Question 1 · 3.1 Enzymes
Enzyme amount, temperature, and pH were held constant across all seven measurements. Which explanation best accounts for the initial rate leveling off near 50 micromoles per minute?
- A Nearly every enzyme molecule is occupied, so extra substrate waits its turn
- B The enzyme has begun to unfold at the highest substrate concentrations tested
- C Product has built up enough to run the reaction backward at the same speed
- D The substrate has been used up, so further product formation has stopped
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A — At low substrate, most enzyme molecules are idle at any moment, so doubling substrate roughly doubles the rate. As substrate climbs, the fraction of enzyme that is busy climbs too, until essentially every molecule is occupied and the rate is set by how fast the enzyme can cycle rather than by how often it meets substrate. That ceiling is the maximum rate, and it depends on the amount of enzyme, which was deliberately held constant here. Note the doubling from 0.5 to 1.0 mM (9 to 17) and the near-absence of a change from 16 to 32 mM (50 to 50) - the same experiment in its two regimes.
Question 2 · 3.2 Environmental Impacts on Enzyme Function
Assays of a purified enzyme run at 30 °C in buffer containing increasing amounts of ethanol show steadily lower activity, and activity remains low after the ethanol is removed by dialysis. Which effect of ethanol best accounts for the result?
- A Ethanol occupies the active site in place of substrate and is slowly converted into a product
- B Ethanol weakens the attractions that keep nonpolar side chains buried, and the protein unfolds
- C Ethanol breaks the covalent bonds that link the enzyme's amino acids into a single chain
- D Ethanol chills the solution as it evaporates, holding the assay far below the enzyme's optimum temperature
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B — A folded enzyme keeps its nonpolar side chains packed in an interior core, away from water. Ethanol is far less polar than water, so it makes the surrounding solution a more comfortable place for those side chains and the core loosens — the protein opens up and the active site is lost. The dialysis result is the decisive clue: something that merely occupied a site or altered the solution would let activity return once removed, and lasting loss after the ethanol is gone points to a structural change the protein cannot undo.
Question 3 · 3.3 Cellular Energy
Removing the third phosphate group from ATP by hydrolysis makes free energy available to a cell. Which account of that release fits the chemistry of the molecule?
- A The bond to the third phosphate is unusually strong, and breaking it gives off energy
- B The adenine base gives up electrons as the third phosphate group leaves the molecule
- C The products are more stable than ATP, whose crowded phosphate groups repel one another
- D The third phosphate group carries stored energy that is set free when the bond opens
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C — Free energy released is a difference between reactants and products, never a quantity parked inside one bond. ATP carries three negatively charged phosphate groups packed next to one another, and like charges repel, which holds the molecule at high free energy. Hydrolysis separates them: ADP and inorganic phosphate are each stabilized by hydration and by spreading their charge over their own oxygen atoms. Because the products are more stable than the reactant, the overall reaction gives off free energy. Note the order of events inside that reaction, because it is where the common picture goes wrong: breaking the bond itself takes energy in, and the greater stability of the products more than repays it.