Q.37 A purified oligomeric protein was analyzed by SDS-PAGE under reducing and non-reducing
conditions. A one litre solution of 1 mg/ml concentration has
4.01 × 1018 molecules of the oligomeric protein.
Based on the data shown below, deduce the total number of polypeptide chains that
constitute this protein.
(A) 2
(B) 4
(C) 6
(D) 12
SDS-PAGE Observation
- Lane 1 (Reducing condition): Two bands at 50 kDa and 25 kDa
- Lane 2 (Non-reducing condition): Single band at ~75 kDa
Step 1: Interpretation of SDS-PAGE
Under reducing conditions, disulfide bonds are broken, revealing individual
polypeptide chains. The appearance of two bands at 50 kDa and 25 kDa indicates
two different polypeptide chains.
Under non-reducing conditions, disulfide bonds remain intact, and the protein
migrates as a single unit of 75 kDa.
Therefore, one oligomeric unit has a molecular weight of:
50 kDa + 25 kDa = 75 kDa
Step 2: Molecular Weight Calculation
Total mass of protein in 1 litre:
1 mg/mL × 1000 mL = 1000 mg = 1 g
Using Avogadro’s number:
4.01 × 1018 = (1 / M) × 6.022 × 1023
M = 6.022 × 1023 / 4.01 × 1018
M ≈ 1.5 × 105 g mol−1 = 150 kDa
Step 3: Number of Polypeptide Chains
Molecular weight from SDS-PAGE = 75 kDa (one oligomer)
Actual molecular weight = 150 kDa
Number of oligomeric units:
150 / 75 = 2
Each oligomer contains two polypeptide chains (50 kDa and 25 kDa).
Total number of polypeptide chains = 2 × 2 = 4
Final Answer
The total number of polypeptide chains constituting the protein is 4.
Correct option: (B)
Conclusion
By combining SDS-PAGE data under reducing and non-reducing conditions with
molecular weight calculations, the oligomeric protein is shown to consist
of two identical oligomeric units, each made of two polypeptide chains.
Hence, the protein contains a total of four polypeptide chains.
