If a mixed inhibitor has a Ki of 5 µM and a Ki' of 10 µM, what does this indicate about its binding affinity?

• A. The inhibitor binds equally to the free enzyme and the enzyme-substrate complex.
• B. The inhibitor has a higher affinity for the enzyme-substrate complex than for the free enzyme.
• C. The inhibitor does not bind to either the free enzyme or the enzyme-substrate complex.
• D. The inhibitor has a higher affinity for the free enzyme than for the enzyme-substrate complex.

Answer: (D)