Lipids: The Basics Notes

Lipids: The Basics Notes

Definition of Lipids

  • Traditional Definition: Substances that are:

  • Insoluble in water

  • Soluble in organic solvents (e.g., chloroform, ether, benzene)

  • Contain long-chain hydrocarbons

  • Present in or derived from living organisms (M. Kates)

  • General Definition: Includes fats and oils (triacylglycerols), waxes, phospholipids, steroids, terpenes. All share water insolubility and solubility in hydrocarbons and alcohols (D.A. Bender).

  • Modern Perspective: No universally accepted definition; typically described as naturally occurring compounds soluble in organic solvents. Includes diverse compounds like fatty acids, steroids, etc. (W.W. Christie & X. Han).

  • Lipidome: The complete collection of distinct lipid species in a biological system. Lipidomics is the field studying lipidomes at a large scale.

Categories of Lipids

  • Fatty Acyls (FA): e.g., hexadecanoic acid
  • Glycerolipids (GL): Glycerol + fatty acids
  • Glycerophospholipids (GP): Phospholipids derived from glycerol
  • Sphingolipids (SP): Lipids containing sphingosine
  • Sterol Lipids (ST): e.g., cholesterol
  • Prenol Lipids (PR): Isoprenoid compounds
  • Saccharolipids (SL): Contain sugar moieties
  • Polyketides (PK): Derived from polyketide synthases

Naming Fatty Acids

  • Linear Hydrocarbons: Classified by carbon number (alkanes) and include names from Methane (1C) to Triacontane (30C).
  • Saturated Fatty Acids: Names and shorthand from Methanoic (1:0) to Hexacosanoic (26:0).
  • Unsaturated Fatty Acids: Designated by their double bonds and relative positions (e.g., 18:3 n-3 for alpha-linolenic acid).
Common Fatty Acids

  • Saturated:

  • Palmitic Acid (C16:0)

  • Stearic Acid (C18:0)

  • Unsaturated:

  • Oleic (C18:1 ω-9)

  • Linoleic (C18:2 ω-6)

  • Arachidonic (C20:4 ω-6)

Structure of Acylglycerides

  • Contains glycerol backbone:

  • Monoacylglyceride (MAG): 1 fatty acid

  • Diacylglyceride (DAG): 2 fatty acids

  • Triacylglyceride (TAG): 3 fatty acids, 99% of fatty acids in plants and animals.

  • Chirality in acylglycerides: Key chiral carbons identified in the glycerol backbone will affect how lipids interact biologically.

Polar vs Neutral Lipids

  • Neutral Lipids: Primarily triacylglycerols (fats)
  • Polar Lipids: Phospholipids; amphipathic nature allows for forming cell membranes.
  • Examples:
  • Phospholipids contain hydrophilic and hydrophobic portions, crucial for membrane structure.
  • Lecithin is a natural emulsifier found in soy oil, consists mainly of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine.

Nutrition and Dietary Fats

  • Sources: Fat appears in various foods, categorized as:

  • Mixed Dishes: 31%

  • Meat: 29%

  • Dairy: 16%

  • Grains, Vegetables, Fruits: Smaller fractions

  • Fat Content Variability: Foods present a wide range of fat percentages.

Summary Points

  • Lipids are primarily hydrophobic and insoluble in water.
  • They are complex but not polymeric; consist mainly of fatty acids and derivatives.
  • Naming systems based on placement and type of double bonds (ω and Δ).
  • Some lipids (like phospholipids) are amphipathic, containing both hydrophobic and hydrophilic components.