Overview of Lipids and Their Functions

Overview of Lipids

• Lipids are substances of biological origin which are insoluble in water.

Structural Features of Lipids

• Fatty Acids are long-chain carboxylic acids
• Cis-Fatty Acids have hydrogen atoms on the same side of the double bond leading to a bend in the molecule.
• Trans-Fatty Acids have hydrogen atoms on opposite sides of the double bond leading to a straight molecule.
• Trans-Fatty Acids have adverse health effects:
  • Increase in low-density lipoprotein in the liver
  • Coronary heart disease
  • Cancer
  • Diabetes
• Essential Fatty Acids are those the human body can’t synthesize and must be obtained through the diet:
  • Linoleic acid (𝝎-6)
  • ɑ-Linoleic acid (𝝎-3)
  • Linoleic acid is the precursor of Arachidonic acid which is important for inflammatory responses and other processes.
  • ɑ-Linoleic acid is the precursor of Docosahexaenoic acid (DHA), an important component of the human brain.
• Glycerol is a naturally occuring alcohol that forms the backbone of triacylglycerols.
• Sphingosine is an 18-carbon amino alcohol that forms the backbone of sphingolipids.

Storage Lipids

• Triacylglycerols (also called triglycerides or fats) are the most abundant class of lipids and serve as energy stores in animals.
  • They consist of a glycerol esterified to three fatty acid residues.
  • There are simple triacylglycerols with the same fatty acid attached to all three positions of glycerol and mixed triacylglycerols with different fatty acid substitutions.
• Reactions of Triacylglycerols
  • Hydrogenation is the reduction of double bonds in unsaturated fatty acids to convert them to solid, saturated fatty acids using hydrogen gas over a platinum or palladium catalyst.
  • Saponification is the process of making soap from fats or oils and involves reacting fats or oils with a base to form a surfactant in a hydrolysis reaction.

Structural Lipids

• Phospholipids are amphiphilic molecules with both a polar and non-polar part making them essential components of cell membranes.
  • Glycerophospholipids are comprised of glycerol with a phosphate group and two fatty acid chains attached.
    • The derivatives of phosphatidic acid (parent compound) include:
      • Phosphatidylcholine
      • Phosphatidylethanolamine
      • Phosphatidylserine
      • Phosphatidylinositol
  • Sphingolipids are lipids with sphingosine as their backbone.
    • Sphingosine is an 18-carbon amino alcohol that forms the backbone of sphingolipids and is a vital part of blood lipoproteins and is the precursor of all sex and steroid hormones.
    • They are found in biological membranes and are important for cell signaling and recognition.
• Glycolipids are lipids with a carbohydrate attached, which are important for cell recognition and communication:
  • Glycosphingolipids have a carbohydrate attached to a sphingosine backbone.
  • Galactolipids are found in plants and have a carbohydrate attached to a glycerol backbone.

Signalling Lipids

• Steroids are lipids with a characteristic four-ring structure.

  • Cholesterol is an important steroid that is essential for survival but excess cholesterol can cause adverse health effects.
    • Cholesterol functions as a structural lipid in the epidermis of the skin and is a precursor of all sex and steroid hormones.
    • Hypercholesterolemia or excess cholesterol has been shown to increase risk of heart attack, stroke, and cancer.
  • Lipoproteins are molecules that transport cholesterol, triacylglycerols, and other hydrophilic biomolecules.
    • Types of lipoproteins:
      • High-Density Lipoprotein (HDL)
      • Low-Density Lipoprotein (LDL)
      • Very Low-Density Lipoprotein (VLDL)
      • Chylomicrons

• Eicosanoids are signaling molecules derived from arachidonic acid.

  • Prostaglandins are involved in inflammation, pain, fever, and blood clotting.
  • Thromboxanes are involved in blood clotting.
  • Leukotrienes are involved in allergic and inflammatory responses.