Enzyme Structure and Catalysis (5E) - Chem & Physics

1. Which option best defines a prosthetic group in enzyme function?

   A tightly bound cofactor that remains associated with the enzyme. Prosthetic groups enhance enzyme function by remaining covalently or tightly bound during catalysis.

2. Which option best defines competitive inhibition in enzyme kinetics?

   Inhibitor binds active site; increases $K_m$; $V_{max}$ unchanged. Competitive inhibitors compete for the active site, requiring higher $[S]$ to achieve the same velocity.

3. What is the lock-and-key model of enzyme-substrate binding?

   Active site is preformed and complementary to the substrate. This rigid model describes perfect geometric complementarity between enzyme and substrate without conformational changes.

4. Which option best defines the active site of an enzyme?

   The specific region where substrate binds and catalysis occurs. The active site facilitates catalysis by providing a microenvironment that stabilizes the transition state through specific substrate interactions.

5. What is the catalytic triad commonly found in serine proteases?

   Serine, histidine, and aspartate. These residues work together to facilitate nucleophilic attack and proton transfer in peptide bond hydrolysis.

6. If $[S] \gg K_m$, what is the approximate value of $v_0$?

   $v_0\approx V_{max}$. High substrate concentrations saturate the enzyme, making the reaction rate independent of $[S]$.

7. What is the relationship between $\Delta G$ and enzyme catalysis for a given reaction?

   Enzymes do not change $\Delta G$ of the reaction. Catalysts like enzymes accelerate reactions without altering the overall free energy change between reactants and products.

8. What is the induced-fit model of enzyme-substrate binding?

   Substrate binding causes enzyme conformational change to fit better. This model explains how enzymes adapt their shape upon substrate binding to enhance catalytic efficiency.

9. What is the relationship between equilibrium constant $K_{eq}$ and enzyme catalysis?

   Enzymes do not change $K_{eq}$. Since enzymes do not affect $\Delta G$, they leave the ratio of products to reactants at equilibrium unchanged.

10. What is the definition of $V_{max}$ in enzyme kinetics?

    Maximum rate when enzyme is saturated with substrate. $V_{max}$ represents the enzyme's turnover rate when all active sites are occupied by substrate.

11. On a Lineweaver-Burk plot, what is the $y$-intercept equal to?

    $\frac{1}{V_{max}}$. The y-intercept corresponds to the reciprocal of the maximum velocity in the Lineweaver-Burk equation.

12. Which option best describes how enzymes increase reaction rate?

    They stabilize the transition state and provide a lower-$E_a$ pathway. Enzymes accelerate reactions by lowering the activation energy through transition state stabilization.

13. What is the definition of $K_m$ in Michaelis-Menten kinetics?

    $[S]$ at which $v_0=\frac{V_{max}}{2}$. $K_m$ reflects the enzyme's affinity for the substrate, where half-maximal velocity is achieved.

14. What is the Lineweaver-Burk (double-reciprocal) form of Michaelis-Menten?

    $\frac{1}{v_0}=\frac{K_m}{V_{max}}\frac{1}{[S]}+\frac{1}{V_{max}}$. This linear transformation allows determination of $K_m$ and $V_{max}$ from intercepts on a double-reciprocal plot.

15. Which option best defines a coenzyme in enzyme catalysis?

    An organic cofactor, often vitamin-derived, required for activity. Coenzymes act as carriers of chemical groups or electrons during enzymatic reactions.

16. What is the primary effect of an enzyme on a reaction energy diagram?

    It lowers the activation energy $E_a$. By stabilizing the transition state, enzymes reduce the energy barrier required for the reaction to proceed.

17. Which option best defines a cofactor in enzyme catalysis?

    A nonprotein helper, often a metal ion, required for activity. Cofactors assist in catalysis by participating in redox reactions or stabilizing intermediates.

18. Which option best defines pure noncompetitive inhibition in enzyme kinetics?

    Inhibitor binds allosteric site; decreases $V_{max}$; $K_m$ unchanged. Noncompetitive inhibitors reduce enzyme activity by binding elsewhere, without affecting substrate binding affinity.

19. If $[S] \ll K_m$, what is the approximate form of $v_0$?

    $v_0\approx \frac{V_{max}}{K_m}[S]$. Low substrate levels make the reaction rate directly proportional to $[S]$, following first-order kinetics.

20. What is the difference between an apoenzyme and a holoenzyme?

    Apoenzyme lacks cofactor; holoenzyme is the active complete enzyme. The apoenzyme requires the cofactor to form the functional holoenzyme for catalytic activity.

21. Which option best defines the transition state in enzyme-catalyzed reactions?

    The highest-energy, unstable configuration between reactants and products. The transition state represents the peak of the energy barrier that must be overcome for the reaction to occur.

22. At $[S]=K_m$, what is $v_0$ expressed in terms of $V_{max}$?

    $v_0=\frac{V_{max}}{2}$. At $[S]=K_m$, the enzyme operates at half its maximum capacity according to Michaelis-Menten kinetics.

23. What is the Michaelis-Menten equation for initial rate $v_0$?

    $v_0=\frac{V_{max}[S]}{K_m+[S]}$. This equation models the hyperbolic relationship between initial velocity and substrate concentration in enzyme kinetics.

24. Which option best defines uncompetitive inhibition in enzyme kinetics?

    Inhibitor binds $ES$; decreases $K_m$ and decreases $V_{max}$. Uncompetitive inhibitors bind only to the enzyme-substrate complex, altering both apparent affinity and maximum rate.

25. On a Lineweaver-Burk plot, what is the $x$-intercept equal to?

    $-\frac{1}{K_m}$. The x-intercept is the negative reciprocal of $K_m$, indicating substrate affinity in the plot.

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