Non-competitive inhibition

This occurs when an inhibitor does not bind to the active site but does bind to a different part of the enzyme and changes the active site shape.

This stops the substrate binding to the enzyme and decreases the reaction rate. Non-competitive inhibition cannot be reversed by increasing the substrate concentration. Examples of non-competitive inhibitors include cyanide, mercury and silver.

Substrate can’t bind because competitive inhibitor prevents it accessing active site.  Non-competitive inhibitor embedded in bottom of enzyme. Active site altered so substrate still can’t bind.

Competitive and non-competitive inhibitors can affect the reaction rates in a metabolic pathway.

3 lines. Top - no inhibitor steep rise starts to level.  Middle - competitive inhibitor straight 45 degree angle.  Bottom - Non-competitive inhibitor rises between first two for a bit then flattens.

Red line (no inhibitor)

The graph levels off because all of the active sites are occupied with the substrate.

Orange line (competitive inhibitor)

There is a gradual increase in reaction rate because competitive inhibitors are occupying only some of the enzyme active sites. As substrate concentration increases, the substrate molecules outnumber the inhibitor so the reaction rate reaches the maximum.

Green line (non-competitive inhibitor)

Most enzyme molecules have become inactive but some are unaffected by the inhibitors so reaction rate remains low. An increase in substrate concentration does not increase reaction rate.