Enzyme Kinetics and Regulation (1A) - Bio & Biochem

1. Which option describes a feedback inhibitor in a metabolic pathway?

   End product inhibits an earlier enzyme, often the first committed step. Feedback inhibition regulates pathways by allosterically inhibiting upstream enzymes to prevent overproduction of end products.

2. State the relationship between $V_{max}$, $k_{cat}$, and total enzyme $[E]_T$.

   $V_{max}=k_{cat}[E]_T$. V_max scales with total enzyme concentration, as k_cat is the turnover rate per enzyme molecule at saturation.

3. What is the definition of the turnover number $k_{cat}$?

   Catalytic cycles per enzyme per second at saturation. k_cat quantifies an enzyme's efficiency by measuring product formation rate per active site when fully saturated.

4. What change in $K_m$ and $V_{max}$ is caused by pure noncompetitive inhibition?

   $V_{max}$ decreases; $K_m$ unchanged. Noncompetitive inhibitors bind elsewhere, reducing effective enzyme concentration and thus V_max, without altering Km.

5. Identify where an uncompetitive inhibitor binds relative to substrate binding.

   Only to the $ES$ complex (not to free enzyme). Uncompetitive inhibition requires substrate-bound enzyme, forming a dead-end complex that lowers apparent Km and V_max.

6. State the Michaelisnten equation for initial velocity $v$.

   $v=rac{V_{max}[S]}{K_m+[S]}$. The equation describes hyperbolic kinetics, where velocity approaches V_max as [S] increases, based on enzyme-substrate binding equilibrium.

7. What is the approximate rate when $[S]K_m$?

   $vV_{max}$. Enzyme saturation at high [S] limits the rate to V_max, independent of further increases in substrate concentration.

8. In a Lineweavernten plot, what is the $y$-intercept equal to?

   $rac{1}{V_{max}}$. The y-intercept in the Lineweaver-Burk plot corresponds to infinite [S], where velocity equals V_max.

9. What is the defining feature of irreversible inhibition in enzyme kinetics?

   Covalent or extremely tight binding permanently inactivates enzyme. Irreversible inhibitors form stable bonds with the enzyme, reducing active enzyme concentration permanently.

10. What is the approximate rate law when $[S]K_m$?

    $vrac{V_{max}}{K_m}[S]$. This linear approximation derives from the Michaelis-Menten equation when [S] is much less than Km, simplifying to a first-order rate.

11. What is the definition of $V_{max}$ in Michaelis–Menten kinetics?

    Maximum initial rate at saturating $[S]$. V_max is achieved when all enzyme active sites are saturated with substrate, reflecting the enzyme's maximum catalytic capacity.

12. Which inhibition type is best overcome by increasing $[S]$ (substrate concentration)?

    Competitive inhibition. Increasing [S] can outcompete competitive inhibitors for the active site, restoring V_max unlike other inhibition types.

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

    Substrate concentration where $v=V_{max}$. Km represents enzyme-substrate affinity, as it is the [S] at which half-maximal velocity is achieved in the Michaelis-Menten model.

14. What change in $K_m$ and $V_{max}$ is caused by uncompetitive inhibition?

    $K_m$ decreases; $V_{max}$ decreases. Uncompetitive inhibitors bind only to ES complex, decreasing both Km and V_max by stabilizing the complex.

15. What does a Hill coefficient $n_H>1$ indicate about substrate binding?

    Positive cooperativity. A Hill coefficient greater than 1 signifies enhanced binding affinity after initial substrate attachment in multimeric enzymes.

16. What is catalytic efficiency, and how is it written mathematically?

    $rac{k_{cat}}{K_m}$. Catalytic efficiency measures how well an enzyme converts substrate to product at low [S], combining turnover and affinity.

17. What is the key kinetic hallmark of allosteric enzymes versus Michaelisnten enzymes?

    Sigmoidal $v$ versus $[S]$ (cooperativity). Allosteric enzymes exhibit cooperative binding, yielding a sigmoidal curve unlike the hyperbolic Michaelis-Menten kinetics.

18. What is the effect of phosphorylation on enzyme activity in general terms?

    Covalent modification that can increase or decrease activity. Phosphorylation adds a phosphate group via kinases, modulating enzyme conformation to either activate or inhibit catalysis.

19. In a Lineweavernten plot, what is the slope equal to?

    $rac{K_m}{V_{max}}$. The slope reflects the ratio of Km to V_max, indicating how quickly velocity approaches maximum as [S] increases.

20. Which condition on $[S]$ makes the rate approximately zero-order in substrate?

    $[S]K_m$. At high [S] relative to Km, the enzyme is saturated, making velocity independent of [S] and zero-order in substrate.

21. Identify where a competitive inhibitor binds relative to the substrate binding site.

    Active site (mutually exclusive with substrate). Competitive inhibitors mimic substrate structure, binding reversibly to the same site and preventing substrate access.

22. What change in $K_m$ and $V_{max}$ is caused by competitive inhibition?

    $K_m$ increases; $V_{max}$ unchanged. Competitive inhibitors compete for the active site, increasing apparent Km but not affecting V_max at high [S].

23. Which condition on $[S]$ makes the rate approximately first-order in substrate?

    $[S]K_m$. At low [S] relative to Km, velocity is directly proportional to [S], following first-order kinetics in the Michaelis-Menten approximation.

24. In a Lineweavernten plot, what is the $x$-intercept equal to?

    $-rac{1}{K_m}$. The x-intercept occurs where 1/v = 0, solving to -1/Km in the Lineweaver-Burk equation.

25. State the Lineweavernten (double-reciprocal) form of the Michaelisnten equation.

    $rac{1}{v}=rac{K_m}{V_{max}}rac{1}{[S]}+rac{1}{V_{max}}$. The double-reciprocal plot linearizes Michaelis-Menten kinetics, allowing determination of Km and V_max from slope and intercepts.

Study these flashcards