Final answer:
A histidine residue (pKa=6.00) would be most advantageous in the vicinity of aspartic acid at pH 7 for enzyme catalysis, as it can act as a proton donor. Therefore, correct option is a.
Step-by-step explanation:
If aspartic acid (pKa=3.90) is required by an enzyme to catalyze the conversion of a ketone to an enol by acid catalysis at a pH of 7, the proximity of certain amino acid residues can be beneficial due to their pH-dependent properties.
At pH 7, the aspartic acid will be deprotonated and carry a negative charge, which may not be ideal for acid catalysis which requires a proton donor. Out of the options listed:
- Histidine residue (pKa=6.00) - Near optimum pH, with some fraction in protonated form able to donate a proton.
- Lysine residue (pKa=10.50) - Will be mostly protonated but has a much higher pKa, acts as a base.
- Glutamic acid residue (pKa=4.07) - Similar to aspartic but slightly better proton donor at pH 7.
- Cysteine residue (pKa=8.18) - Mostly deprotonated and thus not an effective acid catalyst at pH 7.
Considering the need for an amino acid that can act as a proton donor, a histidine residue would be the most advantageous to have in the vicinity of the active site aspartic acid in a pH 7 environment, given its pKa is closest to the working pH and a significant proportion will be in the protonated form, capable of acid catalysis.