BCH 201 - Introductory Chemistry of Lipids

BCH 201 - Introductory Chemistry of Lipids

Introduction

• Lipids (Greek: lipos meaning "fat") are one of the four main biomolecules in nature, alongside proteins, carbohydrates, and nucleic acids.
• Biochemical Characteristics:
  • Biological substances soluble in organic solvents but sparingly or totally insoluble in water.
• Constituents of Lipids:
  • Glycerol
  • Fatty acids
  • Other compounds (e.g., serine, inositol, etc.)
• Examples of lipids:
  • Fats
  • Oils
  • Some vitamins
  • Hormones

Classification of Lipids

• Main Classes:
  • A. Fatty acids
  • B. Glycerolipids (Triacylglycerol)
  • C. Glycerophospholipids
  • D. Sphingolipids
  • E. Sterol lipids
  • F. Prenol lipids
  • G. Saccharolipids
  • H. Polyketides

A. Fatty Acids

• Definition: Fatty acids are diverse hydrophobic organic molecules that contain a carboxylic acid functional group.
• Hydrocarbon Chains: Ranges from 4 (C4) to 36 (C36) carbons in length.
  • Characteristic: Nonpolar hydrocarbon chains make them less soluble or insoluble in water.
  • Variability: Some fatty acids may contain three-carbon rings, hydroxyl groups, or methyl group branches.

Classification of Fatty Acids

• Based on the presence or absence of C=C double bonds:
  • 1. Saturated fatty acids
  • 2. Unsaturated fatty acids

Saturated Fatty Acids

• Composition: Hydrocarbon chains connected by C-C single bonds only, thus they are fully saturated with hydrogens.

Unsaturated Fatty Acids

• Features: Contain one (monounsaturated) or more (polyunsaturated) double bonds.
• Configuration:
  • Cis Configuration: Hydrogens around the double bond are in the same plane.
  • Trans Configuration: Hydrogens are in opposite directions around the double bond.

Impact of Cis and Trans Configurations

• Cis Double Bond: Causes a bend or kink preventing tight packing; keeps them in liquid state at room temperature.
• Trans Double Bond: Produces during fermentation in the rumen of dairy animals.

Examples of Saturated Fatty Acids

• n-Dodecanoic acid (Lauric acid): $CH<em>3(CH</em>2)_{10}COOH$
• n-Tetradecanoic acid (Myristic acid): $CH<em>3(CH</em>2)_{12}COOH$
• n-Hexadecanoic acid (Palmitic acid): $CH<em>3(CH</em>2)_{14}COOH$
• n-Octadecanoic acid (Stearic acid): $CH<em>3(CH</em>2)_{16}COOH$
• n-Eicosanoic acid (Arachidic acid): $CH<em>3(CH</em>2)_{18}COOH$
• n-Tetracosanoic acid (Lignoceric acid): $CH<em>3(CH</em>2)_{22}COOH$

Examples of Unsaturated Fatty Acids

• Cis-9-Hexadecanoic acid (Palmitoleic acid):
  $16:1(A) CH<em>3(CH</em>2)<em>{5}CH=CH(CH</em>2)_{2}COOH$
• Cis-9-Octadecanoic acid (Oleic acid):
  $18:1(A) CH<em>3(CH</em>2)<em>{7}CH=CH(CH</em>2)_{2}COOH$
• Cis-9,12-Octadecadienoic acid (Linoleic acid):
  $18:2(Δ9,12) CH<em>3(CH</em>2)<em>{4}CH=CHCH</em>2CH=CH(CH<em>2)</em>{2}COOH$
• Cis-9,12,15-Octadecatrienoic acid (Alpha-linolenic acid):
  $18:3(A9,12,15) CH<em>3CH=CHCH</em>2CH=CHCH<em>2CH=CH(CH</em>2)_{2}COOH$

Importance of Carbon Numbering in PUFAs

• Polyunsaturated fatty acids (PUFAs) have multiple double bonds:
  • First double bond may occur between the third and fourth carbon from the methyl end (ω-3) or between the sixth and seventh carbon (ω-6).
• Essential fatty acids:
  • Human beings cannot synthesize certain PUFAs and must obtain them from the diet, e.g., Alpha-linolenic acid (ALA).
  • The body can synthesize two other essential omega-3 PUFAs: Eicosapentaenoic acid (EPA; 20:5(Δ5,8,11,14,17)) and Docosahexaenoic acid (DHA; 22:6(Δ4,7,10,13,16,19)).
  • Sources of PUFAs include plant and fish oils.

Differences Between Saturated and Unsaturated Fatty Acids

B. Triacylglycerol/Glycerolipids

• Definition: Triacylglycerols, triglycerides, or neutral fats are the most abundant class of lipids, serving as energy reservoirs in animals.
• Characteristics:
  • Hydrophobic (nonpolar) fatty acid triesters of glycerol.
  • Composed of three fatty acids each linked by ester bonds to a single glycerol.

Types of Triacylglycerols

• 1. Simple Triacylglycerols:
  • All fatty acid residues are the same (e.g., tristearin has three stearic acid residues).
• 2. Mixed Triacylglycerols:
  • Contains two or more different fatty acid residues at different positions on the glycerol backbone.

C. Glycerophospholipids (Phosphoglycerides)

• Definition: Major lipids found in biological membranes.
• Structure:
  • Two fatty acids attached in ester linkage to the first and second carbons of glycerol.
  • A polar or charged group attached through a phosphodiester linkage to the third carbon.
  • Simplest form: Phosphatidic acid.

General Structure of Glycerophospholipids

• $CH<em>2-O-C-R</em>1$
• $CH<em>2-O-C-R</em>2$
• $CH_2-O-P-O-X$
  • R1 and R2 represent fatty acids; X represents the head group.

Examples of Glycerophospholipids and Their Head Groups

D. Sphingolipids

• Definition: A large class of membrane lipids featuring a polar head group and two nonpolar tails, but lacking glycerol.
• Structure:
  • Comprised of sphingosine (or its derivatives) and a long-chain fatty acid with a polar head group.
  • May be attached via glycosidic or phosphodiester linkages.
• Functionality:
  • Present in various human tissues, notably the plasma membranes of neurons.
  • Some sphingolipids define blood groups in humans affecting blood transfusion compatibility.

General Structure of Sphingolipids

• $CH<em>3-CH(NH)-C</em>{17}H_{35}-X$
  • X is the polar head group.

Examples of Sphingolipids

E. Sterol Lipids

• Definition: Sterols are another group of membrane structural lipids characterized by a steroid nucleus consisting of four fused rings—three with six carbon atoms and one with five.
• Major Component: Cholesterol is the predominant sterol in animal tissues.
  • Properties: Amphipathic, containing polar head groups due to the hydroxyl group at C-3 and nonpolar hydrocarbon bodies.
  • Functions:
  • metabolic precursor for steroid hormones, regulating sexual development and carbohydrate metabolism.
  • High levels of cholesterol can pose a risk of heart diseases.