101. A protein sample isolated from natural source was electrophoresed on a non-reducing SDSPAGE. 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?
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.

Detailed Explanation:

When analyzing proteins using gel electrophoresis, SDS-PAGE and native-PAGE serve different purposes:

• SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis) denatures proteins and imparts a uniform negative charge, allowing them to migrate solely based on molecular weight.

• Native-PAGE preserves the protein’s native conformation and quaternary structure, meaning migration is influenced by size, shape, and charge.

Given Observation:

• On non-reducing SDS-PAGE, the protein migrated as a 60 kDa band.

• On native-PAGE, the same protein migrated as a 120 kDa band.

Inference:

This difference in migration strongly suggests that the protein exists as a dimer in its native form (120 kDa) but dissociates into monomers (60 kDa each) under SDS treatment.

Importantly, since non-reducing SDS-PAGE was used (i.e., no reducing agent like DTT or β-mercaptoethanol was added), disulfide bonds would remain intact if present. The fact that the protein still runs as 60 kDa under these conditions indicates that:

• There are no disulfide bonds between the subunits.

• The dimeric structure is maintained non-covalently (e.g., hydrophobic or electrostatic interactions), and is disrupted by SDS but not stabilized by disulfide bridges.

Correct Answer:

A. The protein is a dimer of 60 kDa subunits that are not linked with disulfides.

Understanding this distinction helps in studying protein quaternary structure and stability under different biochemical environments.