Biomolecules & Enzymes

Living systems are composed of four major classes of biomolecules — carbohydrates, proteins, lipids, and nucleic acids — each with distinct structural features and biological roles. Enzymes, primarily proteinaceous catalysts, govern virtually every metabolic reaction with remarkable specificity.

Carbohydrates follow the empirical formula (CH₂O)n and are classified by complexity. Monosaccharides are the simplest sugars: glucose and fructose (C₆H₁₂O₆) are hexoses, while ribose and deoxyribose are pentoses critical to nucleic acid structure. Disaccharides form through glycosidic bonds — sucrose (glucose + fructose), lactose (galactose + glucose), and maltose (glucose + glucose). Polysaccharides serve as storage molecules (starch in plants via $\alpha$-linkages, glycogen in animals) or structural elements (cellulose in plant cell walls via $\beta$-linkages, chitin in fungal walls and arthropod exoskeletons).

Proteins are polymers of twenty different amino acids joined by peptide bonds (formed by dehydration synthesis between the carboxyl group of one amino acid and the amino group of another). Protein structure exists at four hierarchical levels: primary (amino acid sequence), secondary ($\alpha$-helix and $\beta$-pleated sheet stabilized by hydrogen bonds), tertiary (3D folding stabilized by disulphide bonds, hydrophobic interactions, and ionic bonds), and quaternary (multiple polypeptide subunits — e.g., haemoglobin with 4 subunits).

Lipids are hydrophobic molecules including triglycerides (glycerol + 3 fatty acids), phospholipids (structural basis of membranes with hydrophilic head and hydrophobic tail), and steroids (cholesterol, hormones). Saturated fatty acids lack double bonds (solid at room temperature), while unsaturated fatty acids contain one or more double bonds (liquid at room temperature).

Nucleic acids — DNA and RNA — store and transmit genetic information. DNA contains deoxyribose sugar with complementary base pairs adenine-thymine (A-T, two hydrogen bonds) and guanine-cytosine (G-C, three hydrogen bonds) in a double-stranded helix. RNA contains ribose sugar, uses uracil instead of thymine (A-U pairing), and is typically single-stranded.

Enzymes lower the activation energy of reactions without altering the equilibrium or the net free energy change (deltaG). The lock-and-key model proposes a rigid active site complementary to the substrate, whereas the induced fit model (now more accepted) suggests the active site reshapes upon substrate binding. Enzyme classification includes six classes: oxidoreductases (redox reactions), transferases (group transfer), hydrolases (hydrolysis), lyases (non-hydrolytic cleavage), isomerases (isomerization), and ligases (bond formation using ATP). Enzyme activity depends on temperature (optimum ~37 degrees C for human enzymes), pH (pepsin at pH 2, trypsin at pH 8), and substrate concentration (Michaelis-Menten kinetics). Competitive inhibitors bind the active site and can be overcome by excess substrate; non-competitive inhibitors bind allosteric sites and cannot be overcome. Cofactors include coenzymes (organic — NAD⁺, FAD), prosthetic groups (tightly bound — haem), and metal ions (Zn²⁺, Mn²⁺). An apoenzyme (protein part) plus its cofactor forms the functional holoenzyme. Notably, ribozymes are RNA molecules with catalytic activity, proving that not all enzymes are proteins.

The key testable concept is enzyme mechanism, classification, factors affecting enzyme activity, and the distinction between competitive and non-competitive inhibition.