Question

A protein was purified to homogeneity. Determination of the mass by gel-filtration chromatography yields 60 kDa. Chromatography in the presence of 6 M urea yields a 30-kDa species. When the chromatography is repeated in the presence of 6 M urea and 10 mM β-mercaptoethanol, a single molecular species of 15 kDa results. Which of the following best describes the structure of the protein?

A. a dimer of 30 kDa subunits linked by disulfide bonds

B. a tetramer of 15 kDa subunits with all subunits linked to each other by disulfide bonds

C. a tetramer of 15 kDa subunits with two subunits forming 30 kDa dimers through disulfide bonds

D. a tetramer of 30 kDa subunits linked by disulfide bonds

E. none of the above

Answer 1

The correct answer is option C
C. a tetramer of 15 kDa subunits with two subunits forming 30 kDa dimers through disulfide bonds

A protein was purified to homogeneity. Determination of the mass by gel-filtration chromatography yields 60 kDa. Chromatography in the presence of 6 M urea yields a 30-kDa species.
SInce 60/30 = 2, this suggest presence of two subunits with each one having 30 kDa mass/

When the chromatography is repeated in the presence of 6 M urea and 10 mM β-mercaptoethanol, a single molecular species of 15 kDa results.
30/15= 2
THus each subunit contains 2 units of 15 kDa mass.