Enzymes

Key Points:

• Enzymes catalyze reactions by lowering the activation energy.
• They are not changed or consumed during the reaction.
• Enzymes affect the kinetics, not the thermodynamics (ΔG remains unchanged).
• Reactions will occur with or without enzymes, just slower without.

MCAT Practice Problems:

Problem 1:

• True statement: Enzymes lower activation energy but remain unchanged. They affect the kinetics but not the thermodynamics of a reaction.
• Answer: D (The free energy of the catalyzed reaction is the same as for the uncatalyzed reaction).

Problem 2:

• False statement about enzyme kinetics:
• An increase in substrate concentration leads to a proportional increase in reaction rate only initially until the active sites are occupied.
• Answer: A (An increase in the substrate concentration leads to proportional increase in the rate of reaction).

Problem 3:

• Enzyme requiring a non-protein molecule:
• Enzyme without the necessary cofactor is called an apoenzyme (catalytically inactive).
• Answer: B (Apoenzyme).

Problem 4:

• Enzyme specificity:
• Determined by the three-dimensional shape of its active site.
• Answer: A (Three-dimensional shape of its active site).

Problem 5:

• Enzymes increase reaction rates by:
• Decreasing the activation energy.
• Answer: A (Decreasing the activation energy).

Problem 6:

• Substrate C as an allosteric inhibitor:
• Negative feedback: product inhibits a function slowing down the pathway.
• Negative feedback determination:
• Answer: D (Negative feedback).

Problem 7:

• Substrate concentration vs. reaction velocity graph:
• As substrate concentration increases, the change in rate becomes small, indicating Vmax is near.
• Vmax is near 100 millimole/sec.
• KM ≈ 0.5 millimolar.

Problem 8:

• Reaction catalyzed by enzyme A:
• KM = 5 x 10^-6 M, Vmax = 20 millimole/min.
• At concentration of 5 x 10^-6 M, reaction rate = half of Vmax (10 millimole/min).
• Answer: A (10 millimole/min).

Problem 9:

• High substrate concentration (5 x 10^-4 M):
• Enzyme is near its Vmax.
• Answer: A (20 millimole/min).

Problem 10:

• Viral enzyme activity with inhibitors:
• Lineweaver-Burk plot assesses competitive and non-competitive inhibition.
• Tamiflu (competitive inhibitor) increases KM; Relenza (non-competitive inhibitor) changes Vmax.
• Answer: B (Tamiflu increases the KM value for this substrate compared to Relenza).

Problem 11:

• ATP to cyclic AMP catalysis by:
• Lyases break single molecules into two without water/electron transfer.
• Answer: C (Lyases).

Problem 12:

• Not a method by which enzymes decrease activation energy:
• Breaking bonds irreversibly.
• Answer: D (Breaking bonds in the enzyme irreversibly).

Problem 13:

• Cooperative enzyme with 4 subunits, 2 bound to substrate:
• Affinity changes with substrate binding/dissociation.
• Affinity with four bound substrates highest, unbound lowest.
• Answer: D (The affinity of the enzyme for the substrate is greater than with one substrate bound).

Additional Notes:

• Ensure thorough understanding of enzyme function, kinetics, and specificity.
• Review different types of inhibition: competitive, non-competitive, and feedback mechanisms.
• Practice interpreting enzyme kinetics graphs and plots.