Competitive vs Noncompetitive enzyme inhibition (2026)

Enzyme inhibitors are substances that decrease the rate of enzyme-catalyzed reactions by binding to enzymes. These substances can have medical applications, such as treating and controlling metabolic disorders. Competitive and noncompetitive inhibition are two types of enzyme inhibition that have different mechanisms and effects on enzyme activity and kinetics. Today, we will discuss the difference between Competitive and Noncompetitive inhibitors.

Competitive enzymes inhibition

Competitive enzyme inhibition happens when an inhibitor competes with the substrate to bind to the enzyme’s active site. This type of inhibition is characterized by the competitive inhibitor with a structure similar to the substrate, allowing it to bind effectively to the active site. As a result, when the inhibitor is present, it can hinder the substrate from binding to the active site, thereby decreasing the rate of the reaction catalyzed by the enzyme.

Km and Vmax in competitive inhibition

In terms of enzyme kinetics, the enzyme’s Vmax remains unchanged as the inhibitor does not affect its catalytic ability. However, the enzyme’s Km increases because the inhibitor reduces its affinity for the substrate. This means that a higher substrate concentration is needed to achieve the same reaction rate as in the absence of the inhibitor.

\[ \text{Km (apparent)} = K_m \left(1 + \frac{[I]}{K_i}\right) \]

where [I] is the concentration of the inhibitor and \(K_i\) is the inhibition constant.

Competitive inhibitors graph

In the competitive inhibition graph: the maximum reaction rate (Vmax) is unchanged, but the substrate concentration needed to reach half of Vmax (Km) is increased.

Examples of Competitive inhibitors

• Methotrexate: a drug that is used to treat cancer and rheumatoid arthritis. Inhibits the activity of dihydrofolate reductase enzyme.
• Sildenafil: Inhibits the activity of phosphodiesterase type 5 (PDE5) enzyme.
• Malonic acid: Inhibits the activity of succinate dehydrogenase enzyme.
• Sulfanilamide: Inhibits the activity of dihydropteroate synthase enzyme.
• Ritonavir: Inhibits the activity of HIV protease, consequently hindering viral replication.

Noncompetitive enzymes inhibition

Noncompetitive inhibition (allosteric inhibition) occurs when a molecule that does not resemble the normal substrate of an enzyme binds to a site other than the enzyme’s active site. This site is called an allosteric site, and it can influence the shape and function of the enzyme. By binding to the allosteric site, the noncompetitive inhibitor changes the conformation of the enzyme and makes it unable to catalyze the reaction.

Note: The effect of noncompetitive inhibition does not depend on the concentration of the substrate because the inhibitor does not compete with the substrate for binding to the enzyme.

Km and Vmax in Noncompetitive inhibition

The kinetic parameters of an enzyme, such as the V_max and K_m, can be used to quantify the effect of noncompetitive inhibition. The V_max of the enzyme decreases because the inhibitor lowers the amount of active enzyme. However, the K_m of the enzyme does not change because the inhibitor does not affect the enzyme’s affinity for the substrate.

Noncompetitive inhibition graph

In the noncompetitive inhibition graph: the inhibitor reduces the number of active enzyme molecules available for catalysis. As a result, the maximum reaction rate (V_max) is decreased, but the K_m is unchanged.

Examples of Noncompetitive inhibitors

Noncompetitive inhibition is particularly interesting because it affects drug development and therapeutic interventions. Since the inhibitor can bind to the enzyme whether or not the substrate is present, it provides a strategic way to modulate enzyme activity in beneficial ways without completely blocking substrate access. This characteristic can make noncompetitive inhibitors valuable tools in pharmacology, especially for conditions where precise modulation of enzymatic activity is required.

• Nifedipine: Inhibits the activity of the CYP2C9 enzyme.
• Tranylcypromine: Inhibits the activity of monoamine oxidase (MAO) enzyme.
• Phenethyl isothiocyanate: Inhibits the activity of the myrosinase enzyme.
• 6-Hydroxyflavone: Inhibits the activity of the protein kinase C (PKC) enzyme.

In conclusion, Competitive inhibition occurs when a molecule similar to the substrate competes with the substrate for binding to the enzyme’s active site, while noncompetitive inhibition occurs when a molecule that does not resemble the substrate binds to an allosteric site of the enzyme and changes its conformation.

Reference:

1. Brenda-enzymes.org
2. Malvis Romero, A., Pesci, L., Kara, S., Liese, A. (2024). Enzyme Kinetics. In: Jaeger, KE., Liese, A., Syldatk, C. (eds) Introduction to Enzyme Technology. Learning Materials in Biosciences. Springer, Cham. https://doi.org/10.1007/978-3-031-42999-6_4
3. Ring, B., Wrighton, S.A., Mohutsky, M. (2021). Reversible Mechanisms of Enzyme Inhibition and Resulting Clinical Significance. In: Nagar, S., Argikar, U.A., Tweedie, D. (eds) Enzyme Kinetics in Drug Metabolism. Methods in Molecular Biology, vol 2342. Humana, New York, NY. https://doi.org/10.1007/978-1-0716-1554-6_2
4. Ramsay, R. R., & Tipton, K. F. (2017). Assessment of Enzyme Inhibition: A Review with Examples from the Development of Monoamine Oxidase and Cholinesterase Inhibitory Drugs. Molecules (Basel, Switzerland), 22(7), 1192. https://doi.org/10.3390/molecules22071192
5. Pesaresi A. (2023). Mixed and non-competitive enzyme inhibition: underlying mechanisms and mechanistic irrelevance of the formal two-site model. Journal of enzyme inhibition and medicinal chemistry, 38(1), 2245168. https://doi.org/10.1080/14756366.2023.2245168
6. Biorender.com