How tightly does an inhibitor, I, bind E? • I may bind to E and/or ES with an affinity defined by KI – [EI]  [E] + [I] – [ESI]  [ES] + [I] KI = [E][I]/[EI] KI = [ES][I]/[ESI] • KI can be determined from the effect of I on Vmax and/or KM – Changes in Vmax and KM are defined as α – α = 1 + [I]/KI – Determine α from double-reciprocal plots – Knowing [I], you can solve for KI Competitive Inhibition • Competes with substrate for binding – Inhibits substrate binding – does not affect catalysis • No change in Vmax; apparent increase in KM • Lineweaver-Burk: lines intersect at the y-axis. Competitive Inhibition Uncompetitive Inhibition • Only binds to ES complex • does not affect substrate binding • inhibits catalytic function • Decrease in Vmax; apparent decrease in KM • No change in KM/Vmax Uncompetitive Inhibitors • I binds ES ONLY • Aka “anti-competitive”; KM decreases rather than increases • Decreases Vmax and Km Uncompetitive Inhibition Mixed Inhibition • Binds to E or to ES but has a higher affinity for one of these • Decrease in Vmax; increase or decrease in KM • Lineweaver-Burk: lines intersect to the left of the y-axis. Mixed Inhibitors • Competitive AND uncompetitive character • Vmax KM • Competitive ---- αKM • Uncompetitive Vmax/α’ • Sum Vmax/ α’ KM/α’ αKM/α’ • Effect on KM depends on relative competitive vs uncompetitive character • Mixed inhibitors having α = α’ are classified as noncompetitive inhibitors Mixed Inhibition