Competitive inhibition: Which measurable kinetic quantity provides the factor that changes under competitive inhibition (with Vmax unchanged)?

Introduction:
Competitive inhibition elevates the substrate concentration required to reach a given rate by competing for the active site. This item checks whether you know which kinetic parameter captures that effect.

Given Data / Assumptions:

• Competitive inhibitor binds the active site and is overcome by high [S].
• Michaelis–Menten kinetics and initial rates are used.
• Vmax is unchanged; apparent Km increases.

Concept / Approach:
Competitive inhibition increases Km,app by a factor alpha = 1 + [I]/Ki while leaving Vmax unchanged. On a Lineweaver–Burk plot, the y-intercept (1/Vmax) stays constant, but the slope (Km/Vmax) increases, reflecting the larger Km,app.

Step-by-Step Solution:
Start with v = (Vmax * [S]) / (Km + [S]).Introduce inhibitor I; competitive model yields Km,app = alpha * Km where alpha = 1 + [I]/Ki.Since Vmax is unaffected, the observed change is entirely through Km,app.

Verification / Alternative check:
In Lineweaver–Burk plots, lines intersect at the y-axis (same 1/Vmax), confirming that Vmax is unchanged while Km increases (slope increases).

Why Other Options Are Wrong:
Vmax: unchanged in competitive inhibition.

Y-intercept in Lineweaver–Burk: equals 1/Vmax; it does not change in competitive inhibition.

None of these: incorrect because Km is the correct measurable parameter that captures the change.

Common Pitfalls:

• Confusing slope changes with y-intercept changes.
• Mixing up competitive (Km up, Vmax same) with noncompetitive (Vmax down, Km same).

Final Answer:
Km