A protein has a molecular mass of 200 kDa as determined by gel filtration (=nati

Question

A protein has a molecular mass of 200 kDa as determined by gel filtration (=native molecular weight). When separated by SDS-PAGE (without a reducing agent present) only one polypeptide appeared with a mass corresponding to 100 kDa. When determined by SDS-PAGE in combination with a reducing agent (DTT) exactly two polypeptides appeared of sizes 40 and 60 kDa. What is the correct structure of the native protein?
Select one:
a. The native protein is made of one 40 kDA and one 60 kDA polypeptides
b. The native protein is made of two 100 kDa polypeptides
c. The native protein is made of two 40 kDA and two 60 kDA polypeptides
d. The native protein is made of two 100 kDa polypeptides, one 40 kDA and one 60 kDA polypeptides
e. None of the answers is correct.

Explanation / Answer

Answer is (c).

Native protein molecular weight = 200 kDa

Initial SDS-PAGE separation without reducuing agent = polypeptides at 100 kDa (reducing agent separates multi-subunit polypeptides into single polypeptides)

Second SDS-PAGE separation with reducing agents = polypeptides at 40 and 60 kDa

(polypeptides are now separated into individual polypeptides)

Total mass of protein = 200 kDa thus individual polypeptides must = 200 kDa

Thus,

4 total polypeptides at 40, 40, 60, 60 kDa = 200 kDa

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