Biochemistry - Biomolecules, Enzyme Kinetics and Metabolism
What the exam tests
Biochemistry tests the structure and function of biomolecules, enzyme kinetics, bioenergetics and the core metabolic pathways, plus basic molecular biology techniques.
Core rules
- Amino acids: the zwitterion dominates near neutral pH; pI is the average of the two pKa values bracketing the neutral form. The peptide bond is planar and partial double-bonded.
- Protein structure: primary (sequence) → secondary (α-helix, β-sheet) → tertiary (fold) → quaternary (subunits). Denaturation disrupts non-covalent bonds, not the peptide backbone.
- Enzyme kinetics: v = Vmax[S]/(Km + [S]); Km is the substrate concentration at half Vmax and an inverse index of affinity. Lineweaver–Burk: 1/v vs 1/[S], slope Km/Vmax, intercepts 1/Vmax and −1/Km.
- Inhibition: competitive raises Km, Vmax unchanged; non-competitive lowers Vmax, Km unchanged; uncompetitive lowers both.
- Metabolism: glycolysis nets 2 ATP and 2 NADH per glucose; pyruvate dehydrogenase links to the TCA cycle; oxidative phosphorylation yields roughly 30–32 ATP per glucose depending on the shuttle.
Worked example
An enzyme has Vmax = 100 µmol min⁻¹ and Km = 5 mM. At [S] = 15 mM: v = 100 × 15/(5 + 15) = 75 µmol min⁻¹. Check: at [S] = Km the velocity is exactly Vmax/2 = 50, and at 3 Km it should be 75 % of Vmax, which matches.
Traps
Reporting gross instead of net ATP; confusing thymine (DNA) with uracil (RNA); mixing up the inhibition patterns; averaging the wrong pKa values for pI; forgetting that a catalyst changes rate, not ΔG or Keq.