121k views
3 votes
If aspartic acid (pKa=3.90) is being used by an enzyme that catalyzes the conversion of a ketone to an enol by acid catalysis; which of the following would be most advantageous to have in the vicinity of the active site aspartic in a pH=7 environment?

a) A histidine residue (pKa=6.00)
b) A lysine residue (pKa=10.50)
c) A glutamic acid residue (pKa=4.07)
d) A cysteine residue (pKa=8.18)

1 Answer

4 votes

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.

User Akoumjian
by
7.7k points
Welcome to QAmmunity.org, where you can ask questions and receive answers from other members of our community.