A competitive inhibitor increases the apparent Km of an enzyme without changing its Vmax. On a Lineweaver-Burk (double-reciprocal) plot, competitive inhibition is characterized by:
- A Lines intersecting on the x-axis (same x-intercept, different y-intercepts)
- B Parallel lines shifted downward
- C Lines intersecting on the y-axis (same y-intercept, different x-intercepts) ✓
- D Lines intersecting in the second quadrant with both changed intercepts
Correct answer: C. Lines intersecting on the y-axis (same y-intercept, different x-intercepts)
Explanation
A Lineweaver-Burk plot graphs 1/V versus 1/[S]; the y-intercept equals 1/Vmax and the x-intercept equals -1/Km. Since competitive inhibitors increase apparent Km but leave Vmax unchanged, the y-intercept (1/Vmax) remains the same across inhibitor concentrations while the slope and x-intercept change. This produces a family of lines converging at the y-axis, the hallmark graphical pattern of competitive inhibition that can be overcome by increasing substrate concentration.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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