Q.30 Gel filtration profile and corresponding activity data for a pure enzyme are shown in the figure below.
The same enzyme sample on SDS-PAGE runs as a 30 kDa polypeptide.
Which one of the following is the correct interpretation of the data?
(A) Both monomer and dimer are active
(B) Enzyme is active only as a monomer
(C) Protein does not form dimers
(D) Enzyme is active only as a dimer

Correct Answer: (B) Enzyme is active only as a monomer

Understanding the Question

A pure enzyme sample was analyzed using:

• Gel filtration chromatography, showing:

  • One protein peak at ~60 kDa

  • Another protein peak at ~30 kDa

• Enzyme activity profile, plotted alongside absorbance

• SDS-PAGE, where the enzyme runs as a 30 kDa polypeptide

You are asked to interpret which molecular form of the enzyme is catalytically active.

Key Observations from the Data

1. SDS-PAGE Result

• SDS-PAGE separates proteins into individual polypeptide chains

• The enzyme appears as a 30 kDa band
  ✅ Therefore, 30 kDa is the molecular weight of one subunit (monomer)

2. Gel Filtration (Size-Exclusion Chromatography)

• Native (non-denatured) protein separation

• Peaks observed at:

  • 60 kDa → likely dimer (2 × 30 kDa)

  • 30 kDa → monomer

3. Enzyme Activity Profile

• Enzyme activity coincides only with the 30 kDa peak

• The 60 kDa peak shows little or no activity

🔑 Conclusion: Catalytic activity is associated with the monomeric form only

Correct Interpretation

✅ (B) Enzyme is active only as a monomer

• The 30 kDa monomer shows enzyme activity

• The 60 kDa dimer is enzymatically inactive

• SDS-PAGE confirms the enzyme’s subunit size

Explanation of All Options

(A) Both monomer and dimer are active ❌

• Incorrect

• If true, enzyme activity would be seen at both 30 kDa and 60 kDa peaks

• The activity profile shows activity only at 30 kDa

(B) Enzyme is active only as a monomer ✅ (Correct)

• Activity overlaps exclusively with the 30 kDa peak

• SDS-PAGE confirms the monomeric molecular weight

• Dimer formation likely causes loss of active-site geometry

(C) Protein does not form dimers ❌

• Incorrect

• Gel filtration clearly shows a 60 kDa species

• This corresponds to a dimeric form in solution

(D) Enzyme is active only as a dimer ❌

• Incorrect

• The 60 kDa peak does not show enzyme activity

• Activity is absent where the dimer elutes

Final Takeaway

✔ The enzyme exists in both monomeric and dimeric forms in solution
✔ Only the monomer (30 kDa) is catalytically active
✔ Dimerization likely inactivates the enzyme