What are enzyme inhibitors?

Enzyme inhibitors function to slow down the rate of an enzyme catalysed reaction by interfering with the enzyme in some form. There are two main types of inhibitors: 

1. Irreversible inhibitors - these molecules bind permanently to the enzyme. This means the enzyme is changed, and so its active site can no longer bind to its substrate molecule. Therefore, irreversible inhibitors effectively reduce the number of functioning enzymes available, which slows the rate of reaction. An example of an irreversible inhibitor is a metabolic poison called potassium cyanide - this works by irreversibly inhibiting the enzyme cytochrome oxidase, which is found in the electron transport chain (an essential part of respiration in our cells). If there is a reduced number of functioning cytochrome oxidase enzymes, ATP production is limited and this can have fatal consequences. 

2. Reversible inhibitors - these molecules bind temporarily to the enzyme, thus the inhibition is reversible. Competitive reversible inhibitors can be competitive, uncompetitive and non-competitive.

Competitive reversible enzyme inhibitors work by 'competing' with the substrate for the active site on the enzyme, as they have a similar shape to the substrate molecule. Despite fitting into the active site, competitive reversible inhibitors work by remaining unreacted as they have a different structure to the substrate. This means that fewer substrate molecules are able to bind to the enzyme, and so the reaction rate is decreased. As the inhibitor and substrate are competing, the level of inhibition depends on the relative concentrations of inhibitor and substrate i.e. the inhibition can be overcome by increasing the concentration of substrates. 

Non-competitive reversible enzyme inhibitors work by preventing the formation of enzyme-product complexes. They bind to a site on the enzyme that is not the active site, which changes the 3D tertiary structure of the enzyme. This means the enzyme is unable to catalyse the reaction, and so the substrate cannot form the product. The inhibitor is not competing with substrate here, and so non-competitive inhibitors are unaffected by substrate concentrations. It should be noted, non-competitive inhibitors can also be irreversible. 

Uncompetitive reversible enzyme inhibitors bind only to the substrate-enzyme complex. Therefore, these inhibitors require that an enzyme-substrate complex to be formed. 

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Enzyme inhibitors play an important role in organisms, as they are used to regulate metabolic reactions. For example, the final product in a metabolic pathway can feedback and act as a non-competitive inhibitor to one of the earlier enzymes, which will in turn reduce the amount of product being formed. This means that the metabolic reaction can control itself, as more product will cause more pathway inhibition, so less product forms - this is called feedback inhibition.