Biomolecules

Glucose (C₆H₁₂O₆) is the most important monosaccharide. Two forms matter for NEET/JEE.

Open-chain (Fischer projection): glucose is an aldohexose — 6-carbon sugar with an aldehyde group at C1 and 4 chiral centers (C2-C5).

• C1: CHO (aldehyde)
• C2-C5: each has OH and H
• C6: CH₂OH

D-glucose has OH on C5 to the right (Fischer convention). This is the form found in nature.

Cyclic form (Haworth projection): in solution, the C1 aldehyde reacts with C5 OH to form a 6-membered hemiacetal ring (pyranose form). This is the predominant form (~99%).

When C1 closes, it becomes a new chiral center → two anomers:

• α-glucose: OH at C1 points DOWN (in Haworth, axial). Less stable due to anomeric effect, but more present (~36%).
• β-glucose: OH at C1 points UP. More stable. ~64% at equilibrium.

Mutarotation. When pure α or β-glucose is dissolved in water, the specific rotation changes over time until it reaches the equilibrium mix (~+52.5°). Both forms interconvert via the open-chain.

Polysaccharides from glucose:

Disaccharides to know:

Reducing vs non-reducing: if the anomeric C is free (or can open to aldehyde), the sugar reduces Tollens' and Fehling's reagents. Sucrose has both anomeric carbons committed to the glycosidic bond → can't open → non-reducing.

Glucose tests:

• Fehling's and Tollens': positive (reducing sugar).
• Bromine water: oxidizes C1 to gluconic acid.
• HNO₃: oxidizes both C1 and C6 to saccharic (glucaric) acid.
• HI: reduces to n-hexane → confirms straight 6-carbon chain.

Amino acids are the building blocks of proteins. General structure: H₂N-CHR-COOH (R = side chain that defines the amino acid).

20 standard amino acids in human proteins. Classified by R group:

• Non-polar (hydrophobic): Gly, Ala, Val, Leu, Ile, Met, Phe, Trp, Pro.
• Polar uncharged: Ser, Thr, Cys, Tyr, Asn, Gln.
• Acidic (negatively charged at pH 7): Asp, Glu.
• Basic (positively charged at pH 7): Lys, Arg, His.

Essential amino acids (must come from diet, body can't synthesize): Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.

Zwitterion form of amino acid: at physiological pH, the COOH is deprotonated (COO⁻) and NH₂ is protonated (NH₃⁺) → internally neutral but with both charges.

Isoelectric point (pI): pH at which the amino acid has zero net charge. For neutral amino acids, pI ≈ 6. For acidic, pI is low (3); for basic, pI is high (10).

PEPTIDE BOND

Formed between the COOH of one amino acid and NH₂ of another, with release of water:

H₂N-CHR-COOH + H₂N-CHR'-COOH → H₂N-CHR-CO-NH-CHR'-COOH + H₂O

The C-N bond formed is the peptide bond. It has partial double-bond character due to resonance → planar and rigid.

• Dipeptide: 2 amino acids, 1 peptide bond.
• Polypeptide: chain of many.
• Protein: functional polypeptide(s), usually >50 amino acids.

Four levels of protein structure:

1. Primary structure. The linear sequence of amino acids. Determined by the gene (codons). Held by covalent peptide bonds.

2. Secondary structure. Local folding patterns held by H-bonds between backbone atoms (specifically between C=O of one residue and N-H of another).

• α-helix: right-handed coil. H-bond every 4th residue.
• β-sheet: parallel or antiparallel strands. H-bonds perpendicular to backbone direction.
• Random coil: loops, turns.

3. Tertiary structure. Overall 3D folding of the whole polypeptide. Stabilized by:

• Hydrophobic interactions (R-groups cluster inside, away from water).
• H-bonds.
• Disulfide bridges between cysteine residues (-S-S-).
• Salt bridges between charged R-groups.

4. Quaternary structure. Assembly of multiple polypeptide subunits. Example: hemoglobin has 4 subunits (2α + 2β). Insulin has 2 chains (A + B) joined by disulfide bridges.

Denaturation: loss of native structure (tertiary, sometimes secondary) without breaking peptide bonds. Caused by:

• Heat (cooking egg whites — albumin denatures).
• pH extremes (curdling milk with vinegar).
• Heavy metal salts (HgCl₂ binds to -SH groups).
• Detergents.
• Urea (disrupts H-bonds).

Most denaturation is irreversible. Primary structure is preserved; tertiary/quaternary destroyed.

Enzymes are proteins (mostly) that catalyze biological reactions. Highly specific (lock and key model, or induced fit). Active site binds substrate.

Conjugated proteins = protein + non-protein part (prosthetic group). Examples:

• Hemoglobin: globin + heme (with Fe²⁺).
• Glycoproteins: protein + carbohydrate.
• Lipoproteins: protein + lipid.

Fibrous vs globular:

• Fibrous: structural, water-insoluble. Keratin (hair, nails), collagen (skin, bones).
• Globular: roughly spherical, water-soluble, functional. Hemoglobin, enzymes, antibodies.