Enzyme interactions Catechol oxidase causes the browning seen when fruits and ve
ID: 205126 • Letter: E
Question
Enzyme interactions
Catechol oxidase causes the browning seen when fruits and vegetables like apples, bananas, and potatoes are damaged or cut. Catechol oxidase oxidizes catechol to form benzoquinone. A compound called phenylthiourea (PTU) is able to bind to catechol oxidase. You perform an experiment to test the effect of PTU on the enzymatic reaction.
You set up enzyme reactions in 2 test tubes. In tube 1, you add 11 ml water + 1 ml potato juice (contains catechol oxidase), while in tube 2 you add 10 ml water + 1 ml potato juice (contains catechol oxidase) + 1 ml PTU. After adding 1 ml catechol to both tubes, you observe a color change (browning) in tube 1 and no color change in tube 2.
Enzyme binding
The results of your test tubes suggest which of the following?
A) PTU is a colorless chemical.
B) PTU is an inhibitor of catechol oxidase.
C) PTU is a cofactor of catechol oxidase.
D) PTU oxidizes catechol.
Enzyme inhibition
You set up a third test tube containing 8 ml water, 1 ml PTU, 1 ml potato juice, and 3 ml catechol. If this third test tube showed no change, this would suggest which of the following?
A) The enzyme has denatured.
B) PTU is a competitive inhibitor of catechol oxidase.
C) The catechol is not working properly.
D) PTU is a noncompetitive inhibitor of catechol oxidase.
You set up a third test tube containing 8 ml water, 1 ml PTU, 1 ml potato juice, and 3 ml catechol. If this third test tube turned brown, this would suggest which of the following?
A) PTU is a competitive inhibitor of catechol oxidase.
B) PTU is a noncompetitive inhibitor of catechol oxidase.
C) The catechol is not working properly.
D) The enzyme has denatured.
Explanation / Answer
1. Test tube 1 which does not have PTU shows browning while test tube 2 which has PTU does not show browning indicates that PTU acts as an inhibitor of catchol oxidase enzyme. PTU binds to the enzyme there by inhibits its activity. Hence option of B is a correct answer.
2. Third test tube which has increased concentration of substrate i.e., catechol (3 ml against 1ml). Still there is no browning in the test tube. It indicates that increasing concentration of substrate do not have any effect on enzyme activity. It means that binding sites of substrate and inhibitor are different on enzyme. It denotes that PTU is a non-competative inhibitor of enzyme catechol oxidase. Hence option D is correct answer.
3. Third test tube wherein increased concentration of substrate i.e catechol turns the solution into brown. It indicates that increasing concentration of substrate reverses the effect of PTU on enzyme. More substrate concentration compared to inhibitor reverses the effect of inhibitor by removing it from active site. It is because binding sites of inhibitor and substrate are same which is true for competative inhibition. Hence option A is correct answer.
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