Enzyme inhibitors | Competitive and non-competitive inhibitors | Feedback inhibition

Certain chemicals selectively inhibit the action of specific
enzymes. Sometimes, the inhibitor attaches to the enzyme
by covalent bonds, in which case the inhibition is usually
irreversible. Many enzyme inhibitors, however, bind to the
enzyme by weak interactions, and when this occurs the inhibition
is reversible. Some reversible inhibitors resemble the
normal substrate molecule and compete for admission into
the active site. These mimics, called
competitive inhibitors, reduce the productivity of enzymes
by blocking substrates from entering active sites. This kind
of inhibition can be overcome by increasing the concentration
of substrate so that as active sites become available,
more substrate molecules than inhibitor molecules are
around to gain entry to the sites.
In contrast, noncompetitive inhibitors do not directly
compete with the substrate to bind to the enzyme at the
active site. Instead, they impede enzymatic
reactions by binding to another part of the enzyme. This
interaction causes the enzyme molecule to change its shape in such a way that the active site becomes less effective at
catalyzing the conversion of substrate to product.
Toxins and poisons are often irreversible enzyme inhibitors. Many
antibiotics are inhibitors of specific enzymes in bacteria. For
instance, penicillin blocks the active site of an enzyme that
many bacteria use to make their cell walls.
feedback inhibition, a metabolic pathway is halted by the inhibitory binding of
its end product to an enzyme that acts early in the pathway.

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