3.4.3 Feedback Inhibition

Introduction

Feedback inhibition is a regulatory mechanism in which the end product of a metabolic pathway inhibits an enzyme earlier in the pathway. This prevents the overproduction of the end product and maintains metabolic balance. It is a type of reversible non-competitive inhibition and is a normal part of cellular regulation.

Definition

Feedback Inhibition: The activity of almost every enzyme in a cell can be regulated by its product. When the activity of an enzyme is inhibited by its own product, it is called feedback inhibition.

In simple words: Feedback inhibition is when the end product of a pathway inhibits an enzyme earlier in the pathway to prevent overproduction.

Core Concept

• The activity of almost every enzyme in a cell can be regulated by its product.
• When the activity of an enzyme is inhibited by its own product, it is called feedback inhibition.
• This is a type of reversible non-competitive inhibition.
• This phenomenon is a part of the normal regulatory mechanism.
• It usually happens during the regulation of metabolic pathways.
• The end product binds to an allosteric site on an enzyme earlier in the pathway.
• This inhibits the enzyme and stops the pathway.
• When the end product is consumed, the inhibition is removed.
• The pathway resumes its activity.

Detailed Explanation

Mechanism of Feedback Inhibition

• A substrate is converted to an end product through a series of enzymatic reactions.
• Each step is catalyzed by a specific enzyme.

• As the pathway proceeds, the end product accumulates.
• When the end product is present in excess, it needs to be regulated.

• The end product binds to an allosteric site on an enzyme earlier in the pathway.
• Usually, it binds to the first enzyme of the pathway.
• The allosteric site is a site other than the active site.

• The binding of the end product causes a conformational change in the enzyme.
• This changes the shape of the active site.
• The substrate can no longer bind to the active site.
• The enzyme is inhibited.

• The pathway stops.
• No more end product is produced.
• This prevents the overproduction of the end product.

• When the end product is consumed in cellular events, its concentration decreases.
• The end product molecule attached to the allosteric site is also removed.
• The enzyme regains its original shape.
• The pathway resumes its activity.

Example of Feedback Inhibition

Aspartate to Threonine Pathway:

- The amino acid aspartate becomes the amino acid threonine by a sequence of five enzymatic reactions.

Pathway:

Aspartate Enzyme 1→ Intermediate 1 Enzyme 2→ Intermediate 2 Enzyme 3→ Intermediate 3 Enzyme 4→ Intermediate 4 Enzyme 5→ Threonine

Feedback Inhibition:

• When threonine (the end product of this pathway) is present in excess, it binds to an allosteric site on enzyme 1 on this pathway.
• Then the active site is no longer able to bind aspartate.
• The pathway is inhibited.
• When all the threonine is consumed in cellular events, the threonine molecule which is attached to the allosteric site is also removed.
• The pathway resumes its activity once again.

Diagrammatic Representation:

Key Features of Feedback Inhibition

Importance of Feedback Inhibition

• Prevents the accumulation of unnecessary end products.
• Saves energy and resources.

• Keeps the concentration of metabolites within optimal ranges.
• Maintains homeostasis.

• Prevents wasteful synthesis of molecules that are not needed.
• Conserves ATP and raw materials.

• Provides a rapid response to changes in cellular needs.
• Allows the cell to adapt to changing conditions.

• Provides negative feedback control.
• Maintains steady-state levels of metabolites.

Types of Feedback Inhibition

Note: Feedback inhibition is usually negative feedback.

Comparison with Other Types of Inhibition

Clinical Significance

Intermediate Level

Mechanism in Detail

Example: Aspartate to Threonine Pathway

Importance of Feedback Inhibition

Comparison with Other Inhibition Types

Important Points

• 1. The activity of almost every enzyme in a cell can be regulated by its product.
• 2. When the activity of an enzyme is inhibited by its own product, it is called feedback inhibition.
• 3. This is a type of reversible non-competitive inhibition.
• 4. This phenomenon is a part of normal regulatory mechanism and usually happens during the regulation of metabolic pathways.
• 5. For example, the amino acid aspartate becomes the amino acid threonine by a sequence of five enzymatic reactions.
• 6. When threonine, the end product of this pathway, is present in excess, it binds to an allosteric site on enzyme 1 on this pathway and then the active site is no longer able to bind aspartate.
• 7. When all the threonine is consumed in cellular events, the threonine molecule which is attached to the allosteric site is also removed; the pathway resumes its activity once again.

Remember

• - Feedback inhibition = end product inhibits the pathway
• - Type = reversible non-competitive inhibition
• - Site = allosteric site
• - Target = usually the first enzyme
• - Effect = prevents overproduction
• - Reversible = pathway resumes when product is consumed
• - Example = Threonine inhibits Enzyme 1 in the aspartate pathway
• - Importance = prevents overproduction, saves energy, maintains balance

Common Mistakes

• 1. Confusing feedback inhibition with competitive inhibition – Feedback inhibition is non-competitive; the inhibitor binds to the allosteric site.
• 2. Thinking the end product binds to the active site – The end product binds to the allosteric site.
• 3. Thinking feedback inhibition is irreversible – Feedback inhibition is reversible.
• 4. Forgetting the example – Threonine inhibits Enzyme 1 in the aspartate pathway.
• 5. Thinking the first enzyme is always inhibited – Usually, the first enzyme is inhibited, but not always.
• 6. Confusing negative and positive feedback – Feedback inhibition is usually negative feedback.
• 7. Thinking feedback inhibition is only in bacteria – Feedback inhibition occurs in all living organisms.
• 8. Forgetting the importance – Prevents overproduction, saves energy, maintains balance.
• 9. Confusing allosteric site and active site – Allosteric site = regulatory site; Active site = catalytic site.
• 10. Thinking the inhibitor is permanently bound – The inhibitor binds reversibly.
• 11. Forgetting that the pathway resumes – When the end product is consumed, the pathway resumes.
• 12. Thinking feedback inhibition is a type of competitive inhibition – It is a type of non-competitive inhibition.

Quick Revision

• The activity of almost every enzyme in a cell can be regulated by its product.
• When the activity of an enzyme is inhibited by its own product, it is called feedback inhibition.
• This is a type of reversible non-competitive inhibition.
• This phenomenon is a part of normal regulatory mechanism and usually happens during the regulation of metabolic pathways.
• For example, the amino acid aspartate becomes the amino acid threonine by a sequence of five enzymatic reactions.
• When threonine, the end product of this pathway, is present in excess, it binds to an allosteric site on enzyme 1 on this pathway and then the active site is no longer able to bind aspartate.
• When all the threonine is consumed in cellular events, the threonine molecule which is attached to the allosteric site is also removed; the pathway resumes its activity once again.
• Importance of Feedback Inhibition:
• Prevents overproduction of products.
• Saves energy and resources.
• Maintains metabolic balance.
• Regulates metabolic pathways.
• Provides negative feedback control.

60-Second Revision

• Feedback inhibition = end product inhibits the pathway
• Type = reversible non-competitive inhibition
• Site = allosteric site
• Target = usually the first enzyme
• Effect = prevents overproduction
• Reversible = pathway resumes when product is consumed
• Example = Threonine inhibits Enzyme 1 in the aspartate pathway
• Importance = prevents overproduction, saves energy, maintains balance
• Negative feedback = maintains steady-state levels
• Warning: Feedback inhibition is non-competitive, not competitive. The end product binds to the allosteric site, not the active site. It is reversible. Threonine inhibits Enzyme 1 in the aspartate pathway. Feedback inhibition prevents overproduction and saves energy. It is a normal regulatory mechanism.