. A protein has molecular mass of 400 kDa when measured by size exclusion chromatography. When subjected to gel electrophoresis in the presence of sodium dodecyl sulfate (SDS), the protein gives three bands with molecular masses of 180, 160, and 60 kDa. When electrophoresis is carried out in the presence of SDS and dithiothreitol, three bands are again formed, this time with molecular masses of 160, 90, and 60 kDa. Determine the subunit composition of the protein.

The protein has 4 subunits: 2 subunits of 90 kDa, 1 subunit of 160 kDa and 1 subunit of 60 kDa

Explanation:

In gel electrophoresis, the SDS agent produces denaturation of the protein and confers negative charge, so the protein subunits can migrate according to their masses. It produces dissociation of the protein in its subunits but it cannot disrupt disulphyde bridges (S-S) that can bond subunits together.

From the data, with SDS we observe 3 bands ⇒ 180 kDa + 160 kDa + 60 kDa

The addition of dithiotreitol (DTT), a reducing agent, produces the disruption of disulphyde bridges. From the data:

With DTT ⇒ 160 kDa + 90 kDa + 60 kDa

We observe that 160 kDa and 60 kDa subunits are conserved (they are the same as with SDS only), but 180 kDa subunit is missing and in its place appears a band of 90 kDa - a half 180 kDa.

So, the band at 180 kDa is composed by two subunits bonded by a disulphyde bridge.

Therefore, the composition of the protein is: 1 subunit of 160 kDa, 2 subunits of 90 kDa and 1 subunit of 60 kDa.