Final answer:
In a pH=7 environment, the most advantageous amino acid to have in the vicinity of the active site aspartic acid would be glutamic acid (option b).
Step-by-step explanation:
In a pH=7 environment, the most advantageous amino acid to have in the vicinity of the active site aspartic acid would be glutamic acid (option b).
Glutamic acid has a pKa value of approximately 4.07, which means that at pH 7, it will predominantly exist in its deprotonated form (-COO-). This makes it a suitable candidate to donate a proton to the aspartic acid, aiding in the acid catalysis of the conversion of a ketone to an enol by the enzyme.
Furthermore, the close proximity of glutamic acid to the aspartic acid in the active site allows for efficient proton transfer between the two amino acids, facilitating the catalytic process.
If aspartic acid with a pKa of 3.90 is being used by an enzyme for catalysis in a pH=7 environment, it is advantageous for the amino acid histidine to be in the vicinity of the active site. Histidine has a side chain that can function at physiological pH as a proton donor or acceptor, which is essential for acid-base catalysis.
This property allows histidine to participate in proton transfer, which is critical for the conversion of a ketone to an enol. Considering the role of histidine in proton transfer and its ability to facilitate the conversion of a C=O group into a -OH group, as well as activating the C=C bond of the enol for nucleophilic attack, it fits the requirement for the reaction in question.