Question

A protein contains three 60-kD polypeptides and six 20-kD polypeptides. Each 60-kD chain is disulfide bonded to two 20-kD chains. The 100-kD units associate noncovalently to form a protein with a molecular mass of 300 kD. Which of the followings is correct?

a) Molecular mass determined by gel filtration is 300 kD
b) Molecular mass determined by SDS-PAGE without 2-mercaptoethanol is 100 kD
c) Molecular masses determined by SDS-PAGE with 2-mercaptoethanol are 20kD and 60 kD
d) PAGE shows one single band

Answer 1

The correct answer is that the molecular masses determined by SDS-PAGE with 2-mercaptoethanol are 20 kD and 60 kD. 2-mercaptoethanol breaks the disulfide bonds, allowing for separation of the polypeptides. Therefore, correct option is c.

Step-by-step explanation:

The molecular mass of the protein complex described is 300 kD, and the configuration involves disulfide bonds and non-covalent associations between polypeptides. In terms of determining the molecular mass of the protein:

Molecular mass determined by gel filtration would indeed be 300 kD because this technique measures the native state of the protein complex.

Molecular mass determination by SDS-PAGE without 2-mercaptoethanol will not be 100 kD because the disulfide bonds between the 60-kD and 20-kD subunits remain intact, representing the single 300 kD protein complex.

Molecular masses determined by SDS-PAGE with 2-mercaptoethanol are indeed 20 kD and 60 kD since 2-mercaptoethanol would reduce the disulfide bonds, separating the subunits.

PAGE would not show a single band in either case because different electrophoretic techniques would resolve the polypeptides differently based on whether they are in their native form or denatured.

The correct answer to the provided question is c) Molecular masses determined by SDS-PAGE with 2-mercaptoethanol are 20kD and 60 kD. This is because 2-mercaptoethanol breaks the disulfide bonds, allowing the component polypeptides to be separated based on their molecular weight during SDS-PAGE.