Macromolecules: MDCAT Notes and MCQs

Proteins — classification

• Polymers of α-amino acids linked by peptide (amide) bonds — condensation polymers. ≈ 20 amino acids; essential amino acids (≈ 8–10, e.g. valine, leucine, lysine, phenylalanine, tryptophan) must come from diet.
• Amino acids exist as zwitterions (dipolar ions) — amphoteric. Glycine is the only optically inactive amino acid.
• By composition: simple (only amino acids — albumin, globulin), conjugated (protein + prosthetic group — haemoglobin, glycoproteins, lipoproteins, nucleoproteins), derived (from hydrolysis — peptones).
• By shape: fibrous — long, insoluble, structural (keratin of hair/nails, collagen of tendons, myosin, fibroin of silk); globular — compact, water-soluble, functional (enzymes, haemoglobin, insulin, antibodies, albumin).
• By function: structural, enzymatic, transport, hormonal, defensive, contractile, storage.

Structure → function

Primary (sequence) determines folding into secondary (α-helix, β-sheet), tertiary and quaternary structures. Shape determines function — a single amino-acid change (glutamic acid → valine in β-chain of Hb) causes sickle-cell anaemia. Denaturation (heat, pH, heavy metals, alcohol) destroys 2°/3° structure (not peptide bonds) → loss of function (e.g. boiling an egg).

Biological & nutritional importance

• Haemoglobin (O₂ transport), insulin (glucose regulation), antibodies (defence), actin/myosin (contraction), collagen (connective tissue), fibrinogen (clotting), albumin (osmotic pressure), enzymes.
• Dietary protein supplies amino acids for growth and repair; deficiency → kwashiorkor (protein) and marasmus (protein + calorie). Daily need ≈ 0.8–1 g per kg body mass. Sources: meat, eggs, milk, pulses.

Enzymes as biocatalysts

• Mostly globular proteins; highly specific, efficient at body temperature and near-neutral pH; work in minute amounts; lower activation energy; regulated by inhibitors/activators.
• Industrial uses: zymase (fermentation of glucose to ethanol), invertase (sucrose → glucose + fructose), diastase (starch → maltose), urease (urea → NH₃ + CO₂), proteases in detergents, rennin in cheese making.