Inhibitors inhibit (prevent) the activity of enzymes, reducing the rate of their reactions. They are found naturally, but are also used artificially, e.g. antibacterial drugs and pesticides. 

There are two kinds of inhibitors.

1. Competitive inhibitors

2. Non-competitive inhibitors

A competitive inhibitor molecule has a similar structure to the normal substrate molecule, and it can fit into the active site of the enzyme. As such, the competitive inhibitor “competes” with the substrate for the active site, the result is a slower rate of reaction.

Competitive inhibitors increase Km for the enzyme, but have little effect on Vmax. Meaning the rate of reaction can approach a normal rate (normal Vmax) if the substrate concentration is increased high enough to saturate the enzyme (i.e. the Km is increased).

A non-competitive inhibitor molecule has a different structure from the substrate molecule and does not fit into the active site. 

Non-competitive inhibitors binds to another part of the enzyme, resulting in a shape change of the enzyme, including the active site. Meaning the substrate can no longer bind to its enzymes active and no Enzyme-Substrate-Complexes are formed.

So, non-competitive inhibitors reduce the amount of active enzyme available (just like decreasing enzyme concentration). 

Non-competitive inhibitors decrease Vmax, but have no effect on Km. 

Non-competitive inhibitors that bind weakly to the enzymes can be ‘washed out’ and are sometimes called reversible inhibitors, while those that bind tightly and cannot be ‘washed out’ are called irreversible inhibitors. 

Poisons like cyanide, heavy metal ions and some insecticides are examples of non-competitive inhibitors.