A purified protein has a molecular mass of 360 kDa when measured by size exclusion chromatography. When analyzed by gel electrophoresis in the presence of SDS, three bands are observed, with molecular masses of 160, 140, and 60 kDa. When gel electrophoresis is carried out in the presence of SDS and dithiothreitol, three bands are once again observed, with molecular masses of 140, 80, and 60 kDa. What is the subunit composition of the protein?

A protein has four subunits whose molecular masses are 140, 80, and 60 kDa.

A disulfide bond links the two 80 kDa subunits (possibly identical).

Explanation:

Given that:

A protein has four subunits whose molecular masses are 140, 80, and 60 kDa.

A disulfide bond links the two 80 kDa subunits (possibly identical).

As a result of SDS and dithiothreitol analysis treatment, the molecular masses can not be 360 in total. They are 280, which implies that they are in short of 80 kDa. This means that there are possibilities that two groups with a molecular mass of 80 kDa which are joined by a disulfide bond.

The presence of SDS and dithiothreitol acts as a reducing agent, and they can break disulfide bonds whose pH is greater than 7, i.e. those in basic condition.