Topic 3.2 - Enzyme Catalysis

The rate of a chemical reaction in a cell is the measure of how

• often the reaction occurs.

• quickly the substrates are converted to product.

• much energy must be added for the reaction to occur.

• much activation energy is required for the reaction to occur.

• easily the reaction is inhibited.

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NGSS.HS-PS1-5

A reactant reaches the transition state

• right before the energy barrier is overcome.

• when it is ready to receive energy.

• after releasing free energy.

• when there is an input of energy to initiate the reaction.

• only when enzymes are used.

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Knowing the change in free energy (ΔG) of a reaction tells us

• the energy yield of the reaction.

• the rate of the reaction.

• the end concentration of products.

• the optimum temperature for the reaction.

• the activation energy.

The hydrolysis of sucrose to glucose and fructose is exergonic. However, if sucrose is dissolved in water and the solution is kept overnight at room temperature, there is no detectable conversion to glucose and fructose. Why?

• The change in free energy of the reaction is positive.

• The activation energy of the reaction is high.

• The change in free energy of the reaction is negative.

• This is a condensation reaction.

• The free energy of the products is higher than the free energy of the reactants.

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NGSS.HS-PS1-5

The enzyme sucrase increases the rate at which sucrose is broken down into glucose and fructose. Sucrase works by

• increasing the amount of free energy of the reaction.

• lowering the activation energy of the reaction.

• decreasing the equilibrium constant of the reaction.

• supplying energy to speed up the reaction.

• changing the shape of the active site.

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NGSS.HS-PS1-5

The graph shows the change in catalytic reaction rate for an enzyme as a function of temperature. Which statement accurately explains the reasons for the low activity at points A and B?

• At both points A and B, low activity is due to protein denaturation.

• At point A, low activity is due to reduced kinetic energies and collisions of reacting particles at low temperature, whereas low activity at point B is due to protein denaturation.

• At point A, low activity is due to protein denaturation, whereas low activity at point B is due to reduced kinetic energies of reacting particles at a high temperature.

• At both points A and B, low activity is due to temperature-induced conformational shifts in the protein that reduce the affinity of the active site for substrate.

• At both points A and B, low activity is due to reductions in flexibility of the enzyme, which limits its ability to induce strain in the substrate.

A constant amount of enzyme was exposed to increasing amounts of substrate, and the reaction rate was measured at each substrate concentration. The data were plotted in the graph above. How will the data change if the amount of enzyme present in solution was doubled and the measurements repeated?

• The data will form a curve indicating higher reaction rates at low substrate concentrations but eventually reaching the same maximum rate as the one shown.

• The data will form a curve indicating lower reaction rates at low substrate concentrations but eventually reaching the same maximum rate as the one shown.

• The data will form a curve similar to the one shown but leveling off at a maximum rate twice as high.

• The data will form a curve similar to the one shown but leveling off at a maximum rate half as high.

• The graph would not change.

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A particular enzyme catalyzes the conversion of substrate A into product B. If you wished to use this enzyme in a different reaction, the substrate with the best chance of working would be a molecule with the same

• charge as A.

• molecular weight as A.

• atomic composition but different structure as A.

• number of carbons as A.

• three-dimensional shape and charge distribution as A.

Enzyme A catalyzes a cellular reaction found in a wide range of species. The table shows data collected on the turnover numbers of Enzyme A purified from two of these species. In both cases, the same conditions were used to measure turnover numbers. Which statement is consistent with these data?

• ΔG for the two reactions is the same, but the energy of activation for the reaction catalyzed by the mollusk enzyme is lower than the energy of activation for the reaction catalyzed by the bacterial enzyme.

• Energy of activation for the two reactions is the same, but ΔG for the reaction catalyzed by the mollusk enzyme is lower than ΔG for the reaction catalyzed by the bacterial enzyme.

• ΔG for the two reactions is the same, but the energy of activation for the reaction catalyzed by the mollusk enzyme is higher than the energy of activation for the reaction catalyzed by the bacterial enzyme.

• Energy of activation for the two reactions is the same, but ΔG for the reaction catalyzed by the mollusk enzyme is higher than ΔG for the reaction catalyzed by the bacterial enzyme.

• Both energy of activation and ΔG for the reaction catalyzed by the mollusk enzyme are lower than for the reaction catalyzed by the bacterial enzyme.

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NGSS.HS-PS1-5

With the addition of the enzyme alcohol dehydrogenase, the rate of reduction of acetaldehyde to ethanol increases as the concentration of acetaldehyde is increased. Eventually, the rate of the reaction reaches a maximum, at which point further increases in the concentration of acetaldehyde have no effect. Why?

• All the alcohol dehydrogenase molecules are bound to acetaldehyde molecules.

• At high concentrations of acetaldehyde, the activation energy of the reaction increases.

• At high concentrations of acetaldehyde, the activation energy of the reaction decreases.

• The enzyme is no longer specific for acetaldehyde.

• At high concentrations of acetaldehyde, the change in free energy of the reaction decreases.