Enzyme Inhibition (College Board AP® Biology): Study Guide

Competitive & noncompetitive enzyme inhibition

• Enzyme inhibitors slow down or stop enzyme activity, decreasing the rate of reaction

• Increasing the concentration of an inhibitor, therefore, reduces the rate of reaction and eventually, if inhibitor concentration continues to be increased, the reaction will stop completely

Competitive inhibitors

• Competitive inhibitors have a similar shape to that of the substrate molecules and therefore compete with the substrate for the active site

• Competitive inhibitors can bind reversibly or irreversibly to the active site of the enzyme

Reversible inhibitors

• Reversible inhibitors do not form covalent bonds with the enzyme, unlike irreversible inhibitors, therefore their effects can be reduced or reversed by:

  • removing the inhibitor

  • increasing substrate concentration (in the case of competitive inhibitors)

• This can increase the rate of reaction once more

Irreversible inhibitors

• An irreversible inhibitor of enzymes is a molecule that binds to an enzyme and permanently inactivates it, usually by forming a covalent bond in the active site

• The inhibition cannot be reversed by removing the inhibitor or increasing substrate concentration as the inhibitor permanently bonds to the active site, blocking substrates from binding

Noncompetitive inhibitors

• Noncompetitive inhibitors bind to allosteric sites (not the active site) of an enzyme

• They can attach with or without the substrate already bound

• Binding alters the active site's shape, preventing substrate binding or reducing enzyme-substrate complex formation

• Increasing substrate concentration does not overcome inhibition, as the inhibitor does not compete for the active site

• The active site remains altered, blocking enzyme function

Rate of enzyme reactions & inhibitors

• A competitive inhibitor will lower the initial rate of reaction by occupying some of the available active sites

  • Eventually, the same amount of product will be produced as would have been produced without the competitive inhibitor, therefore the maximal rate is not affected

• Non-competitive inhibitors lower the initial rate of reaction and the maximal rate of reaction will be lower, therefore a lower amount of product is produced than would normally be produced