HBF 102: Enzymes II - Inhibition & Regulation
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Questions and Answers

What effect does phosphorylation have on glycogen phosphorylase?

  • Has no effect on its activity
  • Inhibits its synthesis
  • Increases its activity (correct)
  • Decreases its activity

Which statement is true regarding the effect of elevated insulin levels?

  • It represses enzyme synthesis in all physiological conditions
  • It induces the synthesis of enzymes for glucose metabolism (correct)
  • It decreases the synthesis of enzymes for glucose metabolism
  • It has no effect on enzyme synthesis

How long does it typically take for alterations in enzyme levels due to induction or repression of synthesis to occur?

  • Minutes
  • Seconds
  • Hours to days (correct)
  • Weeks

What is the primary change observed in competitive inhibition?

<p>Increased Km (B)</p> Signup and view all the answers

How do noncompetitive inhibitors primarily affect enzyme kinetics?

<p>Only decrease Vmax (B)</p> Signup and view all the answers

Which enzyme's activity is decreased by the addition of phosphate?

<p>Glycogen synthase (D)</p> Signup and view all the answers

What role do heavy metals generally play in enzyme function?

<p>They prevent substrate binding (A)</p> Signup and view all the answers

What is the impact of covalent modification on enzyme activity compared to allosteric regulation?

<p>Covalent modification takes seconds to minutes (C)</p> Signup and view all the answers

What is the main difference between induction and repression of enzyme synthesis?

<p>Induction increases enzyme levels; repression decreases them (B)</p> Signup and view all the answers

What happens to the apparent Km in the presence of a competitive inhibitor?

<p>It increases due to the inhibitor competing with the substrate. (D)</p> Signup and view all the answers

Which of the following best characterizes non-competitive inhibition?

<p>It does not change the Km value of the enzyme. (D)</p> Signup and view all the answers

How does non-competitive inhibition affect the Lineweaver-Burk plot?

<p>The apparent Vmax decreases while Km remains unchanged. (A)</p> Signup and view all the answers

Which of the following statements about non-competitive inhibitors is TRUE?

<p>They prevent the reaction regardless of substrate concentration. (C)</p> Signup and view all the answers

Which scenario exemplifies non-competitive inhibition?

<p>Heavy metal ions binding to enzymes, preventing catalysis. (D)</p> Signup and view all the answers

Which of the following is NOT a characteristic of competitive inhibitors?

<p>They decrease the Vmax of the reaction. (D)</p> Signup and view all the answers

In the context of competitive inhibition, what does a change in apparent Km indicate?

<p>The inhibitor affects the enzyme's active site efficiency. (C)</p> Signup and view all the answers

How do non-competitive inhibitors influence enzyme kinetics?

<p>They decrease Vmax without affecting Km. (D)</p> Signup and view all the answers

What role does glucose-6-phosphate play in the regulation of hexokinase?

<p>It acts as a non-competitive inhibitor. (B)</p> Signup and view all the answers

What is the primary characteristic of competitive inhibition in enzyme reactions?

<p>The inhibitor competes with the substrate for the active site. (A)</p> Signup and view all the answers

How does a competitive inhibitor affect the Km value of an enzyme-catalyzed reaction?

<p>It increases the Km value. (A)</p> Signup and view all the answers

In a Lineweaver-Burk plot, the lines for competitive inhibition intersect at which axis?

<p>y-axis (C)</p> Signup and view all the answers

What type of bond do reversible inhibitors typically use to bind with enzymes?

<p>Noncovalent bonds (B)</p> Signup and view all the answers

Which of the following statements about irreversible inhibitors is true?

<p>They bind covalently to enzymes. (A)</p> Signup and view all the answers

What is the effect of increasing substrate concentration in the presence of a competitive inhibitor?

<p>It can reverse the effect of the inhibitor. (D)</p> Signup and view all the answers

Which of the following correctly describes the effect of a non-competitive inhibitor?

<p>It decreases Vmax without affecting Km. (A)</p> Signup and view all the answers

Which type of inhibition is characterized by the formation of an enzyme-inhibitor complex that does not compete for the active site?

<p>Non-competitive inhibition (D)</p> Signup and view all the answers

What is the effect of a competitive inhibitor on the rate of formation of the enzyme-substrate complex?

<p>It decreases the formation rate. (C)</p> Signup and view all the answers

What is a key indicator that a reaction is undergoing competitive inhibition when analyzing Lineweaver-Burk plots?

<p>Lines have different slopes but intersect on the y-axis. (C)</p> Signup and view all the answers

What is the role of allosteric effectors in enzyme regulation?

