Enzyme kinetics analysis of a reaction shows the following in the presence of compound X: Vmax is unchanged but Km increases significantly. What type of inhibition does compound X exhibit, and on a Lineweaver-Burk (double reciprocal) plot, where do the lines with and without inhibitor intersect?

• A Competitive inhibition; lines intersect on the y-axis (same Vmax, different x-intercept representing Km) ✓
• B Uncompetitive inhibition; lines are parallel (both Km and Vmax decrease proportionally)
• C Non-competitive inhibition; lines intersect on the x-axis
• D Allosteric inhibition; lines intersect at the origin

Explanation

Competitive inhibitors compete with substrate for the active site. They do not alter Vmax (can be overcome by excess substrate) but increase apparent Km (more substrate needed to reach half-Vmax). On a Lineweaver-Burk plot (1/v vs. 1/[S]), the y-intercept (= 1/Vmax) is unchanged while the x-intercept (= −1/Km) shifts leftward, so lines with and without inhibitor intersect on the y-axis. Uncompetitive inhibition gives parallel lines (both Vmax and Km decrease). Non-competitive inhibition decreases Vmax only; lines converge on the x-axis.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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