Enzyme Regulation and Inhibitors
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Questions and Answers

What occurs to the Vmax when an uncompetitive inhibitor is present?

  • Vmax increases significantly
  • Vmax becomes lower than in the absence of the inhibitor (correct)
  • Vmax is eliminated completely
  • Vmax remains unchanged

What effect does an uncompetitive inhibitor have on the apparent KM value?

  • It decreases the apparent KM value (correct)
  • It increases the apparent KM value
  • It alters the KM value in an unpredictable manner
  • It does not affect the KM value at all

Which statement is true regarding the effect of a non-competitive inhibitor on the enzyme-substrate interaction?

  • The non-competitive inhibitor increases the KM value
  • The non-competitive inhibitor does not affect the substrate binding to the enzyme (correct)
  • The non-competitive inhibitor completely prevents substrate binding
  • The non-competitive inhibitor enhances the substrate binding affinity

How does the presence of a non-competitive inhibitor affect Vmax and KM?

<p>Vmax decreases but KM remains unchanged (A)</p> Signup and view all the answers

What happens to the concentration of functional enzymes when a non-competitive inhibitor is introduced?

<p>It decreases because the inhibitor binds at a different site (A)</p> Signup and view all the answers

Which statement accurately describes the role of allosteric inhibitors in enzyme function?

<p>They require increased substrate or activator concentration to counteract their effects. (C)</p> Signup and view all the answers

What is the significance of the regulatory enzyme in a metabolic pathway?

<p>It typically catalyzes the rate-limiting step, affecting overall pathway flux. (B)</p> Signup and view all the answers

Which method of enzyme regulation occurs through the activation of proteolytic enzymes?

<p>Zymogen cleavage (A)</p> Signup and view all the answers

Which type of regulation allows an end product to influence its own synthesis?

<p>Feedback regulation (B)</p> Signup and view all the answers

What impact does covalent modification, such as phosphorylation, have on enzyme activity?

<p>It leads to a conformational change that can enhance enzyme activity. (A)</p> Signup and view all the answers

What is the defining feature of a reversible inhibitor?

<p>Can dissociate from the enzyme at a significant rate. (C)</p> Signup and view all the answers

Which type of inhibitor competes with the substrate for binding to the active site?

<p>Competitive inhibitor (A)</p> Signup and view all the answers

How does a competitive inhibitor affect the apparent Km (KM,app) of an enzyme?

<p>Increases KM,app. (C)</p> Signup and view all the answers

What occurs when an uncompetitive inhibitor binds to an enzyme-substrate complex?

<p>The enzyme-substrate-inhibitor complex does not progress to product formation. (B)</p> Signup and view all the answers

Which characteristic is true for a noncompetitive inhibitor?

<p>It decreases the Vmax of the enzyme. (D)</p> Signup and view all the answers

Which of the following best describes the effect of increasing substrate concentration in the presence of a competitive inhibitor?

<p>It increases the likelihood of substrate binding to the active site. (C)</p> Signup and view all the answers

What type of binding site do competitive inhibitors typically target?

<p>Substrate-recognition site. (C)</p> Signup and view all the answers

Which statement about irreversible inhibitors is true?

<p>They decrease the concentration of functional enzymes. (C)</p> Signup and view all the answers

Which of the following inhibitors does not affect Vmax but alters the Km?

<p>Competitive inhibitor (D)</p> Signup and view all the answers

What effect does Penicillin have on enzymes?

<p>It forms a covalent bond with serine in the enzyme’s active site. (C)</p> Signup and view all the answers

How does Aspirin function at the biochemical level?

<p>It acetylates a serine residue, making enzyme inaccessible. (C)</p> Signup and view all the answers

What is a characteristic of allosteric activators?

<p>They increase the enzyme's affinity for the substrate. (C)</p> Signup and view all the answers

What is the effect of irreversible inhibition on Km?

<p>It has no effect on Km. (C)</p> Signup and view all the answers

Which of the following statements best defines non-competitive inhibition?

<p>It can occur irrespective of substrate concentration. (D)</p> Signup and view all the answers

In terms of enzyme affinity, what happens when an allosteric inhibitor is present?

<p>The enzyme affinity decreases, stabilizing the low-affinity state. (C)</p> Signup and view all the answers

Flashcards

Allosteric Inhibition

A type of enzyme regulation where a molecule binds to an enzyme at a site other than the active site, changing its conformation and decreasing its activity.

Rate-Limiting Enzyme

The enzyme in a metabolic pathway with the slowest reaction rate, controlling the overall speed of the pathway.

Feedback Regulation

A mechanism where the end product of a metabolic pathway inhibits an earlier enzyme in the same pathway, controlling its own synthesis.

Zymogen

An inactive precursor of an enzyme that requires activation by proteolysis.

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Covalent Modification

A type of enzyme regulation involving the attachment or removal of a chemical group, like a phosphate group, changing its conformation and activity.

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Reversible Inhibitor

A compound that decreases the velocity of an enzyme reaction by binding to the enzyme, but this binding is NOT covalent and can readily dissociate.

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Competitive Inhibitor

An inhibitor that binds to the same active site as the substrate, competing for the same spot on the enzyme.

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How does a Competitive Inhibitor affect Km?

A competitive inhibitor INCREASES the apparent Km of an enzyme (KM,app). This means more substrate is needed to reach half-maximal velocity.

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How does a Competitive Inhibitor affect Vmax?

