# DBT JRF 2022 — Question 22

Exam: DBT JRF 2022 · Section: B · Topic: Biochemistry › Protein Characterization - SDS-PAGE & Native PAGE · Difficulty: Medium

## Question

A protein sample isolated from natural source was electrophoresed on a non-reducing SDS-PAGE. It migrated as a band corresponding to 60 kDa. When the same protein was electrophoresed on a native-PAGE its migration corresponded to a 120 kDa marker protein. What is a reasonable inference?

## Options

- **A.** The protein is a dimer of 60 kDa subunits that are not linked with disulfides.
- **B.** The protein has two 60 kDa subunits that are linked with disulfides.
- **C.** No inference concerning protein size can be drawn since proteins don't run according to their masses in native PAGE.
- **D.** The protein is unusually rich in aspartic and glutamic acid.

## Answer

Option C

## Explanation

SDS-PAGE and native-PAGE interpretation: Observations: • Non-reducing SDS-PAGE: single band at 60 kDa — SDS denatures protein and separates subunits; all subunits are 60 kDa, non-reducing conditions leave disulfides intact but SDS disrupts non-covalent interactions. • Native-PAGE: migration corresponds to 120 kDa protein — under native conditions, the protein's quaternary structure is preserved. Conclusion: The protein is a DIMER of two 60 kDa subunits (120 kDa total), held together by NON-COVALENT interactions (not disulfides), because: • Non-reducing SDS still gives 60 kDa (if disulfides held the dimer, non-reducing SDS would give 120 kDa). • If it were disulfide-linked, non-reducing SDS-PAGE would show 120 kDa. Answer: The protein is a dimer of 60 kDa subunits that are NOT linked by disulfides (Option 1).

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