Reversible inhibition of an enzyme is achieved via three main mechanisms. This article explores the difference between two of those mechanisms, i.e., competitive and non-competitive inhibition.
| Naturally occurring poisons are enzyme inhibitors that have evolved as a defense mechanism to protect the plant or animal against predators. |
Reversible inhibitions involve the attachment of inhibitor molecules or compounds to bind to a specific target enzyme with the help of non-covalent interactions that include hydrogen bonds, ionic bonds, and hydrophobic interactions. Multiple instances of such bonds help in establishing an overall strong and specific bond between the inhibitor and the enzyme.
However, as the name suggests, this inhibitory binding is reversible, i.e. it is temporary in nature. The inhibitor itself does not experience any change in structure or composition when it is bound to the enzyme, and hence can be released from it either by dilution or by getting recruited by some other molecule. These reversible inhibitors are differentiated from each other on the basis of their binding affinities and also according to the effect that the concentration of the substrate has on them. The two extreme types are competitive and non-competitive inhibitors, which are differentiated in the following table.
Competitive Inhibition Vs. Non-competitive Inhibition
| Definition | |
| ❖ The enzyme undergoes competitive inhibition when the inhibitor and the substrate both compete to bind to the active site of the enzyme. | ❖ The enzyme undergoes non-competitive inhibition when the inhibitor inactivates the enzyme by binding to a site different from the active site. |
| Binding Site | |
| ❖ Same as the active site for substrate. | ❖ An allosteric site, that is not the active site. |
| Structure | |
| ❖ Similar to that of substrate. | ❖ Dissimilar to that of substrate. |
| Effect on Binding | |
| ❖ Blocks active site, hence substrate cannot bind. | ❖ Alters the structure of active site to prevent activation of enzyme by substrate. |
| Binding Affinity | |
| ❖ Affinity to enzyme only. | ❖ Affinity to enzyme as well as enzyme-substrate complex. |
| Kinetics of Inhibition | |
| ❖ Inhibition constant Kmincreases. ❖ Velocity of reaction Vm is constant. ❖ Effect of inhibition can be decreased by increasing substrate concentration. ❖ In a Lineweaver-Burk plot, the Y-intercept of Vm remains constant. |
❖ Inhibition constant Km is unchanged. ❖ Velocity of reaction Vmdecreases. ❖ Effect of inhibition is independent of substrate concentration. ❖ In a Lineweaver-Burk plot, the X-intercept of Km remains constant. |
| Example | |
| ❖ Sulfanilamide competes with para-aminobenzoic acid (PABA) in bacteria, in order to inhibit folic acid synthesis. | ❖ Cyanide attaches to the prosthetic group of cytochrome oxidase, and thus inhibits the electron transport chain. |
An enzyme in a system may be inactive or active depending on the action of these inhibitors. Hence, it is measured in terms of its activity rather than weight. Here, activity refers to the unit quantity of substrate that is converted to product per unit of time.