Biomolecules and Enzymes
Biomolecules and Enzymes tests the chemistry of life: monomers and polymers, confirmatory tests, protein structure, and the kinetics and control of enzyme action.
Biomolecule rules
- Carbohydrates: glucose and fructose are monosaccharides; sucrose (glucose + fructose), lactose and maltose are disaccharides; starch, glycogen and cellulose are polysaccharides. Cellulose is β-1,4-linked and indigestible to humans.
- Proteins are polymers of amino acids joined by peptide bonds, with primary, secondary (α-helix, β-pleated sheet), tertiary and quaternary structure; haemoglobin is quaternary.
- Lipids are glycerol plus fatty acids; phospholipids are amphipathic. Nucleic acids are nucleotide polymers: DNA has deoxyribose and thymine, RNA has ribose and uracil.
- Tests: Benedict's for reducing sugars (brick-red), iodine for starch (blue-black), Biuret for peptide bonds (violet), Sudan III for lipids, Millon's for proteins.
Enzyme rules
Enzymes are globular protein catalysts that lower activation energy, are specific to substrate, and have an active site. The lock-and-key and induced-fit models describe binding. Rate rises with temperature and substrate concentration until a maximum; each enzyme has an optimum pH and temperature. Competitive inhibitors raise Km without changing Vmax; non-competitive inhibitors lower Vmax. Cofactors are inorganic ions; coenzymes such as NAD and FAD are organic; prosthetic groups are firmly bound.
Worked example
An enzyme has Vmax = 100 units and Km = 5 mM. Use v = Vmax[S]/(Km + [S]). At [S] = 5 mM, v = 100 × 5/(5 + 5) = 50 units, i.e. half Vmax at Km. At [S] = 15 mM, v = 100 × 15/20 = 75 units. This shows why doubling substrate does not double rate near saturation.
Traps
Benedict's does not detect sucrose; Biuret detects peptide bonds, not free amino acids; enzymes are not consumed; competitive inhibition changes Km, not Vmax; high temperature denatures rather than simply slowing the enzyme.