Test: Enzymes - 1 - MCAT MCQ

Enzymes catalyze reactions by lowering their activation energy, and are not changed or consumed during the course of the reaction. While the activation energy is lowered, the free energy of the reaction, ΔG, remains unchanged in the presence of an enzyme. A reaction will continue to occur in the presence or absence of an enzyme; it simply runs slower without the enzyme, eliminating choice (A). Most physiological reactions are optimized at body temperature, 37°C, eliminating choice (B). Finally, dehydrogenases catalyze oxidation–reduction reactions, not transfer reactions, eliminating choice (C).

An enzyme devoid of its necessary cofactor is called an apoenzyme and is catalytically inactive.

Enzymes increase the rate of a reaction by decreasing the activation energy. They do not affect the overall free energy, ΔG, of the reaction.

By limiting the activity of enzyme 1, the rest of the pathway is slowed, which is the definition of negative feedback. Choice (A) is incorrect because there is no competition for the active site with allosteric interactions. While many products do indeed competitively inhibit an enzyme in the pathway that creates them, this is an example of an allosterically inhibited enzyme. There is not enough information for choice (B) to be correct because we aren’t told whether the inhibition is reversible. In general, allosteric interactions are temporary. Choice (C) is incorrect because it is the opposite of what occurs when enzyme 1 activity is reduced.

Based on the graph, when the substrate is present, Tamiflu results in the same v_max and a higher K_m compared to when no inhibitor is added. These are hallmarks of competitive inhibitors. Noncompetitive inhibitors result in decreased v_max and the same K_m as the uninhibited reaction, which is shown by the Relenza line in the graph. Because the question is only comparing the values between the two inhibitors, and not the enzyme without inhibitor, the mechanism of inhibition is less important to determine than the values of K_m and v_max. This is a great example of why previewing the answer choices works well in the sciences.

Lyases are responsible for the breakdown of a single molecule into two molecules without the addition of water or the transfer of electrons. Lyases often form cyclic compounds or double bonds in the products to accommodate this. Water was not a reactant, and no cofactor was mentioned; thus lyase, choice (C), remains the best answer choice.

Cooperative enzymes demonstrate a change in affinity for the substrate depending on how many substrate molecules are bound and whether the last change was accomplished because a substrate molecule was bound or left the active site of the enzyme. Because we cannot determine whether the most recent reaction was binding or dissociation, choices (A) and (B) are eliminated. We can make absolute comparisons though. The unbound enzyme has the lowest affinity for substrate, and the enzyme with all but one subunit bound has the highest. The increase in affinity is not linear, and therefore choice (C) is not necessarily true. An enzyme with two subunits occupied must have a higher affinity for the substrate than the same enzyme with only one subunit occupied; thus, choice (D) is correct.

Enzymes are not altered by the process of catalysis. A molecule that breaks intramolecular bonds to provide activation energy would not be able to be reused.

Triglycerides are unlikely to act as coenzymes for a few reasons, including their large size, neutral charge, and ubiquity in cells. Cofactors and coenzymes tend to be small in size, such as metal ions like choice (C) or small organic molecules. They can usually carry a charge by ionization, protonation, or deprotonation. Finally, they are usually in low, tightly regulated concentrations within cells. Metabolic pathways would be expected to include both oxidation–reduction reactions and movement of functional groups, thus eliminating choices (B) and (D).

The rate of reaction increases with temperature because of the increased kinetic energy of the reactants, but reaches a peak temperature because the enzyme denatures with the disruption of hydrogen bonds at excessively high temperatures. In the absence of enzyme, this peak temperature is generally much hotter. Heating a reaction provides molecules with an increased chance of achieving the activation energy, but the enzyme catalyst would typically reduce activation energy. Keep in mind that thermodynamics and kinetics are not interchangeable, so we are not considering the impact of heat on the equilibrium position.