Lipids and Their Biological Functions

Lecture Overview

• Topics Covered:

  • Biological functions of lipids

  • Sterol and derivatives

  • Lipidic hormones

  • Lipidic vitamins

  • Synthesis of lipids

Definition and Classification of Lipids

• Lipids: Refers to a collection of organic molecules of varying chemical composition, including:

  • Fatty Acids:

  • Saturated

  • Unsaturated

  • Glycerides: Lipids that contain glycerol.

  • Nonglyceride Lipids:

  • Sphingolipids

  • Steroids

  • Waxes

  • Complex Lipids: Such as lipoproteins.

  • Sterol Derivatives: Varieties of sterols which serve as precursors to other compounds.

Biological Functions of Lipids

• Energy Source: When oxidized, each gram of fat releases 9 kcal of energy, which is more than twice the energy released from a gram of carbohydrates.

• Energy Storage: Store energy in the form of triglycerides (TAG) within adipocytes (fat cells).

• Cell Membrane Structural Components:

  • Phosphoglycerides: Major component of cellular membranes.

  • Sphingolipids: Important for membrane integrity.

  • Steroids: Contribute to fluidity and signaling within the membrane.

Functions of Lipids Continued

• Hormones: Lipids include eicosanoids and steroid hormones.

• Vitamins:

  • Lipid-soluble vitamins include A, D, E, and K.

  • Vitamin Absorption: Dietary fats function to carry these lipid-soluble vitamins.

• Protection: Fats act as shock absorbers for organs.

• Insulation: Subcutaneous fat insulates the body.

Eicosanoids

• Definition: Eicosanoids can act as paracrine signals, meaning they send signals nearby to other cells.

• Eicosanoids Characteristics:

  • Derived From Arachidonic Acid: A 20-carbon fatty acid.

  • Role as Drug Targets: The derivatives can be targets for pharmacological agents.

Eicosanoid Terminology

• Eíkosi: Greek for “twenty.”

• Para: Greek for “nearby.”

• Ekkrino: Greek for “to secrete.”

• Significant Message: Eicosanoids are molecules that secrete signals in a localized area.

Importance of Root Words in Biology

• Tip: Learning root words can facilitate understanding of complex scientific terms through combinations rather than memorizing entire definitions.

Sterols and Derivatives

• Sterol: Basis for various lipids, characterized by having a polar head and hydrophobic tail.

• Properties of Sterols:

  • Amphipathic: Contains both polar (hydrophilic) and nonpolar (hydrophobic) parts.

  • Planar (flat) structure, no rotation around carbon bonds.

Cholesterol

• Sterol Derivative: Cholesterol serves as an important component in biological membranes.

• Molecular Structure: Characteristic structure detailed with carbon and hydrogen arrangements, displaying its polarity and hydrophobic characteristics.

Bile Salts

• Function: Emulsification of dietary lipids, thereby allowing for their solubilization and absorption.

• Molecular Behavior: Behave similarly to detergents (caution advised against misunderstanding).

Steroid Hormones

• Source: Derived from sterols.

• Function: Serve as cellular messengers which relay information throughout bodily systems.

• Connection to Drugs: Many pharmaceutical agents mimic steroid structures (steroid analogues).

Types of Steroid Hormones

• Example Hormones:

  • Testosterone: Influences male sexual development through various hormonal pathways involving the hypothalamus and pituitary gland.

  • Estradiol: Key female sex hormone involved in the menstrual cycle.

  • Progesterone: Plays role in the female fertility cycle, particularly during the luteal phase.

  • Corticosteroids: Includes cortisol and aldosterone, involved in stress response and regulation of fluids and electrolytes.

Steroid Drugs

• Synthetic Analogs: Utilized for treatment of inflammatory and autoimmune disorders.

Vitamins

• Types: Categorized as either water-soluble or fat-soluble.

  • Water-soluble Vitamins:

  • B vitamin class (1, 2, 3, 5, 6, 7, 9, 12)

  • Vitamin C

  • Fat-soluble Vitamins:

  • A, D, E, K

Vitamin A (Retinol)

• Function: Critical for vision and skin cell health; converts from oxidation of aldehyde to acid (retinoic acid), serving as a hormonal signal to epithelial cells.

• Dietary Sources: Encouraged to consume carrots for health benefits.

Historical Context of Vitamin A

• During WWII: Misinformation promoted by the British Ministry about carrots improving night vision due to military tactics surrounding radar interception.

Vitamin D

• Synthesis: Produced from 7-dehydrocholesterol and modified to 1,25-dihydroxycholecalciferol.

• Function: Essential for calcium metabolism and bone health.

Vitamin K

• Use in Medicine: Acts as an antidote for rat poisons and is necessary for countering anticoagulant overdoses (e.g., warfarin).

Lipid Synthesis

• Building Blocks:

  • Acetate is converted into Acetyl-Coenzyme A (Acetyl-CoA).

  • Fatty acids and sterols are synthesized from Acetyl-CoA.

Malonyl-CoA

• Role: Malonyl-CoA is an intermediate in the fatty acid synthesis process.

Assembly of Fatty Acids

• Process: Fatty acids are constructed two carbons at a time using Acetyl-CoA as the starting point.

Palmitate

• Importance:

  • Precursor of fatty acids

  • Synthesized by Fatty Acid Synthase

  • Longer chain fatty acids are generated through fatty acid elongation systems.

Smooth Endoplasmic Reticulum (ER)

• Function:

  • Site of synthesis for longer-chain fatty acids.

  • Involved in phospholipid and cholesterol synthesis.

  • Prokaryotic cells lack an endoplasmic reticulum.

Making Sterols

• Synthesis Pathway: Sterols are synthesized from acetate, with isoprene as an intermediate.

Final Recap of Key Concepts

• Lipids function as signaling molecules.

• Eicosanoids and sterol derivatives are pivotal classes of lipid hormones.

• Vitamins can exhibit hydrophilic or hydrophobic properties.

• Lipid hormones, whether natural or synthetic, can be applied in pharmaceutical contexts.

• Overall, lipids are constructed from smaller substrates (Acetate and Acetyl-CoA) with Palmitate acting as a fatty acid precursor and Isoprene as a sterol precursor.