1. 20.5 Enzyme Inhibition
The structure of a noncompetitive inhibitor does not resemble the substrate and does not compete for the active site.
Learning Goal Describe competitive and noncompetitive inhibition and reversible and irreversible inhibition.

2. Inhibitors Inhibitors
are molecules that cause a loss of catalytic activity.
prevent substrates from fitting into the active sites.
can be classified as either reversible inhibitors or irreversible inhibitors.

3. Reversible Inhibition
Reversible inhibitors
cause a loss of enzyme activity that can be restored.
can act in different ways but do not form covalent bonds with the enzyme.
Reversible inhibition can be competitive or noncompetitive.
Competitive inhibitors compete for the active site.
Noncompetitive inhibitors act on another site that is not the active site.

4. Competitive Inhibitors
A competitive inhibitor
has a chemical structure and polarity similar to the substrate.
competes with the substrate for the active site.
has its effect reversed by increasing substrate concentration.

5. Antimetabolites: Competitive Inhibitors in Medicine
Some bacterial infections are treated with competitive inhibitors called antimetabolites.
Sulfanilamide competes with p-aminobenzoic acid (PABA), an essential metabolite in the growth cycle of bacteria.

6. Noncompetitive Inhibitors
A noncompetitive inhibitor
has a structure that is much different from that of the substrate.
does not compete for the active site.
distorts the shape of the enzyme, which prevents the binding of the substrate at the active site.
cannot have its effect reversed by adding more substrate.

7. Irreversible Inhibition
In irreversible inhibition, enzyme activity is destroyed when
the inhibitor covalently bonds with R groups of an amino acid that may be near the active site.
the inhibitor changes the shape of the enzyme, which prevents the substrate from entering the active site.

8. Irreversible Enzyme Inhibitors

9. Enzyme Inhibition Summary

10. Study Check
Identify each description of an inhibitor that is either competitive or noncompetitive.
Increasing substrate reverses inhibition.
It binds to the enzyme’s surface but not to the active site.
Its structure is similar to that of the substrate.
Inhibition is not reversed by adding more substrate.

11. Solution
Identify each description of an inhibitor that is either competitive or noncompetitive.
Increasing substrate reverses inhibition. competitive
It binds to the enzyme’s surface but not to the active site noncompetitive
Its structure is similar to that of the substrate competitive
Inhibition is not reversed by adding more substrate noncompetitive