<p>They bind at a site other than the active site and can alter enzyme affinity for substrate. (D)</p> Signup and view all the answers

What distinguishes a homotropic effector from a heterotropic effector?

<p>Homotropic effectors are identical to the substrate while heterotropic effectors are different. (A)</p> Signup and view all the answers

Which of the following best describes negative effectors?

<p>They inhibit enzyme activity. (C)</p> Signup and view all the answers

How does feedback inhibition regulate enzyme activity?

<p>By allowing the end-product to inhibit an earlier enzyme in the pathway. (B)</p> Signup and view all the answers

What is the primary function of protein kinases in enzyme regulation?

<p>To catalyze the addition of phosphate groups using ATP. (C)</p> Signup and view all the answers

What effect does phosphorylation generally have on enzymes?

<p>It can either increase or decrease enzyme activity. (A)</p> Signup and view all the answers

What do phosphoprotein phosphatases do in enzyme regulation?

<p>They remove phosphate groups from phosphorylated enzymes. (C)</p> Signup and view all the answers

Which statement about allosteric enzymes is true?

<p>Allosteric enzymes demonstrate cooperativity among substrates. (B)</p> Signup and view all the answers

What happens when the concentration of the end-product G increases in a metabolic pathway?

<p>It inhibits the enzyme responsible for its own synthesis. (A)</p> Signup and view all the answers

Which of the following is a limiting factor in enzyme activity regulation through phosphorylation?

<p>The concentration of ATP in the reaction. (D)</p> Signup and view all the answers

Flashcards

Non-competitive Inhibition

A type of enzyme inhibition where the inhibitor binds to a site on the enzyme different from the active site, preventing substrate binding.

Effect of Non-competitive Inhibition on Km

Non-competitive inhibitors do not affect the binding of the substrate to the enzyme, meaning the enzyme still has the same affinity for the substrate.

Effect of Non-competitive Inhibition on Vmax

Non-competitive inhibitors reduce the maximum rate at which the enzyme can catalyze a reaction (Vmax).

Lineweaver-Burk Plot for Non-competitive Inhibition

The Lineweaver-Burk plot displays the relationship between enzyme activity and substrate concentration. In non-competitive inhibition, the x-intercept (1/Km) stays the same, but the y-intercept (1/Vmax) changes, indicating a lower Vmax.

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Overcoming Non-competitive Inhibition

In non-competitive inhibition, increasing the substrate concentration cannot overcome the effect of the inhibitor because the inhibitor binds to a different site.

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Example: Heavy Metal Inhibition

Heavy metals like lead can act as non-competitive inhibitors by binding to enzymes and altering their structure. This disrupts the enzyme's function, for example, ferrochelatase, which is crucial for heme production.

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Example: Allosteric Inhibition of Hexokinase

Glucose 6-phosphate is a molecule that can bind to a site on hexokinase, an enzyme involved in glucose metabolism. This binding causes a conformational change in the enzyme, reducing its activity and slowing down the reaction.

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What is an enzyme inhibitor?

A substance that reduces the rate of an enzyme-catalyzed reaction.

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Irreversible Inhibitors

Inhibitors that form a permanent bond with the enzyme, often through covalent bonds.

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Reversible Inhibitors

Inhibitors that bind to the enzyme through non-covalent bonds, allowing them to detach and be removed.

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Competitive Inhibition

A type of reversible inhibition where the inhibitor competes with the substrate for the active site.

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Vmax

The maximum rate at which an enzyme can catalyze a reaction.

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Km

The concentration of substrate at which the reaction rate is half of Vmax.

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Lineweaver-Burk Plot

A graphical representation that plots the reciprocal of reaction velocity against the reciprocal of substrate concentration.

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How does increasing substrate concentration affect competitive inhibition?

In competitive inhibition, increasing substrate concentration can overcome the effect of the inhibitor because the substrate outcompetes the inhibitor for the active site.

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How does a competitive inhibitor affect Km?

A competitive inhibitor increases the apparent Km of the enzyme, meaning more substrate concentration is needed to reach half of Vmax.

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Allosteric Effectors

Molecules that bind to an enzyme at a site other than the active site, affecting the enzyme's activity.

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Negative Effectors

Allosteric effectors that decrease enzyme activity.

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Positive Effectors

Allosteric effectors that increase enzyme activity.

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Homotropic Effect

When the substrate itself acts as an effector, typically enhancing enzyme activity.

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Heterotropic Effect

When a molecule other than the substrate acts as an effector, regulating enzyme activity.

