues
      - HDL: Scavenges excess cholesterol and returns it to the liver

Cholesterol Levels and Lipid Profiles

Desirable Lipid Levels

  • HDL Cholesterol:
      - Desirable: >60 mg/dl
      - Borderline: 35–45 mg/dl
      - High Risk: <35 mg/dl

  • LDL Cholesterol:
      - Desirable: 60–130 mg/dl
      - Borderline: 130–159 mg/dl
      - High Risk: 160–189 mg/dl

  • Total Cholesterol:
      - Desirable: <200 mg/dl
      - Borderline: 200–239 mg/dl
      - High Risk: 240 mg/dl

HDL Functions

  • HDL scavenges cholesteryl esters and transports them to the liver for conversion into bile acids, being labeled as ‘good cholesterol’

Lecithin-Cholesterol Acyltransferase (LCAT) Reaction

  • The LCAT reaction involves lecithin and cholesterol to form cholesterol esters and lysolecithin

Membranes

Overview of Membrane Structure

  • Membranes consist of lipid bilayers and associated proteins arranged in a fluid mosaic model

  • Proportions of lipid, protein, and carbohydrate vary among cell types and organelles

Membrane Lipids and Fluidity

  • Phospholipids form bimolecular layers, responsible for many membrane properties

  • Membrane fluidity depends on unsaturated fatty acids, cholesterol presence stabilizes the membrane

Selective Permeability and Movement Through Membrane

  • Membranes are selectively permeable, aiding in regulating ion and polar substance movement

  • Rapid lateral diffusion is necessary for normal membrane functioning

Membrane Proteins

  • Functions associated with membranes require membrane proteins, categorized as:
      - Integral proteins: Embedded in or pass through membrane
      - Peripheral proteins: Bound primarily through non-covalent interactions, sometimes linked covalently

Asymmetry in Membrane Structure

  • Each side of the membrane has a different lipid composition, necessary for proper function.