Functional Biochemistry II Lecture 1 - Lipids Notes

Lipids

Overview

Lipids are a broad group of organic compounds including fats, waxes, sterols, fat-soluble vitamins (A, D, E, and K), glycerides (monoglycerides, diglycerides, triglycerides), phospholipids, and cholesterol. They are formed from alcohol and fatty acids combined by an ester linkage.

Classification of Lipids Based on Function

  1. Energy-storage lipids: Triacylglycerols
  2. Membrane lipids: Phospholipids, sphingoglycolipids, and cholesterol
  3. Emulsification lipids: Bile acids
  4. Messenger lipids: Steroid hormones and eicosanoids
  5. Protective-coating lipids: Biological waxes

Lipids Functions

  • Storing energy
  • Signaling
  • Acting as structural components of cell membranes
  • Used in cosmetic and food industries

Cholesterol and Lipoproteins

Cholesterol aids in the absorption of fats and vitamins and the production of hormones. Because cholesterol and triglycerides are hydrophobic, they combine with proteins to form lipoproteins for transport in the blood.

Hydrophobic and Amphiphilic Nature of Lipids

Lipids can be either hydrophobic (nonpolar) or amphiphilic (e.g., phospholipids).

Lipoproteins: HDL, VLDL, and LDL

  • HDL (High-Density Lipoprotein): Produced by the liver and small intestine, HDL transports cholesterol to the liver for conversion into bile acid for excretion. It also combats inflammation, blood clots, and oxidation; often referred to as "good" cholesterol.
  • VLDL (Very Low-Density Lipoprotein): Synthesized by the liver, VLDL carries triglycerides from the liver to other cells. After VLDLs deliver triglycerides and cholesterol, they become denser and transform into LDL.
  • LDL (Low-Density Lipoprotein): Known as the "bad" cholesterol, LDL can accumulate on artery walls, impeding blood flow.

Hydrophilic Groups

  • Anionic groups:
    • Carboxylates: RCO2−RCO_2^−
    • Sulfates: RSO4−RSO_4^−
    • Sulfonates: RSO3−RSO_3^−
    • Phosphates (in phospholipids)
  • Cationic groups:
    • Ammoniums: RNH3+RNH_3^+

Polar, Uncharged Groups

Examples include alcohols with large R groups, such as diacyl glycerol (DAG), and oligo ethylene glycol with long alkyl chains.

Amphiphilic Molecules

  • Defensins, pepsin, and lysozyme in the GIT
  • Lysozyme in tears
  • Lysozyme in serum and tears breaks down bacterial cell walls (peptidoglycan).
  • Interferon induces an antiviral state in uninfected cells and activates other cells to kill pathogens.
  • Integrins are surface proteins that promote cell-to-cell interaction in inflammation and immune response.

Lipid Building Blocks and Categories

Biological lipids are derived from ketoacyl and isoprene groups and are divided into eight categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and polyketides.

Triglyceride Formation

Glycerol + 3 Fatty acids →\rightarrow Triglyceride + 3H2O

Lipid-Related Disorders

The production of abnormal amounts of lipids can lead to:

  • Hyperlipidemia (high lipid levels)
  • Hypolipidemia (low lipid levels)
  • Hypercholesterolemia
  • Atherosclerosis
  • Hypertriglyceridemia
  • Coronary artery disease

Compound Lipids: Phospholipids (Glycerophospholipids)

Phospholipids are simple lipids with other compounds attached that contain phosphoric acid. They feature:

  • One hydrophilic "head" containing a phosphate group (1st Carbon)
  • Two hydrophobic "tails" derived from fatty acids (2nd and 3rd carbons)
  • Hydrophobic "tails" joined by an alcohol residue (usually a glycerol molecule).

Marine phospholipids typically have omega-3 fatty acids EPA and DHA.

Structure and Function of Phospholipids

Phospholipids are essential for cell membranes, forming lipid bilayers due to their amphiphilic nature. In eukaryotes, cell membranes also contain sterols interspersed among phospholipids, providing fluidity and mechanical strength.

Glycolipids

Glycolipids are lipids with a carbohydrate attached. They:

  • Maintain cell membrane stability
  • Facilitate cellular recognition (important in immune response)
  • Enable cell connections to form tissues

Glycolipids are found on the surface of eukaryotic cell membranes, extending from the phospholipid bilayer into the extracellular environment.

Types of Glycolipids

  • Cerebrosides: Found mainly in the brain and peripheral nervous tissue, acting as insulators and providing a protective coating to nerve cells.
  • Gangliosides: Neutral compounds composed of sialic acid residues, found abundantly in the plasma membrane of neurons and act as main components of animal cell membranes.

Functions of Glycolipids

  • Providing energy to cells
  • Serving as essential components of cell membranes
  • Determining blood group of an individual
  • Acting as receptors on red blood cells' surfaces
  • Acting as antigens or PAMPs in pathogens

Sterols

Sterols are structural lipids present in the membranes of most eukaryotic cells. They are forms of steroids with a hydroxyl group at position three (e.g., Cholesterol).

Lab Analysis of Lipids

Blood samples are analyzed for:

  • Total Cholesterol
  • Triglycerides
  • HDL (high-density lipoproteins)
  • Calculated LDL (low-density lipoproteins)
  • Coronary Risk