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Feedback Inhibition

A regulatory mechanism where the end product of a metabolic pathway inhibits an earlier enzyme in the pathway, preventing overproduction.

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Covalent Modification

The process of adding or removing phosphate groups from an enzyme, often regulating its activity.

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Protein Kinases

Enzymes that catalyze the addition of phosphate groups to proteins.

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Phosphoprotein Phosphatases

Enzymes that remove phosphate groups from proteins.

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Phosphorylation

The process of adding a phosphate group to a protein.

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Glycogen phosphorylase

An enzyme that breaks down glycogen, a stored form of glucose. Its activity is increased when phosphorylated.

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Glycogen synthase

An enzyme that synthesizes glycogen, the storage form of glucose. Its activity is decreased when phosphorylated.

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Enzyme induction

The process of increasing the production of an enzyme in response to a specific signal. This leads to more active enzyme molecules and a faster reaction rate.

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Enzyme repression

The process of decreasing the production of an enzyme in response to a specific signal. This results in fewer enzyme molecules and a slower reaction rate.

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Allosteric regulation

The regulation of enzyme activity by binding of molecules to sites other than the active site. These molecules can either activate or inhibit the enzyme's function.

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Multi-subunit allosteric enzymes

Enzymes with multiple subunits that can bind to regulatory molecules, affecting their activity. These enzymes often control rate-limiting steps in metabolic pathways.

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Rate-limiting step

The slowest step in a metabolic pathway, which determines the overall rate of the pathway. Often regulated by allosteric enzymes.

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Study Notes

Course Information

  • Faculty: Medicine
  • Academic Year: 2024-2025
  • Year: 1
  • Semester: 1
  • Module: Human Body Function (HBF) 102
  • Module Specific Topic: Enzymes II: Enzyme Inhibition & Regulation

Enzyme Inhibition

  • Enzyme inhibition occurs when a substance reduces the speed of an enzyme-catalyzed reaction.
  • Inhibitors can be irreversible (bind covalently) or reversible (bind non-covalently).
  • Reversible inhibitors are further categorized as competitive and non-competitive.

Competitive Inhibition

  • In competitive inhibition, the inhibitor competes with the substrate for the active site.
  • The inhibitor binds reversibly to the active site that the substrate would normally occupy.
  • Consequently, the substrate cannot bind.
  • Increasing substrate concentration reverses this effect
  • Competitive inhibition increases the apparent Km but does not change Vmax.
  • In a Lineweaver-Burk plot, competitive inhibition shows a change in the x-intercept, but not the y-intercept.

Non-Competitive Inhibition

  • In non-competitive inhibition, the inhibitor does not compete with the substrate for the active site.
  • It binds to a different site on the enzyme.
  • This binding changes the enzyme's shape, impacting its ability to bind the substrate.
  • Non-competitive inhibition reduces Vmax but does not change Km.
  • In a Lineweaver-Burk plot, non-competitive inhibition shows a change in the y-intercept, but not the x-intercept.
  • Heavy metals are an example of non-competitive inhibitor

Allosteric Inhibition

  • Allosteric enzymes are regulated by molecules called effectors (or modifiers).
  • Modifiers bind non-covalently at a site other than the active site.
  • This binding can alter the enzyme's affinity for its substrate or its maximal catalytic activity.
  • Positive effectors increase enzyme activity, while negative effectors decrease it.
  • Homotropic effectors are the substrate itself;
  • Heterotropic effectors are different from the substrate. Examples are feedback inhibition.

Covalent Modification

  • Many enzymes can be regulated by adding or removing phosphate groups from specific amino acid residues (serine, threonine, or tyrosine).
  • Phosphorylation reactions are catalyzed by protein kinases.
  • Dephosphorylation is catalyzed by phosphatases.
  • Phosphorylation can increase or decrease an enzyme's activity.

Induction and Repression

  • Enzymes may be regulated by the rates of their synthesis or degradation.
  • Increased or decreased synthesis affects the total number of active enzyme sites.
  • This process is slower than allosteric or covalent modification.

Interactive Question Examples

  • Correct answer: Captopril inhibition can be relieved by increasing substrate concentration.
  • Correct answer: Heavy metal enzyme inhibitors bind to different sites than the active site.

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Related Documents

Enzyme Inhibition Lecture Notes PDF

Description

Explore the concept of enzyme inhibition in this quiz focusing on competitive and non-competitive inhibitors. Understand how these inhibitors affect enzyme-catalyzed reactions, including their impact on kinetic parameters like Km and Vmax. Test your knowledge on the mechanisms and implications of enzyme regulation.

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