Final answer:
Arginine would be most advantageous to have near the active site histidine in a pH 5 environment, as it would be in a form capable of donating a proton to ensure the histidine remains in its basic form for effective catalysis.
Step-by-step explanation:
If histidine (pKa=6.04) is being used by an enzyme that catalyzes the conversion of a ketone to an enol by base catalysis, which of the following would be most advantageous to have in the vicinity of the active site histidine in a pH=5 environment?
The most advantageous would be an amino acid that can donate a proton to the histidine, to ensure it is in its basic form which is necessary for the catalysis.
At pH 5, histidine is more likely to accept a proton and be in its protonated form, as its pKa is slightly above 6, which is less favourable for base catalysis.
Glutamic acid (option b) and aspartic acid (option e), both have side chains with carboxyl groups that can donate protons.
However, their pKas are too low (< 5) which means in a pH=5 environment, they are deprotonated and will not be in their acidic form. Arginine (option d), with its guanidine functional group, has a pKa well above 6, meaning it is more likely to be in its protonated form at pH 5 and thus can donate a proton.
Therefore, arginine would be the most advantageous to have in the vicinity of the active site histidine in a pH 5 environment to facilitate the enzyme-mediated conversion of a ketone to an enol.