What is the Difference Between Non-Competitive and Allosteric Inhibition

The key difference between non-competitive and allosteric inhibition is that in non-competitive inhibition, the maximum rate of catalyzed reaction (Vmax) decreases and substrate concentration (Km) remains unchanged, while in allosteric inhibition, Vmax remains unchanged and Km increases.

Enzymes are essential for most reactions taking place in organisms. Usually, an enzyme catalyzes a reaction by reducing the activation energy required for the reaction. But enzymes should be regulated carefully in order to control the levels of end products rising to undesired levels. It is controlled by enzyme inhibition. An enzyme inhibitor is a molecule that disturbs the normal reaction pathway between an enzyme and a substrate.

An active site is the region of an enzyme where the substrates bind and undergo a chemical reaction. An allosteric site is where it allows molecules to either activate or inhibit enzyme activity. Enzyme kinetics plays an important role during enzyme inhibition. The maximum rate of reaction characteristic of a particular enzyme at a particular concentration is known as the maximum velocity or Vmax. The substrate concentration that gives the rate that is half of Vmax is Km.

CONTENTS

1. Overview and Key Difference
2. What is Non-Competitive Inhibition
3. What is Allosteric Inhibition
4. Similarities – Non-Competitive and Allosteric Inhibition
5. Non-Competitive vs Allosteric Inhibition in Tabular Form
6. Summary – Non-Competitive vs Allosteric Inhibition

What is Non-Competitive Inhibition?

Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the enzyme activity and binds equally well to the enzyme, whether it is bound to the substrate or not. In other words, non-competitive inhibition is where the inhibitor and substrate both bind to the enzyme at any given time. When both substrate and inhibitor bind with the enzyme, it forms an enzyme-substrate-inhibitor complex. Once this complex is formed, it cannot produce any product. It can only convert back to the enzyme-substrate complex or the enzyme-inhibitor complex.

In non-competitive inhibition, the inhibitor has equal affinity for the enzyme and enzyme-substrate complex. The most common mechanism of a non-competitive inhibitor is the reversible binding of the inhibitor to an allosteric site. But the inhibitor also has the ability to bind directly to the active site. An example of a non-competitive inhibitor is the conversion of pyruvate kinase into pyruvate. The conversion of phosphoenolpyruvate to yield pyruvate is catalyzed by pyruvate kinase. An amino acid called Alanine, which is synthesized from pyruvate, inhibits the enzyme pyruvate kinase during glycolysis. Alanine acts as a non-competitive inhibitor.

What is Allosteric Inhibition?

Allosteric inhibition is a type of enzyme inhibition where the inhibitor slows down the enzyme activity by deactivating the enzyme and binding to the enzyme at the allosteric site. Here, the inhibitor does not directly compete with the substrate at the active site. But, it indirectly changes the composition of the enzyme. Once the shape is changed, the enzyme becomes inactive. Thus, it can no longer bind with the corresponding substrate. This, in turn, slows down the formation of final products.

Allosteric inhibition prevents the formation of unnecessary products, reducing energy wastage. An example of allosteric inhibition is the conversion of ADP to ATP in glycolysis. Here, when there is excess ATP in the system, ATP serves as an allosteric inhibitor. It binds to phosphofructokinase, which is one of the enzymes involved in glycolysis. This slows down the ADP conversion. As a result, ATP prevents the unnecessary production of itself. Therefore, excess production of ATP is not needed when there are adequate amounts.

What are the Similarities Between Non-Competitive and Allosteric Inhibition?

• Both types of enzyme inhibitions slow down enzyme activity.
• The inhibitors in both enzyme inhibitions do not compete with the substrate at the active site.
• The inhibitors change the composition of the enzyme indirectly.
• Both inhibitors change the shape of the enzyme.

What is the Difference Between Non-Competitive and Allosteric Inhibition?

In non–competitive inhibition, the Vmax of the reaction decreases while leaving the Km value unchanged. In contrast, in allosteric inhibition, the Vmax remains unchanged, and the Km value increases. So, this is the key difference between non–competitive and allosteric inhibition. Allosteric inhibition focuses more on the usage of chemicals which alters the enzyme activity by binding at an allosteric site, while non-competitive inhibitors always stop the working enzyme by directly binding at an alternative site.

The following infographic tabulates the differences between non-competitive and allosteric inhibition for side by side comparison.

Summary – Non-Competitive vs Allosteric Inhibition

Non-competitive inhibition is an enzyme inhibition where the inhibitor reduces the enzyme activity and binds equally well to the enzyme whether it is bound to the substrate or not. Allosteric Inhibition is a type of enzyme inhibition where the inhibitor slows down the enzyme activity by deactivating the enzyme and binds to the enzyme at the allosteric site. The key difference between non–competitive and allosteric inhibition is that the maximum rate of catalyzed reaction (Vmax) is reduced, and the substrate concentration (Km) remains unchanged in non-competitive inhibition while the Vmax remains unchanged, and Km is increased in allosteric inhibition.

Reference:

1. “Allosteric regulation: Protocol.” Jove.
2. “Enzymes.” Boundless microbiology.

Image Courtesy:

1. “Competitive&NonCompetitive Enzyme Inhibition” By California16 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Allosteric inhibition” By [[:en:User:{{{1}}}|{{{1}}}]] at the English-language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia