Exercises: Enzyme Structure, Function, and Environmental Effects

Define substrate, active site, and activation energy. Then explain in one or two sentences how an enzyme speeds up a reaction without being used up.

Most human enzymes work best near 37 °C and a pH near 7.4, while pepsin in the stomach works best near pH 2. Explain what happens to the shape and function of a typical human enzyme if it is heated well above its optimum temperature. Name the term used for this change.

Compare competitive and noncompetitive inhibitors. For each, state where it binds on the enzyme and whether adding more substrate can overcome its effect.

An enzyme's activity is measured at different temperatures, with all other conditions held constant.

(a) Identify the approximate optimum temperature. (b) Explain why the rate rises from 10 °C to 40 °C. (c) Explain why the rate falls after 40 °C.

A fixed amount of enzyme is mixed with different concentrations of substrate.

(a) Explain why the rate increases at low substrate concentrations. (b) Explain why the rate levels off at 100. (c) Predict what would happen to the maximum rate if the amount of enzyme were doubled.

Two enzymes were tested at different pH values. Relative activity (% of maximum) is shown.

One enzyme comes from the stomach and the other from the small intestine. Decide which is which, and explain at the molecular level why a change in pH changes enzyme activity.

A researcher measures reaction rate ($\mu$mol/min) at several substrate concentrations with no inhibitor, with inhibitor A, and with inhibitor B. The same amount of enzyme is used each time.

(a) Identify which inhibitor is competitive and which is noncompetitive, citing evidence from the data. (b) Explain, using the structure of the enzyme, why a very high substrate concentration nearly overcomes one inhibitor but not the other. (c) Predict which kind of inhibitor an allosteric site would produce.

An exergonic reaction has an activation energy of 75 kJ/mol without a catalyst and 40 kJ/mol with an enzyme. The overall free energy change is $-20$ kJ/mol. (a) Explain what the enzyme changes and what it does not change about this reaction (consider activation energy, $\Delta G$, and the equilibrium position). (b) A student plans to test how temperature affects the enzyme amylase using starch and iodine solution. Describe the independent variable, the dependent variable, two variables to hold constant, and one appropriate control (for example, one using boiled enzyme). (c) Predict the results at 0 °C, 37 °C, and 90 °C and justify them.