What is the difference between competitive and non-competitive enzyme inhibitors?

Competitive enzyme inhibitors are structurally similar to substrates, and binds to the active site of the enzyme, and work by blocking the normal function at the active site. Non-competitive inhibitors decrease enzyme activity by binding to the enzyme whether or not substrate has bound to the active site, and inhibit the change in enzyme conformational shape necessary for activity.

The difference lies in the site that the inhibitor binds to the enzyme at. A competitive enzyme inhibitor will bind to the active site of the enzyme and so directly prevents the substrate from binding to the enzyme. On the other hand, a non-competitive enzyme inhibitor binds to a site other than the active site; consequently causing the conformation of the active site to change, thus the substrate is no longer able to bind to the active site of the enzyme.

Both competitive and non competitive inhibitors can affect the rate of an enzyme-substrate reaction. Competitive inhibitors occupy the active site of the enzyme, preventing the substrate from binding. If you increase the substrate concentration in the reaction, the rate of reaction still gradually increases.

A non competitive inhibitor differs, as it binds to the enzyme and changes the shape of the active site, therefore substrate enzyme complexes can not form. An increase in substrate does not increase the rate of reaction

Competitive inhibitors have a similar shape to the enzyme’s substrate and therefore, they can fit into the active site. This prevents the normal substrate from binding to the active site - the inhibitor directly “competes” with the substrate for the active site. The inhibition is reversible if the inhibitor is released from the active site. Increasing the concentration of substrates can reduce the effect of competitive inhibitors as the presence of more substrate molecules increases their chance of out-competing the inhibitor for the active sites.

Non-competitive inhibitors bind to the enzyme away from the active site (the place they bind is called an “allosteric” site). This distorts the shape of the active site so that the substrate cannot bind to it. The effect is often irreversible, because as long as the inhibitor stays bound to the enzyme, no substrate molecules can bind to the active site. Increasing the substrate concentration makes no difference to this type of inhibition.