Practice question from Med school applicants: Passage-based bio/biochem, experimental reasoning (Medium -> Hard)
Passage: Enzyme Kinetics Experiment Researchers studied an enzyme X that catalyzes the conversion of substrate S to product P. They measured initial velocity (V₀) at different substrate concentrations [S] while keeping enzyme concentration constant. Data Table: The researchers note that at [S] = 0.5 mM, adding a competitive inhibitor at concentration [I] = 2 mM reduces V₀ to 1.1 μmol/min. Based on this data, what kinetic parameter can be most directly estimated from the plateau region of the velocity curve? A. The Michaelis constant (Km) of the enzyme B. The maximum velocity (Vmax) of the enzyme C. The catalytic efficiency (kcat/Km) of the enzyme D. The dissociation constant (Ki) of the inhibitor E. The turnover number (kcat) of the enzyme
[S] (mM) V₀ (μmol/min)
0.5 2.1
1.0 3.8
2.0 6.2
5.0 9.1
10.0 10.8
20.0 11.4Answer
B
Explanation
The plateau region (at high [S] = 10-20 mM) where velocity stops increasing significantly represents saturation of the enzyme, giving a direct estimate of Vmax (~11-11.4 μmol/min). While Km can be estimated (as [S] where V₀ = Vmax/2, around 2-5 mM), it requires additional calculation. kcat requires knowing enzyme concentration. kcat/Km requires both parameters. Ki requires comparing inhibited vs uninhibited conditions at multiple concentrations.