Enzymes: Structure and Function Quiz
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Enzymes: Structure and Function Quiz

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Questions and Answers

How does histidine function as an acid-base catalyst in enzymatic reactions?

  • It only functions in its non-ionized form to stabilize substrates.
  • It always acts as a proton donor regardless of its ionized state.
  • It exclusively acts as a proton sink without providing protons.
  • It can serve as both a proton donor and proton acceptor depending on its ionization state. (correct)

    • Which amino acid is most likely a nucleophile in enzyme catalytic mechanisms?

  • L-Cysteine
  • Histidine
  • L-Arginine
  • L-Serine (correct)

    • What role does water serve in the enzymatic reaction involving serine as a nucleophile?

  • It participates in hydrolysis to generate the product. (correct)
  • It stabilizes the enzyme-substrate intermediate.
  • It serves as a proton donor to activate serine.
  • It acts as a substrate to form the product.

    • Which of the following best describes the simultaneous use of acid and base catalysis in enzymes?

    <p>Acid and base catalysis can enhance reaction rates and facilitate substrate interactions.</p> Signup and view all the answers

    In which situations would a serine residue be involved in the formation of covalent bonds with substrates?

    <p>When the substrate has an electrophilic character that allows nucleophilic attack.</p> Signup and view all the answers

    Which statement correctly describes the role of enzymes in catalyzing reactions?

    <p>Enzymes remain chemically unchanged after the reaction is complete.</p> Signup and view all the answers

    What is the sequence of events in enzyme action?

    <p>Substrate approaches the active site, forming a complex, which is then transformed into products.</p> Signup and view all the answers

    What happens to the activation energy in the presence of an enzyme?

    <p>It decreases, allowing reactions to occur more readily.</p> Signup and view all the answers

    What does the symbol ∆G represent in an enzymatic reaction?

    <p>The change in free energy of the reaction.</p> Signup and view all the answers

    Which property is true about enzymes in relation to substrates?

    <p>They transform substrates into products without altering themselves.</p> Signup and view all the answers

    What is the significance of the enzyme-substrate complex?

    <p>It is a temporary structure where substrate conversion occurs.</p> Signup and view all the answers

    Which of the following statements about enzyme specificity is correct?

    <p>Enzymes have a unique active site that fits specific substrates.</p> Signup and view all the answers

    Which factor does NOT affect the rate of an enzyme-catalyzed reaction?

    <p>The mass of the enzyme in grams</p> Signup and view all the answers

    Which amino acid acts as a nucleophile in the mechanism for chymotrypsin?

    <p>Serine (Ser)</p> Signup and view all the answers

    What role does Histidine play in the mechanism of chymotrypsin?

    <p>It acts as a proton donor.</p> Signup and view all the answers

    Which of the following statements about chymotrypsin's mechanism is false?

    <p>The active site of chymotrypsin is exclusively hydrophobic.</p> Signup and view all the answers

    What is the nature of the bond formed between the enzyme and the substrate during chymotrypsin's action?

    <p>Covalent bond</p> Signup and view all the answers

    In the chymotrypsin mechanism, which component is responsible for the formation of the tetrahedral intermediate?

    <p>The serine hydroxyl group</p> Signup and view all the answers

    Which of the following describes the ultimate fate of the tetrahedral intermediate formed during the chymotrypsin reaction?

    <p>It collapses to release the first product.</p> Signup and view all the answers

    Which amino acid primarily stabilizes the transition state during chymotrypsin's catalytic cycle?

    <p>Aspartate (Asp)</p> Signup and view all the answers

    During which step in the chymotrypsin mechanism does the enzyme undergo a significant conformational change?

    <p>Formation of the acyl-enzyme intermediate</p> Signup and view all the answers

    What defines a reversible enzyme inhibitor?

    <p>Blocks substrate access to the active site</p> Signup and view all the answers

    Which statement about competitive inhibition is true?

    <p>It can be reversed by increasing substrate concentration.</p> Signup and view all the answers

    How does sulfanilamide act as a competitive inhibitor?

    <p>By mimicking the structure of p-aminobenzoic acid</p> Signup and view all the answers

    Which type of inhibitor does not affect enzyme-substrate binding positively?

    <p>Non-competitive inhibitor</p> Signup and view all the answers

    What role does p-aminobenzoic acid (PABA) play in the action of sulfanilamide?

    <p>Inhibits folic acid biosynthesis when coadministered</p> Signup and view all the answers

    Which statement about non-competitive inhibitors is correct?

