In a Lineweaver–Burk plot, a non-competitive inhibitor shows which change in Vmax and Km?

• A. Lower Vmax, same Km
• B. Same Vmax, higher Km
• C. Lower Vmax, lower Km
• D. Higher Vmax, lower Km

Answer: (A)

Non-competitive inhibition lowers the maximum rate without changing how tightly the substrate binds. The inhibitor binds at a site other than the active site, reducing the amount of active enzyme available, so the reaction can’t reach the same maximum velocity even when substrate is plentiful. However, it doesn’t alter the substrate’s binding affinity to the enzyme, so Km stays the same.

In a Lineweaver–Burk plot, the y-intercept equals 1/Vmax, so a reduced Vmax shifts the line upward (larger 1/Vmax). The x-intercept equals −1/Km, so with Km unchanged the x-intercept remains at the same position. Thus the pattern is lower Vmax with Km unchanged.