An inhibitor binds equally well to the free enzyme and the enzyme-substrate complex at a site distant from the active site. On a double reciprocal (Lineweaver-Burk) plot obtained at increasing inhibitor concentrations, what pattern is expected?

• A Lines intersect at the same y-intercept, x-intercept shifts toward zero
• B Lines are parallel to the control line
• C Lines intersect at the same point on the x-axis, y-intercept increases ✓
• D Lines intersect at the same point on the y-axis, slope increases

Explanation

Pure noncompetitive inhibition reduces Vmax because inhibitor-bound enzyme cannot form product, but affinity for substrate is unchanged since the inhibitor binds E and ES equally, so Km stays constant. On a Lineweaver-Burk plot all lines cross on the x-axis at -1/Km while the y-intercept (1/Vmax) rises. Option D describes competitive inhibition, where lines share the y-intercept because Vmax is preserved. Parallel lines occur only in uncompetitive inhibition.

Reference: Lippincott Biochemistry, 8th ed.

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