Enzyme Inhibition C483 Spring 2013

Questions 1. An inhibitor binds to a site other than the active site of the enzyme. Which statement below correlates with this observation? A) It must be a competitive inhibitor. B) The inhibition must be irreversible. C) It could be noncompetitive or uncompetitive inhibition. D) It could be irreversible, classical competitive, noncompetitive or nonclassical competitive, or mixed inhibition. 2. Which equilibrium below applies to noncompetitive inhibition?

3. The following data were obtained in the presence and absence of inhibitor. What type of inhibition is shown? A) Competitive. B) Uncompetitive. C) Noncompetitive. D) Irreversible. E) Cannot tell inhibition type from the information given. Substrate concentration (millimolar) Rate without inhibitor (mmol/min) Rate with inhibitor (mmol/min)

Enzyme Inhibition Reversible inhibitors – Substrate analogs – Rational Design Irreversible inhibitors – Affinity labels – Mechanism-based inhibitors

Reversible Inhibition Kinetics Know types of Reversible Inhibition Know effect on kinetic parameters Understand why Interpret MM plots – NOT Lineweaver-Burk!

Competitive Inhibition Added substrate can outcompete inhibitor “Feels like…” – Same amount of Enzyme at high [S] – Needs more S to bind (lowers affinity) Draw altered MM plot

Non-classical Competitive Inhibition Ultimately, same as classical competitive Generally, inhibitor is not a substrate analog

Uncompetitive Inhibition Added substrate increases inhibitor effect “Feels like…” – Less enzyme at high [S] – Enzyme has greater affinity for substrate Draw altered MM plot

Noncompetitive Inhibition Assumes simple case of inhibitor binding equally to E and ES “Feels like…” – Less enzyme at all [S] – No effect on substrate affinity (no equil shift) Draw altered MM plot

Mixed Inhibition Like noncompetitve, but not the simple case “Feels like…” – Less enzyme at all [S] – Overall lowering OR raising of affinity for substrate

Fill in the Chart InhibitionEffect on K M Effect on V max Effect on V max /K M Competitive Uncompetitive Noncompetitive Mixed

Answers 1.C 2.B 3.A