    <p>They bind only to free enzymes, not enzyme-substrate complexes.</p> Signup and view all the answers

    What is a common characteristic of reversible enzyme inhibitors?

    <p>They can be displaced by excessive substrate.</p> Signup and view all the answers

    Which of the following is an example of a reversible enzyme inhibitor?

    <p>Sulfa drugs</p> Signup and view all the answers

    What is the consequence of an allosteric inhibitor binding to an enzyme?

    <p>It changes the shape of the active site so that the substrate can no longer bind.</p> Signup and view all the answers

    Which characteristic defines transition-state inhibitors compared to regular substrates?

    <p>They mimic the transition state and bind more strongly than substrates.</p> Signup and view all the answers

    What role do renin inhibitors play in the body?

    <p>They act as antihypertensives by blocking the synthesis of angiotensin I and II.</p> Signup and view all the answers

    Which statement about the induced fit model in enzymes is correct?

    <p>The active site changes shape to better fit the substrate.</p> Signup and view all the answers

    What happens to the active site when an allosteric inhibitor binds?

    <p>It distorts and becomes unrecognizable to the substrate.</p> Signup and view all the answers

    Why can transition states not be isolated or synthesized?

    <p>They are high-energy, transient species.</p> Signup and view all the answers

    What is a key property of allosteric inhibitors in relation to substrate concentration?

    <p>Their inhibition is unaffected by substrate concentration.</p> Signup and view all the answers

    What feature distinguishes the design of transition-state inhibitors from substrate analogs?

    <p>Transition-state inhibitors mimic the stereochemistry and properties of reaction intermediates.</p> Signup and view all the answers

    Study Notes

    Enzymes: Structure and Function

    • Enzymes are special types of receptors that interact with substrates, forming complexes.
    • Unlike other receptors, enzymes catalyze reactions, transforming substrates into products.
    • Most enzymes are soluble and found in the cytosol of cells.

    Enzymatic Reactions

    • Steps:
      • Substrate approaches the active site.
      • Enzyme-substrate complex forms.
      • Substrate transforms into products.
      • Products are released.
      • Enzyme is recycled.

    Examples:

    • Sucrase: Catalyzes the breakdown of sucrose into glucose and fructose.
    • Protein Tyrosine Kinase: Catalyzes the phosphorylation of tyrosine residues in proteins using ATP as a phosphate source.

    Enzyme Properties:

    • Activation energy: Amount of energy needed to disrupt a stable molecule before the reaction can occur.
    • Enzymes lower the activation energy, but the free energy change (ΔG) remains the same.
    • Equilibrium constant (K) = [Product]/[Reactant]

    Catalysis Mechanisms:

    • Nucleophilic residues: Enzymes can use nucleophilic amino acid side chains (e.g., Serine or Cysteine) in the active site to form covalent bonds with the substrate.
    • Acid/base catalysis: Enzymes can utilize acid and base catalysis simultaneously, utilizing the protonation and deprotonation properties of amino acid residues like histidine.

    Example: Chymotrypsin Mechanism

    • Chymotrypsin utilizes a catalytic triad consisting of Serine, Histidine, and Aspartate.
    • The mechanism involves a series of steps involving nucleophilic attack, proton transfer, and release of the product.

    Enzyme Inhibitors:

    • Reversible Enzyme Inhibitors: Slow down or block enzyme catalysis by reversibly binding to the active site.
    • Competitive inhibition: Inhibitor competes with the substrate for binding to the active site.
    • Non-competitive inhibition: Inhibitor binds to a site other than the active site, altering the enzyme's shape and preventing substrate binding.
    • Example: Sulfonamides are competitive inhibitors of dihydrofolate synthetase, an enzyme involved in folic acid biosynthesis.
    • Irreversible Inhibitors: Bind irreversibly to the active site, permanently inactivating the enzyme.
    • Allosteric Inhibitors: Bind to a site other than the active site (allosteric site), causing a conformational change that alters the active site, preventing substrate binding.

    Transition-State Inhibitors:

    • Designed to mimic the transition state of an enzyme-catalyzed reaction.
    • Since transition states are high-energy and transient, transition-state inhibitors bind more strongly than substrate or product analogs.
    • Example: Renin inhibitors, which mimic the transition state of the reaction catalyzed by renin, are used as antihypertensives to lower blood pressure.

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    Description

    Test your knowledge on enzymes, their structure, function, and the enzymatic reaction process. This quiz covers key concepts like substrate interaction, activation energy, and examples of specific enzymes like sucrase and protein tyrosine kinase. Perfect for students studying biochemistry or related subjects.

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