In a laboratory setting, why might studying an enzyme at saturating substrate concentrations not provide an accurate measure of its binding affinity?

• A. Because enzyme-substrate complexes do not form at high concentrations.
• B. Because enzyme activity is too low to measure accurately.
• C. Because substrate concentration is irrelevant to enzyme function.
• D. Because all enzyme molecules are bound to substrate, masking differences in binding affinity.

Answer: (D)