A protein was purified to homogeneity. The determination of the mass by gel‑filtration chromatography yields a value of 60 kDa. Chromatography in the presence of urea results in a 30 kDa species. Repeating the chromatography in the presence of both urea and β‑mercaptoethanol, however, results in a single molecular species of 15 kDa. What do the data suggest about the structure of the protein?

In the given case, the homogeneity of the protein is done. By performing, the technique of gel-filtration chromatography, the mass of the protein obtained is 60 kilodalton. When the chromatography is performed in the existence of urea, a protein of mass 30 kilodalton is obtained. The ratio between the two obtained mass is 60/30 = 2, thus, it shows that the protein comprises two subunits and each of them weighs 30 kilodaltons.  

When the same technique of chromatography is performed in the existence of beta-mercaptoethanol and urea, a single protein of mass 15 kilodalton is produced. Now, the ratio is 30/15 = 2. Therefore, each subunit of the protein comprises two units of mass 15 kilodalton.  

Hence, it shows that the protein is a tetramer of 15 kilodalton subunits and each subunit produces dimers of mass 30 kilodalton and is attached by disulfide bonds.