Unit 3: Lipids and Nucleic Acids

1. Lipids: Classification and Properties

Lipids are a diverse group of biological molecules defined by their hydrophobicity (insoluble in water) and solubility in nonpolar organic solvents (e.g., ether, chloroform).

Classification of Lipids

1. Simple Lipids: Esters of fatty acids with various alcohols.
   • Fats & Oils (Triacylglycerols): Esters of fatty acids with glycerol.
2. Complex Lipids: Esters of fatty acids containing groups in addition to an alcohol and fatty acids.
   • Phospholipids: Contain a phosphate group.
   • Glycolipids: Contain a carbohydrate.
3. Derived Lipids: Substances derived from the hydrolysis of simple and complex lipids (e.g., fatty acids, steroids, fat-soluble vitamins).

2. Fatty Acids: Classification and Properties

Fatty acids are the simplest lipids. They are long-chain carboxylic acids (—COOH head) with a long hydrocarbon tail.

a) Saturated Fatty Acids

• Structure: Contain no carbon-carbon double bonds (C=C).
• Properties: The chains are straight, allowing them to pack tightly. This results in a higher melting point (solid at room temp).
• Example: Palmitic acid (16C), Stearic acid (18C).

b) Unsaturated Fatty Acids

• Structure: Contain one or more C=C double bonds.
  • Monounsaturated: One C=C bond (e.g., Oleic acid).
  • Polyunsaturated (PUFA): Two or more C=C bonds (e.g., Linoleic acid).
• Properties: The cis-double bonds create "kinks" or "bends" in the chain. This prevents tight packing, resulting in a lower melting point (liquid at room temp - "oils").

3. Essential Fatty Acids

Essential Fatty Acids (EFAs) are polyunsaturated fatty acids that the human body cannot synthesize and must be obtained from the diet.

The two primary EFAs are:

1. Linoleic Acid (Omega-6): Found in vegetable oils, nuts.
2. α-Linolenic Acid (Omega-3): Found in flaxseeds, walnuts, fish oils.

They are precursors for important signaling molecules like prostaglandins.

4. Phospholipids

Phospholipids are the main component of all cell membranes.

Structure

They are amphipathic, meaning they have both hydrophilic and hydrophobic parts.

• Hydrophilic "Head": A phosphate group linked to a polar alcohol (e.g., choline, serine).
• Hydrophobic "Tail": Two fatty acid chains.

This structure causes them to spontaneously form a lipid bilayer in water, which is the basis of the cell membrane.

5. Glycolipids

Glycolipids are lipids with a carbohydrate attached by a glycosidic bond. They are also amphipathic and found in cell membranes, particularly on the outer surface.

Function

• Cell Recognition: They act as surface markers for cell-cell identification.
• Blood Groups: The ABO blood group antigens are examples of glycolipids on red blood cells.

6. Steroids

Steroids are derived lipids characterized by a specific four-ring carbon structure called the steroid nucleus.

Examples and Functions

• Cholesterol: A vital component of animal cell membranes, affecting fluidity. It is also the precursor for all other steroids.
• Steroid Hormones:
  • Glucocorticoids (e.g., Cortisol): Regulate metabolism and inflammation.
  • Sex Hormones (e.g., Testosterone, Estrogen): Regulate secondary sexual characteristics.
• Bile Acids: Help in the digestion and absorption of fats in the intestine.

7. Nucleic Acids: Nucleosides and Nucleotides

Nucleic acids (DNA and RNA) are polymers of nucleotides. They store and transmit genetic information.

A nucleotide has three components:

1. A Pentose Sugar: Deoxyribose (in DNA) or Ribose (in RNA).
2. A Nitrogenous Base: A purine or pyrimidine.
3. A Phosphate Group: One or more phosphate groups.

Nucleoside = Sugar + Base
Nucleotide = Sugar + Base + Phosphate

8. Purines and Pyrimidines

The nitrogenous bases are the "letters" of the genetic code.

• Purines (Two rings):
  • Adenine (A)
  • Guanine (G)
• Pyrimidines (One ring):
  • Cytosine (C)
  • Thymine (T) (Found only in DNA)
  • Uracil (U) (Found only in RNA)

9. Physical and Chemical Properties of Nucleic Acids

a) Chemical Properties

• Phosphodiester Bonds: Nucleotides in a strand are linked by phosphodiester bonds, which connect the 5' carbon of one sugar to the 3' carbon of the next. This creates the sugar-phosphate backbone.
• Hydrogen Bonds: In DNA, the two strands are held together by hydrogen bonds between the bases.

b) Physical Properties

• UV Absorption: The nitrogenous bases strongly absorb UV light at a wavelength of 260 nm. This property is used to measure nucleic acid concentration.
• Denaturation (Melting): The two strands of DNA can be separated by heat or chemicals. The temperature at which 50% of the DNA is denatured is called the melting temperature (T_m).
  • G-C pairs (3 H-bonds) are stronger than A-T pairs (2 H-bonds). Therefore, DNA with a higher G-C content has a higher T_m.
• Renaturation (Annealing): Separated DNA strands can re-associate and re-form the double helix when cooled.

10. Double Helical Model of DNA (Watson-Crick Model)

In 1953, James Watson and Francis Crick proposed the 3D structure of DNA. Its key features are:

1. It consists of two polypeptide chains coiled around a common axis, forming a right-handed double helix.
2. The two strands are anti-parallel (one runs 5' → 3', the other runs 3' → 5').
3. The sugar-phosphate backbone is on the outside of the helix.
4. The nitrogenous bases are stacked on the inside, perpendicular to the helix axis.
5. The strands are held together by hydrogen bonds according to specific base-pairing rules (Chargaff's Rules):
   • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
   • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.
6. The helix has a major groove and a minor groove on its surface, which are important for protein-DNA interactions.

11. Types of DNA

While the Watson-Crick model describes B-DNA, other conformations exist.