LIPIDS

Objectives

• Understand the chemical and structural features of different classes of lipids.

• Discuss the distribution in the body and biological functions of lipids.

• Differentiate between saponifiable and non-saponifiable lipids.

Overview of Lipids

• Definition of Lipids:

  • A group of heterogeneous biomolecules.

  • Insoluble in water but soluble in non-polar solvents.

• Biological Functions of Lipids:

  • Energy source.

  • Insulation of vital organs.

  • Covering of nerve fibers.

  • Components of cell membranes.

  • Chemical messengers (such as hormones).

Classification of Lipids

• LIPIDS

  • Saponifiable:

  • Glyceryl Esters:

    • Triacylglycerols.

    • Phospholipids.

  • Non-glyceryl Esters:

    • Waxes.

    • Glycolipids.

  • Non-Saponifiable:

  • Steroids.

  • Eicosanoids.

  • Terpenes.

  • Pheromones.

  • Fat-soluble vitamins.

Fatty Acids

• Definition:

  • Monocarboxylic acids with long, unbranched hydrocarbon chains, typically consisting of 12 to 25 carbons.

  • Smallest Fatty Acid:

  • Butanoic acid (4 carbons).

• Types of Fatty Acids:

  • Saturated FA: No double bonds.

  • Monounsaturated FA: One double bond.

  • Polyunsaturated FA: Two or more double bonds.

Fatty Acid Formula

• Condensed and Line-bond Formula Examples:

  • Caprylic Acid (C8, Saturated):

  • Condensed: CH₃CH₂CH₂CH₂CH₂CH₂CH₂COOH

  • Line-bond: CH₃―(CH₂)₆―COOH

  • Octanoic acid (8:0).

  • Palmitoleic Acid (C16, Monounsaturated):

  • Condensed: CH₃CH₂CH₂CH₂CH₂CH₂CH=CHCH₂CH₂CH₂CH₂CH₂CH₂COOH

  • Line-bond: CH₃(CH₂)₅CH=CH(CH₂)₂COOH

  • Name: Cis-9-hexadecenoic acid 16:1(A⁹).

Saturated vs. Unsaturated Fatty Acids

• Saturated Fatty Acids:

  • Higher melting point (MP).

  • Structure characterized by no double bonds in hydrocarbon chain.

• Unsaturated Fatty Acids:

  • Lower melting point (MP).

  • Structure characterized by the presence of double bonds.

Essential Fatty Acids

• Definition:

  • Fatty acids that mammals cannot synthesize.

  • Examples: Omega-3 and Omega-6 fatty acids.

  • Importance:

  • Essential for normal visual and central nervous system development.

  • Omega Notation:

  • Indicates the position of the first double bond from the methyl end (last carbon).

Omega-3 and Omega-6 Fatty Acids

• Omega-3 Fatty Acids:

  • Alpha-linolenic acid (ALA, C18:3).

  • Eicosapentaenoic acid (EPA, C20:5).

  • Docosahexaenoic acid (DHA, C22:6).

• Omega-6 Fatty Acids:

  • Linoleic acid (LA, C18:2).

  • Arachidonic acid (AA, C20:4).

Saponifiable Lipids

• Definition:

  • Lipids that produce fatty acid salts upon treatment with a base.

• Categories:

  • Glyceryl Esters:

  • Triglycerides:

    • Composed of glycerol and three fatty acids.

  • Glycerophospholipids:

  • Common constituents of cell membranes, where the first two -OH groups of glycerol are esterified with fatty acids and the third with a phosphate group.

  • Non-Glyceryl Esters:

  • Sphingophospholipids, waxes, glycolipids.

Triacylglycerols (Triglycerides)

• Structure:

  • Esters of glycerol and fatty acids (TAG).

  • Example Structure:

    

• Simple vs. Mixed Triglycerides:

  • Simple Triglyceride (Fat): Contains the same fatty acid.

  • Mixed Triglyceride (Oil): Contains different fatty acids.

Fats vs. Oils

• Fats:

  • TAGs with a high percentage of saturated fatty acids.

  • Solid at room temperature.

  • Typically derived from animal sources.

• Oils:

  • TAGs with a high percentage of unsaturated fatty acids.

  • Liquid at room temperature.

  • Typically derived from plant sources.

Glycerophospholipids

• Function:

  • Form significant components of cell membranes and exhibit amphiphilic properties (having both hydrophilic heads and hydrophobic tails).

• Structure:

  • Triesters with varying R groups attached to glycerol and phosphate groups.

Sphingophospholipids

• Definition:

  • Derivatives of ceramide which combine sphingosine with fatty acids via amide linkages, typically featuring a polar group connected through the terminal carbon.

Waxes

• Definition:

  • Esters formed from fatty acids and long-chain monohydric alcohols.

• Functions:

  • Found on leaf surfaces to prevent excessive water loss.

  • Composed in feathers of birds and hair.

Glycolipids

• Definition:

  • Lipids that contain monosaccharides bonded through a glycosidic bond to the -OH group of ceramide.

• Types:

  • Cerebrosides: Simple glycolipids in the brain/myelin sheath.

  • Gangliosides: Complex glycolipids with multiple sugar units.

Non-Saponifiable Lipids

• 1. Steroids:

  • Lipids containing a steroid nucleus, a fused structure of four rings (e.g., cholesterol, bile salts, hormones).

• 1.1 Cholesterol:

  • Most abundant steroid, synthesized in the liver and food source.

• 1.2 Bile Salts:

  • Synthesized from cholesterol; emulsifies fats to enhance lipid digestion.

• 1.3 Steroid Hormones:

  • Various hormones derived from steroid structure (e.g., cortisol, aldosterone).

Eicosanoids

• Definition:

  • Signaling molecules derived from C20 essential fatty acids.

• Types:

  • Prostaglandins, thromboxanes, leukotrienes.

Prostaglandins

• Function:

  • Regulate contraction and relaxation of smooth muscles.

Thromboxanes

• Function:

  • Promote blood clot formation.

Leukotrienes

• Function:

  • Involved in inflammatory responses.

Terpenes

• Definition:

  • Hydrocarbons constructed from isoprene units, contributing to plant scents and tastes.

• Types of Terpenes:

  • Monoterpenes (C10), Diterpenes (C20), Triterpenes (C30), Tetraterpenes (C40).

Pheromones

• Definition:

  • Chemicals used by insects for communication within the species, often long chain alkenes with functional groups.

Fat Soluble Vitamins

• Vitamin A:

  • C20H30; involved in visual pigment function.

• Vitamin D₂:

  • C28H44; important for calcium metabolism and bone growth.

• Vitamin E:

  • C29H50O2; acts as an antioxidant.

• Vitamin K₁:

  • C31H46O2; essential for blood clotting.

References

• S. Nelson and Cox. Lehninger's Principles of Biochemistry, 4th ed.

• Stryer, Berg, and Tymoczko. Biochemistry, 4th ed.