Determine the subunit composition of a protein from the following information: a. Molecular mass by gel filtration: 200 kD
b. Molecular mass by SDS-PAGE: 100 kD
c. Molecular mass by SDS-PAGE with 2-mercaptoethanol: 40 kD and 60 kD

Explanation:

The protein comprises of two 60-kD polypeptides and two 40-kD polypeptides. Each one of the 40-kD chains has a disulfide-bond which is directly bonded to a 60-kD chain.

The 100-kD units attach noncovalently to produce a protein with a molecular mass of 200 kD. The interaction among the different subunits in the protein is not affected by the Gel filtration chromatography.

Hence, the protein seems to be 200 kD in size. When SDS-PAGE takes place samples are being subjected to heat and therefore undergoing denaturation conditions. The aftermath effect of this causes disorganization in the 100 kD units.

However, the reducing agent (BME) main task is the reduction of disulfide bonds in a protein.

It implies that BME is responsible for the reduction between the R1-S-S-R2 bond between 40 kD and 60 kD to R1-SH and R2-SH, resulting in seperate proteins.