What is the Difference Between Competitive and Noncompetitive Inhibition

The main difference between competitive and noncompetitive inhibition is that competitive inhibition is the binding of the inhibitor to the active site of the enzyme whereas noncompetitive inhibition is the binding of the inhibitor to the enzyme at a point other than the active site. Furthermore, competitive inhibitors compete with the substrate for the binding to the active site while noncompetitive inhibitors change the conformation of the active site upon binding.

Competitive and noncompetitive inhibitors are two mechanisms of enzyme inhibitions whose binding to the enzyme decreases the activity.

Key Areas Covered

1. What is Competitive Inhibition
     – Definition, Mechanism of Inhibition, Examples
2. What is Noncompetitive Inhibition
     – Definition, Mechanism of Inhibition, Examples
3. What are the Similarities Between Competitive and Noncompetitive Inhibition
     – Outline of Common Features
4. What is the Difference Between Competitive and Noncompetitive Inhibition
     – Comparison of Key Differences

Key Terms

Active Site, Allosteric Site, Competitive Inhibition, Enzyme, Enzyme-Substrate Complex, Noncompetitive Inhibition

What is Competitive Inhibition

Competitive inhibition is a type of reversible inhibition in which the inhibitor molecules bind to the active site of the enzyme. For this to happen, these inhibitor molecules have to compete with the substrate molecules. Therefore, the conformation of the inhibitor molecules is similar to the substrate molecule. Also, they are chemically similar to the substrate molecules; hence, they can bind to the active site of the enzyme chemically. However, the binding of the inhibitor molecule blocks the active site of the enzyme. And, this results in the accumulation of the substrate, increasing the concentration of the substrate.

Figure 1: Competitive Inhibition

Furthermore, some of the medicines used to treat diseases including Relenza serve as competitive inhibitors. Relenza is a neuraminidase inhibitor used to treat influenza. Here, neuraminidase is a viral enzyme which cleaves docking proteins of the host. The inhibition of the activity of this enzyme by the drug prevents the cleavage of the enzyme, preventing the spreading of the virus.

What is Noncompetitive Inhibition

Noncompetitive inhibition is a type of reversible inhibition in which the inhibitor molecules bind to the enzyme-substrate complex at an allosteric site, a site other than the active site. Here, the binding of the inhibitor molecules to the allosteric site results in the conformational change in the active site of the enzyme. It alters the specificity of the active site to the corresponding substrate, making the active site unavailable for the binding to the substrate. However, since noncompetitive inhibitors do not directly compete with the substrate, they do not change the substrate concentration.

Figure 2: Noncompetitive Inhibition

Moreover, cyanide is a poison which binds to an allosteric site of cytochrome oxidase, a carrier protein in the electron transport chain. It prevents the ATP production through aerobic respiration, leading to death eventually.

Similarities Between Competitive and Noncompetitive Inhibition

• Competitive and noncompetitive inhibition are two mechanisms of enzyme inhibition.
• Both are types of reversible enzyme inhibition mechanisms in which the inhibitor molecules dissociate from the enzyme at a certain point.
• Also, they both are responsible for decreasing the activity of enzymes upon binding molecules to the enzyme.
• Besides, they play a key role in the regulation of enzyme activity.

Difference Between Competitive and Noncompetitive Inhibition

Definition

Competitive inhibition refers to the blockage of the action of an enzyme on its substrate by replacing the substrate with a similar but inactive compound, which can combine with the active site of the enzyme but is not acted upon or split by the enzyme. In contrast, noncompetitive inhibition refers to the enzyme inhibition in which the inhibiting compound does not compete with the natural substrate for the active site on the enzyme but inhibits reaction by combining with the enzyme-substrate complex after forming the complex. Thus, this is the main difference between competitive and noncompetitive inhibition.

Conformation of the Inhibitor Molecule

Another difference between competitive and noncompetitive inhibition is that the competitive inhibitors are similar in conformation to the substrate while noncompetitive inhibitors have a different conformation to the substrate.

Binding to the Active Site

In competitive inhibition, molecules compete with the substrate for binding to the active site of the enzyme while in noncompetitive inhibition, molecules bind to the enzyme at a site other than the active site of the enzyme. Hence, this is one other difference between competitive and noncompetitive inhibition.

Type of Binding of the Inhibitor Molecules

One more difference between competitive and noncompetitive inhibition is that competitive inhibitors compete with the substrate for the binding to the active site of the enzyme while noncompetitive inhibitors bind to the enzyme-substrate complex.

Effect

Moreover, competitive inhibitors block the active site of the enzyme while noncompetitive inhibitors are responsible for the distortion in size or the shape of the active site of the enzyme, destabilizing the enzyme-substrate complex.

Duration

Duration is another difference between competitive and noncompetitive inhibition. Competitive inhibitors dissociate from the enzyme within a short period of time while noncompetitive inhibitors remain binding to the enzyme for a considerable time period until the substrate becomes unavailable.

Effect on the Substrate Concentration

While competitive inhibition increases the substrate concentration, noncompetitive inhibition does not change the substrate concentration. This is also a difference between competitive and noncompetitive inhibition.

Conclusion

Competitive inhibition is a type of reversible enzyme inhibition in which inhibitor molecules compete with the substrate for the binding to the active site of the enzyme. It increases the substrate concentration in the medium. On the other hand, noncompetitive inhibition is another type of reversible enzyme inhibition in which inhibitor molecules bind to the enzyme-substrate complex at a site other than the active site of the enzyme. Here, it changes the conformation of the active site, dissociating the enzyme-substrate complex before forming the product. Therefore, the main difference between competitive and noncompetitive inhibition is the type of binding of the inhibitor molecule to the enzyme.

References:

1. Cornell, Brent. “Enzyme Inhibition.” BioNinja, Available Here

Image Courtesy:

1. “Competitive inhibition” By TimVickers – en:Image:Competitive_inhibition.png (Public Domain) via Commons Wikimedia  
2. “Allosteric competitive inhibition 3” By srhat (talk · contribs)File:Comp_inhib.svg:SVG version:Srhat (talk · contribs)PNG version:Jerry Crimson Mann at en.wikipedia – File:Comp_inhib.svg (Public Domain) via Commons Wikimedia