Lipids Lecture 1 - Introduction

Introduction to Lipids

• Lipids are a broad category of biological molecules involved in various functions in the body.

• They are defined as biological nonpolar molecules, which makes them soluble in nonpolar solvents but insoluble in water.

• Key functions of lipids include:

  • Moisture retention and exclusion

  • Chemical messengers (hormones)

  • Long-term energy storage

  • Protective padding

  • Thermal insulation

  • Cell membrane structure

  • Cell signaling

  • Electrical insulation

• Types of lipids discussed include: waxes, eicosanoids, triglycerides, cholesterol, phospholipids, glycerolipids, glycolipids, and sphingolipids.

• These lipids often contain carboxylic acids, esters, or amide derivatives.

Fatty Acids

A) Structures

• Fatty acids have a carboxylic acid functional group attached to a long nonpolar tail.

• Common fatty acids include:

  • Saturated Fats:

    • Lauric Acid (C-12) - coconut oil

    • Myristic Acid (C-14) - butter fat

    • Palmitic Acid (C-16) - common

    • Stearic Acid (C-18) - common

  • Unsaturated Fats:

    • Oleic Acid (C-18, ω-9) - olive oil

    • Linoleic Acid (C-18, ω-6) - vegetable oil

    • α-Linolenic Acid (C-18, ω-3) - flaxseed oil

    • Arachidonic Acid (C-20, ω-6) - animal fats

• Linoleic acid and α-linolenic acid are essential fatty acids needed in the diet.

B) Classification

• Length Classification:

  • Short Chain (up to 5 carbons)

  • Medium Chain (6-12 carbons)

  • Long Chain (more than 12 carbons)

  • Very Long Chain (more than 22 carbons)

• Saturation Classification:

  • Saturated with no double bonds

  • Unsaturated with one or more double bonds:

    • Monounsaturated (one double bond)

    • Polyunsaturated (more than one double bond)

• Naming convention for unsaturation:

  • For determining ω-position, counting starts from the last carbon from the carboxylic acid.

  • Example: Oleic acid is an ω-9 fatty acid; linoleic and arachidonic acids are ω-6 fatty acids; α-linolenic acid is an ω-3 fatty acid.

• Cis vs. Trans Configuration:

  • Naturally occurring unsaturated fats are usually cis-configured, causing molecular kinks due to spatial differences in double bonds.

  • Trans fats are more linear, affecting metabolic pathways differently.

C) Location

• Fatty acids are integral to the structure of various lipids (e.g. waxes, eicosanoids, triglycerides).

• Rarely found free as individual fatty acids; oils that release free fatty acids often taste unpalatable.

Waxes

• Biological waxes are esters of fatty acids (usually C-16 to C-36) and fatty alcohols (C-24 to C-36).

• Waxes possess unique properties:

  • Solid at room temperature yet plastic (moldable).

  • Low melting points (usually above 45 °C).

  • Low viscosity when melted, allowing for easy flow.

• Use cases include:

  • Plants use waxes for moisture control.

  • Insects like bees use waxes for structural needs.

  • Animals secrete waxes for waterproofing and thermal regulation.

• Examples: carnauba wax, beeswax, lanolin (up to 30% by weight in unwashed sheep's wool).

Eicosanoids

• Eicosanoids are hormones that regulate various biological functions, derived from 20-C fatty acids (Eicosa means "20").

• Produced locally where they exert their effects (local hormones).

• Four families of eicosanoids to know:

  1. Prostaglandins

     • Involved in smooth muscle activation, pain and swelling during inflammation, and influencing various physiological responses.

  2. Prostacyclins

     • Prevent platelet aggregation and act as vasodilators, leading to lower blood pressure.

  3. Thromboxanes

     • Promote platelet aggregation and act as vasoconstrictors, raising blood pressure.

  4. Leukotrienes

     • Trigger responses associated with asthma, smooth muscle contraction, and severe allergic reactions.

• Each eicosanoid family has a unique structure built from a 20-C fatty acid, influencing how they function in the body.