9. Gel electrophoresis. The total mass of a protein containing quaternary struct

Question

9. Gel electrophoresis. The total mass of a protein containing quaternary structure was measured by centrifugation, using the technique of sedimentation equilibrium. Under physiological conditions its total mass is 140,000 g/mol. The same protein complex was analyzed by SDS-PAGE in the absence or presence of the reducing agent . mercaptoethanol (BME); see the results in lanes A and B in the figure below. (Note that lane C shows a standard of known protein masses.) ID /19_%) From these results, describe the native protein complex, in terms of the kinds of subunits present, the stoichiometry of the subunits, and the kinds of bonding (covalent, non-covalent) existing between the subunits.

Explanation / Answer

Ans: From the SDS gel electrophoresis, it is clear that the protein has two subunits; one of mass 47,000 Da and other of mass 30,000 Da. The two subunits are held together by covalent disulphide bonds, which are cleaved by beta mercaptoethanol in the SDS gel electrophoresis.

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