Biomolecules — Notes

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give these on hydrolysis.

• Monosaccharides (no further hydrolysis): glucose, fructose, ribose. Named by carbonyl type and carbon count — glucose is an aldohexose, fructose a ketohexose.
• Oligosaccharides (2–10 units): sucrose, maltose, lactose. Polysaccharides: starch, cellulose, glycogen.
• Reducing sugars reduce Tollens’ and Fehling’s reagents (free aldehyde or ketone group available): all monosaccharides, maltose, lactose. Sucrose is non-reducing.

Preparing glucose: boil sucrose with dilute acid, $\ce{C12H22O11 + H2O ->[H+] C6H12O6 + C6H12O6}$ (glucose + fructose); commercially, hydrolyse starch with dilute H₂SO₄ at 393 K under pressure.

Open-chain structure — evidence: HI (prolonged heating) → n-hexane (straight chain of six C); NH₂OH → oxime and HCN → cyanohydrin (C=O); bromine water → gluconic acid (the C=O is an aldehyde); ethanoic anhydride → pentaacetate (five –OH); HNO₃ → saccharic acid (primary –OH at C6). The –OH on C5 is on the right: D-configuration; (+) means dextrorotatory.

The open chain does not explain why glucose gives no Schiff’s test or NaHSO₃ adduct, or why the pentaacetate does not react with NH₂OH. Glucose is mainly a six-membered ring (pyranose): the C5 –OH adds to the C1 aldehyde (hemiacetal). The two ring forms, α (m.p. 419 K) and β (m.p. 423 K), differ only at C1 — they are anomers. In water each slowly changes to an equilibrium mixture (mutarotation). Fructose (ketohexose, laevorotatory) forms a five-membered furanose ring.

Glycosidic linkage: two monosaccharides join with loss of water.

• Sucrose: α-D-glucose C1 to β-D-fructose C2 — both reducing carbons used, so non-reducing. Hydrolysis changes the rotation from + to − (invert sugar).
• Maltose: two α-D-glucose, C1–C4 — reducing. Lactose (milk sugar): β-D-galactose C1 to glucose C4 — reducing.

Polysaccharides: starch = amylose (15–20%, unbranched α-1,4, water-soluble, blue with iodine) + amylopectin (80–85%, α-1,4 chains branched by α-1,6 links); cellulose = unbranched β-1,4 chains of β-glucose (plant cell walls; humans cannot digest it); glycogen = highly branched, animal store in liver and muscle. Each glucose unit in a chain has mass $180 - 18 = 162$.

Importance: energy source and store (starch, glycogen), structure (cellulose in wood and cotton), raw material for paper, textiles and ethanol; ribose and deoxyribose in nucleic acids.

α-Amino acids carry –NH₂ and –COOH on the same carbon, with a side chain R; all except glycine are optically active (L-form in proteins). Essential amino acids (e.g. valine, leucine, lysine) must come from food. In water they exist as zwitterions, $\ce{H3N+-CHR-COO-}$: amphoteric, crystalline, water-soluble, high-melting.

Peptide bond: –COOH of one amino acid + –NH₂ of the next → –CO–NH– + H₂O. A chain of $n$ amino acids has $n - 1$ peptide bonds and is formed with loss of $n - 1$ water molecules. More than about 100 residues: a protein.

Structure levels: primary (sequence; covalent peptide bonds), secondary (α-helix or β-pleated sheet; H-bonds between C=O and N–H), tertiary (overall 3D fold; H-bonds, disulfide links, ionic and van der Waals forces), quaternary (arrangement of subunits, e.g. haemoglobin).

Fibrous proteins (keratin, myosin): parallel chains, insoluble. Globular proteins (insulin, albumin): compact, soluble.

Denaturation: heat, pH change or some chemicals disrupt H-bonds and other weak forces, uncoiling secondary and tertiary structure; biological activity is lost but the primary structure stays. Examples: boiling an egg, curdling milk.

Enzymes are biocatalysts, almost all globular proteins. They are highly specific (one substrate or reaction), work at body temperature and near-neutral pH, and lower the activation energy. Examples: invertase (sucrose → glucose + fructose), maltase (maltose → glucose), amylase (starch → maltose), pepsin and trypsin (proteins → peptides), urease (urea → NH₃ + CO₂).

Vitamins are organic compounds needed in small amounts for normal growth and health.

• Fat-soluble: A, D, E, K — stored in liver and adipose tissue.
• Water-soluble: B group and C — excreted in urine, so must be supplied regularly (except B₁₂, stored in the liver). Vitamin C is destroyed by heat.

Nucleic acids are polymers of nucleotides. Nucleoside = base + pentose sugar; nucleotide = nucleoside + phosphate (at C5′). Nucleotides are joined by phosphodiester links (C5′ of one sugar to C3′ of the next).

• DNA: 2-deoxy-D-ribose; bases A, G, C, T; double helix of two antiparallel complementary strands; A pairs with T (two H-bonds), G with C (three H-bonds), so %A = %T and %G = %C.
• RNA: D-ribose; bases A, G, C, U; usually single-stranded; three types — mRNA (message), tRNA (carries amino acids), rRNA (in ribosomes).

Functions: DNA stores hereditary information and replicates; RNA directs protein synthesis (transcription to mRNA, translation at ribosomes).

Hormones are chemical messengers from endocrine glands, carried in blood: steroids (oestrogen, testosterone, glucocorticoids), polypeptides (insulin, glucagon), amino acid derivatives (adrenaline, thyroxine). Insulin lowers and glucagon raises blood glucose; thyroxine controls metabolic rate (iodine deficiency → goitre, hypothyroidism); adrenaline drives the fight-or-flight response; sex hormones control sexual development.