Enzyme Inhibition

C483 Spring 2013

Questions1. 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)

0.100 2.5 1.60.200 4.2 2.90.500 6.6 5.10.750 7.4 6.21.000 9.9 9.82.000 10.0 10.1

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 ChartInhibition Effect on KM Effect on Vmax Effect on Vmax/KM

Competitive

Uncompetitive

Noncompetitive

Mixed

Answers

1. C2. B3. A