Lineweaver–Burk Plot: What is the characteristic effect of a competitive inhibitor?

Introduction:
Competitive inhibition is a fundamental concept in enzyme kinetics. On a Lineweaver–Burk (double-reciprocal) plot, competitive inhibitors alter specific parameters in a predictable way. This question checks recognition of which intercept and slope are affected.

Given Data / Assumptions:

• Michaelis–Menten single-substrate kinetics.
• Classic competitive inhibitor competes with substrate at the active site.
• Lineweaver–Burk plotting: 1/v versus 1/[S].

Concept / Approach:

In competitive inhibition, Vmax is unchanged because sufficiently high substrate concentrations can outcompete the inhibitor. However, the apparent affinity decreases, so Km,app increases. On the Lineweaver–Burk plot, the y-intercept equals 1/Vmax (unchanged) and the x-intercept equals −1/Km,app (moves toward zero as Km,app increases). The slope equals Km,app/Vmax, which therefore increases.

Step-by-Step Solution:

Verification / Alternative check:

Graphical families of lines for competitive inhibition intersect at the same y-intercept but have different slopes, confirming y-intercept constancy and x-intercept shift.

Why Other Options Are Wrong:

A: Both intercepts do not shift; y-intercept stays constant. B: There is no uniform horizontal shift with intercepts fixed. D: Slope increases; it does not remain unchanged. E: y-intercept (1/Vmax) does not decrease since Vmax is unchanged.

Common Pitfalls:

Confusing competitive with non-competitive or uncompetitive patterns; forgetting that only apparent Km changes in competitive inhibition.

Final Answer:

It changes the x-intercept (moves it toward zero) while the y-intercept remains unchanged.