AQA A-Level Chemistry Paper 3, 2023: Question 35

Question

Which statement about enzymes is not correct?

• A: The tertiary structure of an enzyme influences which molecules can bind to the active site.
• B: The action of enzymes can be inhibited by a molecule or ion that binds to the active site.
• C: Enzymes work equally well on both optical isomers of a substrate.
• D: Computers can be used to design drugs to block active sites on enzymes.

Correct Answer: C

C: Enzymes work equally well on both optical isomers of a substrate. This is incorrect because enzymes are highly specific to the structure of their substrate, including its chirality (optical isomerism). Enzymes typically catalyse reactions for only one optical isomer.

Explanation

Enzymes are biological catalysts that are specific to their substrates due to the unique shape of their active sites. This specificity includes stereospecificity, meaning enzymes generally act on only one enantiomer (optical isomer) of a chiral substrate.

• A: Correct. The tertiary structure of an enzyme determines the shape of its active site and its specificity for substrates.
• B: Correct. Competitive inhibitors bind to the active site, preventing the substrate from binding.
• C: Incorrect. Enzymes are stereospecific and usually work on only one optical isomer of a chiral substrate.
• D: Correct. Modern drug design involves computational methods to model enzyme active sites and design molecules that can bind to them, acting as inhibitors.

Teacher Tip: Highlight the concept of enzyme specificity and stereospecificity when discussing enzyme activity. Provide examples of how enzymes distinguish between enantiomers, such as L- and D-amino acids.