Which of the following is true of an enzyme?

A. It catalyzes a series of reactions.
B. It breaks down immediately following the reaction it catalyzes.
C. It changes the amount of energy released at the end of a reaction.
D. It binds a single set of substrates.****

D is correct

To determine which of the given choices is true of an enzyme, we can break down each option and analyze its accuracy:

A. It catalyzes a series of reactions.
- This statement is not always true of enzymes. While some enzymes can catalyze a series of reactions, others are specific to a single reaction.

B. It breaks down immediately following the reaction it catalyzes.
- This statement is not true. Enzymes themselves are not consumed or broken down during a reaction. They remain unchanged and are typically reused.

C. It changes the amount of energy released at the end of a reaction.
- This statement is true. Enzymes can affect the energy released in a reaction by lowering the activation energy required for the reaction to occur. This can speed up the reaction and increase the amount of energy released.

D. It binds a single set of substrates.
- This statement is true. Enzymes are specific to particular substrates, meaning they bind only to certain molecules (substrates) to catalyze a reaction.

Therefore, the correct statement is D. It binds a single set of substrates.