, the active site
of an enzyme
is a region to which another molecule
or molecules can bind. This molecule is called the substrate
. The shape of the active site allows this particular substrate to fit correctly, and to be held in place by temporary bonds
The active site in many enzymes can be inhibited or suppressed by the presence of another molecule. There are two ways in which this can occur. In competitive inhibition, the active site itself is blocked when a molecule similar in shape to the substrate bonds to the active site and cannot be processed by the enzyme. In noncompetitive inhibition, a molecule bonds to the enzyme at another site, the allosteric site, and this bond causes the enzyme to transform such that the active site is rendered useless. Often certain metabolic pathways are designed such that after several stages negative feedback
will cause the inputs of the system to become reduced.
There are two theories of how enzymes work: the lock and key mechanism and an alternative theory whereby the active site binds and encloses onto the substrate molecule. Often enzymes bond to their substrate by Van der Waals forces or hydrogen bonds between the R' groups in the amino acid monomers.