Q.15 An enzyme shows highest activity in the pH range 2.0 – 3.0. At pH 4.0 and pH 7.0, the enzyme exhibits 50% and 1%, respectively, of its highest activity. Which of the following states of an amino acid residue in the catalytic site is most responsible for its activity profile?
(A) A protonated Asp
(B) A deprotonated Asp
(C) A deprotonated Asn
(D) A protonated Asn
Pepsin-like enzymes exhibit peak activity at extremely low pH (2.0-3.0), where specific protonation states of catalytic residues enable acid-base catalysis. The activity drop to 50% at pH 4.0 and near-zero (1%) at pH 7.0 reflects titration of a key residue with pKa around 3-4.
Option Analysis
Enzyme pH profiles depend on the protonation state of active-site residues, governed by their side-chain pKa values (Asp ~3.9, Asn none).
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(A) Protonated Asp: Aspartic acid (pKa ≈3.9) is protonated (COOH, neutral) below pH 3.9, ideal for general acid catalysis by donating H⁺ in low-pH environments like pepsin’s Asp32/Asp215 dyad. At pH 2-3 (below pKa), it’s fully protonated for max activity; 50% at pH 4.0 (near pKa); ~1% at pH 7.0 (deprotonated COO⁻).
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(B) Deprotonated Asp: Requires pH > pKa (COO⁻ form), suiting neutral-alkaline enzymes (e.g., serine proteases), not acidic optima. Activity would rise with pH, opposite the profile.
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(C) Deprotonated Asn: Asparagine lacks ionizable side chain (neutral amide, no pKa), so no pH-dependent protonation; cannot drive sharp acidic profile.
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(D) Protonated Asn: Asn side chain (CONH₂) non-ionizable; “protonated” irrelevant, no catalysis modulation by pH.
Correct: (A) A protonated Asp matches pepsinogen-derived enzymes active at gastric pH 1.5-2.5.
Introduction to Enzyme pH Activity Profile
Enzyme pH activity profile determines catalytic efficiency via protonation states of active-site residues like protonated Asp (pKa ~3.9), crucial for acidic enzymes in CSIR NET Life Sciences. This profile peaks where key residues match substrate needs, dropping sharply outside.
Protonated Asp in Acidic Enzyme Catalysis
In enzymes with pH 2-3 optimum (e.g., pepsin), protonated Asp (COOH) acts as general acid, donating protons. One Asp protonated, one deprotonated in dyad enables hydrolysis; full deprotonation above pH 4 halves activity. Matches query: 100% at pH 2-3, 50% at 4.0, 1% at 7.0.
Why Not Deprotonated Asp or Asn?
Deprotonated Asp (COO⁻) fits neutral pH enzymes; acidic conditions protonate it, killing activity. Asn (amide) has no pKa, so no profile shift—protonated/deprotonated states irrelevant.
CSIR NET Implications
For competitive exams, recognize acidic profiles signal protonated Asp; plot log(activity) vs pH shows pKa ~3.5 inflection. Pepsin exemplifies gastric adaptation.