A competitive inhibitor has NO effect on Vmax. The enzyme can still reach its maximum velocity, just with a higher concentration of substrate.

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Uncompetitive Inhibitor

An inhibitor that binds ONLY to the enzyme-substrate complex (ES) and prevents the formation of product.

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How does an Uncompetitive Inhibitor affect Km?

An uncompetitive inhibitor DECREASES the apparent Km of an enzyme (KM,app). This means less substrate is needed to reach half-maximal velocity.

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How does an Uncompetitive Inhibitor affect Vmax?

An uncompetitive inhibitor DECREASES Vmax. The maximal velocity of the reaction is reduced.

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Key Features of Competitive Inhibitor

A Competitive Inhibitor (CI) increases KM,app but has no effect on Vmax. The inhibitor can be overcome by increasing substrate concentration.

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How Irreversible Inhibitors Affect Enzyme Kinetics

Irreversible inhibitors reduce the concentration of functional enzymes, leading to a lower Vmax (maximum reaction rate). However, the Km (substrate affinity) remains unchanged.

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Penicillin: Example of Irreversible Inhibitor

Penicillin acts as a 'suicide inhibitor' by covalently bonding to the active site of the enzyme Glycopeptide transpeptidase, which is essential for bacterial cell wall formation.

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Aspirin: How it Works as an Inhibitor

Aspirin irreversibly inhibits the COX enzyme by attaching an acetyl group to its active site. This blocks the production of prostaglandins, involved in pain and inflammation.

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Allosteric Regulation

A type of enzyme regulation where molecules bind to a site other than the active site, called the allosteric site, causing a conformational change that affects enzyme activity.

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Allosteric Activators

Molecules that bind to the allosteric site and increase the enzyme's affinity for its substrate, leading to higher activity.

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How does a non-competitive inhibitor affect Vmax and Km?

A non-competitive inhibitor lowers the Vmax of the reaction because it decreases the amount of active enzyme. However, it does not affect the Km because the affinity between the enzyme and substrate remains unchanged. The inhibitor just makes a portion of the enzyme less active.

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Study Notes

Enzyme Regulation

  • Reversible Inhibitors: A compound that reduces the reaction velocity by binding to the enzyme. It's reversible if it's not covalently bound and can dissociate readily.
    • Classified by their relationship to the substrate:
      • Competitive: Competes with the substrate for binding at the enzyme's substrate-recognition site. Usually a close structural analog of the substrate. Increasing substrate concentration overcomes the inhibitor.
      • Noncompetitive: Binds to the enzyme at a site other than the active site. Decreases Vmax, but Km remains unchanged.
      • Uncompetitive: Binds only to the enzyme-substrate complex (ESI). Decreases both Vmax and Km.

Competitive Inhibitor (CI)

  • Both inhibitor and substrate bind to the same site on the enzyme.
  • This binding for both is reversible.
  • To overcome the inhibitor, increase the substrate concentration.
  • Increasing substrate concentration raises the apparent KM of the enzyme (KM,app).
  • Increasing substrate concentration is necessary to saturate the enzyme.
  • Competitive inhibitors have no effect on Vmax.

Uncompetitive Inhibitor

  • Binds only to the ES complex (enzyme-substrate complex).
  • This enzyme-substrate-inhibitor complex (ESI) does not form any product.
  • Decreases both Vmax and KM.

Non-competitive Inhibitor

  • Inhibitor binds to the enzyme on a different binding site.
  • Decreases Vmax, but KM remains unchanged.
  • The concentration of functional enzymes is lower.

Irreversible Inhibitor

  • Binds to the enzyme's active site irreversibly via a covalent bond.
  • Decreases Vmax, but Km remains unchanged.
  • Concentration of functional enzymes is lower.

Examples of Irreversible Inhibitors

  • Penicillin: A covalent bond with the OH group of Serine residue at the active site of Glycopeptide transpeptidase; inhibits cell wall synthesis in bacteria.
  • Aspirin: Non-competitive and irreversible acetylation of COX enzyme; blocks the biotransformation of arachidonic acid into prostaglandins.

Allosteric Activators and Inhibitors

  • These are compounds that bind to sites other than the active catalytic site on an enzyme.
  • Allosteric Activators: Increase enzyme affinity for the substrate by changing the conformation of the catalytic site. Increased enzyme in the active state facilitates substrate binding.
  • Allosteric Inhibitors: Bind more tightly to the T state (inactive form), so either the substrate concentration or activator concentration must increase to overcome the inhibitor's effects.

Other Ways for Enzyme Regulation

  • Regulation of Gene Expression: Controls the quantity and rate of enzyme synthesis (induction or repression).
  • Zymogen Cleavage: Some enzymes are synthesized as inactive precursors (zymogens), which are activated by proteolysis.
  • Covalent Modification: Chemical modification (e.g., phosphorylation) of serine, threonine, or tyrosine residues on the enzyme.

Feedback Inhibition

  • The end product of a metabolic pathway controls its own synthesis rate.
  • The end product might allosterically inhibit an earlier enzyme or inhibit the transcription of the gene for an earlier enzyme in the pathway and thus control the rate.

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Description

This quiz explores the mechanisms of enzyme regulation, focusing on reversible inhibitors. Understand the differences between competitive, noncompetitive, and uncompetitive inhibitors, and how they affect enzyme kinetics. Test your knowledge of these critical biochemical concepts.

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