Biomolecules

Introduction & D/L Configuration

Carbohydrates are optically active polyhydroxy aldehydes or ketones. They are also called Saccharides.

• D and L Nomenclature: It is a relative configuration based on Glyceraldehyde. If the -OH group on the lowest chiral carbon (bottom-most chiral center in Fischer projection) is on the Right side, it is D-sugar. If it is on the Left side, it is L-sugar.
• Note: D/L has NO relation with optical rotation (+ or dextrorotatory, - or laevorotatory). Naturally occurring glucose is D(+)-Glucose.

Monosaccharides (Glucose & Fructose)

• Glucose (Aldohexose): Monomer of starch, cellulose. Prepared industrially by acid hydrolysis of starch. Known as blood sugar or dextrose.
• Fructose (Ketohexose): Found in fruits, honey. It is an important ketohexose. It is D(-)-Fructose (Laevorotatory).
• Epimers: Carbohydrates that differ in configuration at exactly one chiral center (e.g., Glucose and Galactose are C-4 epimers).

Cyclic Structure & Mutarotation

• Haworth Projections: Cyclic structures represented as Pyranose (6-membered, 5 Carbon + 1 Oxygen) for Glucose, and Furanose (5-membered, 4 Carbon + 1 Oxygen) for Fructose.
• Anomers: Isomers that differ ONLY in the configuration of the hydroxyl group at C-1 (the anomeric carbon). Known as α-form and β-form.
• Mutarotation: Spontaneous change in the specific optical rotation of an optically active compound in aqueous solution until equilibrium is reached. (e.g., α-D-Glucose +111° changes to equilibrium value +52.5°).

Disaccharides & Glycosidic Linkage

Two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule. This linkage is called a Glycosidic Linkage.

• Sucrose (Invert Sugar): Formed by α-D-Glucose (C1) + β-D-Fructose (C2). Non-reducing sugar. Hydrolysis converts dextrorotatory sucrose to laevorotatory mixture.
• Maltose: Formed by two α-D-Glucose units (C1-C4 linkage). Reducing sugar.
• Lactose (Milk Sugar): Formed by β-D-Galactose (C1) + β-D-Glucose (C4). Reducing sugar.

Polysaccharides

• Starch: Main storage in plants. Mixture of:
  - Amylose (15-20%): Water soluble, straight chain (C1-C4 α-glycosidic).
  - Amylopectin (80-85%): Insoluble, branched chain (C1-C4 and branching at C1-C6).
• Cellulose: Structural material of plant cell walls. Straight chain polymer of β-D-Glucose (C1-C4 linkage). Human body cannot digest it due to lack of cellulase enzyme.
• Glycogen: Animal starch stored in liver, muscles, brain. Highly branched (similar to amylopectin).

Amino Acids & Zwitterion Mechanism

Proteins are polymers of α-amino acids connected by Peptide Bonds (-CO-NH-).

• Essential Amino Acids: Body cannot make them, required through diet (e.g., Valine, Leucine, Isoleucine).
• Non-Essential Amino Acids: Body can synthesize them (e.g., Glycine, Alanine).
• Zwitterion: In aqueous solution, the -COOH group loses a proton and -NH2 group accepts a proton, forming a dipolar ion (Zwitterion). This makes them amphoteric in nature.
• Isoelectric Point (pI): The pH at which the amino acid zwitterion does not migrate in an electric field.

Structure of Proteins

• Primary: Exact sequence of amino acids. Any change creates a different protein (e.g., Sickle cell anemia is caused by a single amino acid change).
• Secondary: Folding of polypeptide backbone.
  - α-Helix: Coiled like a spring, stabilized by H-bonds between >C=O and -NH groups.
  - β-Pleated sheet: Chains stretched out and laid side-by-side, held by intermolecular H-bonds.
• Tertiary: Overall 3D folding (Fibrous & Globular). Stabilized by Hydrogen bonds, Disulfide linkages (-S-S-), van der Waals forces, and electrostatic forces.
• Quaternary: Arrangement of two or more polypeptide sub-units (e.g., Hemoglobin has 4 subunits).

Classification & Denaturation

• Fibrous Proteins: Thread-like, insoluble in water. Provide structure (e.g., Keratin in hair, Myosin in muscles).
• Globular Proteins: Spherical shape, soluble in water. (e.g., Insulin, Albumin).
• Denaturation: When protein is subjected to physical change (heat) or chemical change (pH), H-bonds are disturbed. 2° and 3° structures are destroyed but 1° structure remains intact. Uncoiling occurs. (Example: Coagulation of egg white on boiling, curdling of milk).

Enzymes (Biocatalysts)

Almost all enzymes are globular proteins. They are highly specific and work optimally at physiological pH (7.4) and body temperature (37°C).

• Mechanism: Follows 'Lock and Key' or 'Induced Fit' model. The substrate fits into the active site of the enzyme forming an E-S complex, which reduces Activation Energy.
• Coenzymes & Prosthetic Groups: Some enzymes require a non-protein part to function. If organic and loosely bound, it's a coenzyme (like Vitamins). If tightly bound, it's a prosthetic group.
• Examples: Zymase (Glucose to Ethanol), Maltase (Maltose to Glucose), Pepsin (Proteins to Peptides in stomach).

Vitamins (Complete Chart)

Structure of Nucleic Acids

Nucleic acids are polymers of nucleotides, responsible for heredity and protein synthesis.

• Nucleoside: Base + Pentose Sugar (Linked at 1' position via N-glycosidic linkage).
• Nucleotide: Base + Pentose Sugar + Phosphate Group (Linked at 5' OH of sugar).
• Phosphodiester Linkage: Nucleotides are joined together by linking the 5' phosphate of one nucleotide to the 3' hydroxyl of the next, forming the sugar-phosphate backbone.

Deoxyribonucleic Acid (DNA)

• Sugar: β-D-2-deoxyribose (Lacks oxygen at 2' Carbon).
• Nitrogenous Bases:
  - Purines: Adenine (A), Guanine (G)
  - Pyrimidines: Cytosine (C), Thymine (T)
• Double Helix Model (Watson & Crick): Two anti-parallel strands held together by Hydrogen bonds.
• Base Pairing Rule:
  - Adenine pairs with Thymine (A = T) via 2 H-bonds.
  - Guanine pairs with Cytosine (G ≡ C) via 3 H-bonds.
• Chargaff's Rule: Amount of Purines always equals amount of Pyrimidines (A+G = T+C).

Ribonucleic Acid (RNA) & Hormones

• Sugar: β-D-ribose.
• Nitrogenous Bases: Contains Uracil (U) instead of Thymine. (A, G, C, U).
• Structure: Usually single-stranded, but can fold onto itself.
• Types of RNA:
  - mRNA (Messenger): Carries genetic info from DNA.
  - tRNA (Transfer): Transports amino acids to ribosomes.
  - rRNA (Ribosomal): Structural component of ribosomes.