Enzymes and Enzyme Kinetics
Enzymes are biological catalysts that speed reactions by lowering activation energy. This domain tests active-site models, cofactors, Michaelis-Menten kinetics, Km and Vmax, and inhibition patterns.
Core rules
- A holoenzyme is an apoenzyme plus its cofactor; cofactors may be metal ions or coenzymes.
- Lock-and-key describes a rigid active site; induced fit describes a site that changes shape on binding.
- The Michaelis-Menten equation is v = Vmax[S]/(Km + [S]). Km is the substrate concentration at half Vmax.
- Competitive inhibitors raise apparent Km but leave Vmax unchanged; non-competitive inhibitors lower Vmax.
- Temperature and pH affect activity; extremes denature the enzyme. Allosteric enzymes do not follow simple Michaelis-Menten kinetics.
Worked example
An enzyme has Vmax = 120 µmol min−1 and Km = 5 mM. Find v at [S] = 15 mM. Substitute: v = 120 × 15/(5 + 15) = 1800/20 = 90 µmol min−1. At [S] = Km = 5 mM, v would be 60 µmol min−1, exactly half Vmax. This confirms the definition of Km.
Traps that cost marks
Km is not a velocity and is not directly the affinity constant, although lower Km usually means higher apparent affinity. A competitive inhibitor does not change Vmax. Enzymes are not consumed in the reaction. Do not apply the Michaelis-Menten equation to allosteric enzymes without qualification.