Inhibitors:.
As shown above the active site of an enzyme only fits with one particular substrate, but it is possible for other molecules to bind to an enzyme's active site, if it is very similar in shape to the substrate. This is called an inhibitor, as it inhibits the enzyme's function. .
If the inhibitor molecule binds only for a short time to the active site, it is in competition with the normal substrate for the active site. While it is attached to the enzyme molecule it prevents the normal substrate from doing so, it becomes less likely that the substrate will collide with an empty active site. If there is much more of the substrate than the inhibitor present, substrate molecules can easily bind to the active site in the usual way and so the enzyme's function is unaffected, because it is a higher chance of the substrate colliding with an active site, rather than the inhibitor. If the inhibitor is sufficiently concentrated, a high proportion of the enzyme molecules will combine with it and the reaction will be slowed. This is therefore known as competitive inhibition. The degree of inhibition depends on the relative concentrations of substrate and inhibitor. It is reversible because increasing the concentration of the substrate can reverse it. .
If the inhibitor binds permanently to the active site of the enzyme there will be no possibility of the normal substrate taking the active site. The extent of inhibition depends on the concentration of inhibitor and cannot be varied by changing the amount of substrate present. As the substrate and inhibitor are not competing with each other for the active site, this is known as non-competitive inhibition. This type is inhibition is irreversible because once the inhibitor molecule has combined with the active site it cannot be dislodged.
Not all non-competitive inhibition is irreversible. Some inhibitors have no likeness with the active site and so can bind with parts of the enzyme other than the active site.