Which statement about enzymes and substrates is true?

Which statement about enzymes and substrates is true?

• A. The substrate forms a covalent bond at the active site
• B. The enzyme is acted upon by the substrate
• C. Enzymes are specific about the substrates they can act upon
• D. If the product is present, the enzyme is not acting on the substrate

Answer: (C)

Enzymes are specific about the substrates they can act upon because their active sites have shapes and chemical environments that fit only certain molecules. This precise fit, either described by the lock-and-key idea or the induced-fit idea, aligns the substrate’s reactive groups and stabilizes the transition state, which lowers the activation energy and speeds up the reaction. The enzyme binds the substrate to form an enzyme–substrate complex, then releases the product and is ready to catalyze more substrate molecules. Most interactions at the active site are noncovalent, not permanent covalent bonds, which is why the substrate is released as product and the enzyme remains unchanged. The idea that the enzyme is acted upon by the substrate is incorrect—the substrate binds to the enzyme, not the other way around. And the mere presence of product doesn’t mean the enzyme isn’t acting on substrate; the enzyme continues to catalyze as long as substrate is available. This specificity is what makes the statement true.

Enzymes are specific about the substrates they can act upon because their active sites have shapes and chemical environments that fit only certain molecules. This precise fit, either described by the lock-and-key idea or the induced-fit idea, aligns the substrate’s reactive groups and stabilizes the transition state, which lowers the activation energy and speeds up the reaction. The enzyme binds the substrate to form an enzyme–substrate complex, then releases the product and is ready to catalyze more substrate molecules. Most interactions at the active site are noncovalent, not permanent covalent bonds, which is why the substrate is released as product and the enzyme remains unchanged. The idea that the enzyme is acted upon by the substrate is incorrect—the substrate binds to the enzyme, not the other way around. And the mere presence of product doesn’t mean the enzyme isn’t acting on substrate; the enzyme continues to catalyze as long as substrate is available. This specificity is what makes the statement true.