Competitive Inhibitor IC50 Titration

Reaction Conditions

Substrate [S] 40 µM

Michaelis constant K_m 20 µM

Inhibitor affinity K_i 5.0 µM

Inhibitor [I] 0 µM

Live Kinetics

Substrate Inhibitor Product

Rate v / V_max

Apparent K_m

20.0 µM

IC₅₀ (Cheng–Prusoff)

— µM

Turnovers (products formed)

How it works

A competitive inhibitor has the same shape-fit as the substrate, so it binds the free enzyme's active site directly and blocks substrate binding — but a competitive inhibitor never binds the enzyme-substrate complex, and raising [S] can always out-compete it. The 3D scene shows exactly one molecule occupying the active site at a time, chosen by a race weighted by relative concentration and affinity.

v = Vmax·[S] / ( Km·(1 + [I]/Ki) + [S] )
Km,app = Km·(1 + [I]/Ki)          (apparent Km rises; Vmax unchanged)
IC50 = Ki·(1 + [S]/Km)            (Cheng–Prusoff, competitive case)

At a fixed [S], the rate falls from v₀ = Vmax·[S]/(Km+[S]) toward zero as [I] rises past the IC₅₀ — the inhibitor concentration that halves the rate. Because IC₅₀ scales with [S]/Km, running the assay at higher substrate concentration always makes a competitive inhibitor look weaker, which is the diagnostic signature separating it from non-competitive or uncompetitive inhibition.

[S], Km, Ki, [I] sliders — set the true kinetic parameters; every readout is computed live from the equations above (Vmax is normalised to 1).
Active-site scene — blue substrate and red inhibitor spheres orbit the enzyme; whichever wins the binding race snaps into the pocket. Substrate that binds converts to green product after a short hold; inhibitor just blocks the site and releases unchanged.
Dose-response curve — v/Vmax plotted against log[I] at the current [S]; the marker follows the [I] slider and the dashed line marks IC₅₀.

[I] = 0.01·KiDose–response v/Vmax vs log[I][I] = 100·Ki

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.