Question

A titratable group is in the active site of an enzyme and must be protonated for the enzyme to function. Assume that the group has a pKa of 6.5 and that the activity is proportional to the fraction of molecules in the protonated state. Which of the following are true?

1. At pH 9.0 close to 100% of the enzyme molecules are active.
2. At pH 9.0 close to 100% of the enzyme molecules are inactive.
3. At pH 6.5 the enyzme is 50% active.
4. At pH 6.8 the enzyme is 66.6% inactive.5. At pH 7.2 there are about 5 inactive enzymes for every active enzyme.

Answer 1

The correct option is 3. "At pH 6.5 the enyzme is 50% active"

Step-by-step explanation:

For the titratable group to be protonated and cause the enzyme to be in the active state, it needs to have gained a hydrogen cation (H+). In order for that to happen, there must be enough hydrogen cations in the environment of the enzyme, and hence, an acidic pH is required in this case.