Question

determine the subunit composition of a protein from the following information:

Molecular mass by gel filtration: 250 kDa

Molecular mass by SDS-PAGE(w/oBME): 55kDa, 50kDa,45kDa

Molecular mass by SDS (wBME):50kDa,45kDa,30kDa,25kDa

Answer 1

the protein contains two 60-kD polypeptides and two 40-kD polypeptides. Each 40-kD chain is disulfide bonded to a 60-kD chain. the 100-kD units associate noncovalently to form a protein with a molecular mass of 200 kD.


Gel filtration does not affect the interaction between the various subunits in your protein.
Thus, your protein appears to be 200 kDa in size.
SDS-PAGE involves boiling the samples, and running them under denaturing conditions. This will disrupt the association between the two 100 kDa subunits.
BME is a reducing agent, that is it reduces the disulfide bonds in proteins. In other words the R1-S-S-R2 bond between the 40 and 60 kDa fragments is reduced to R1-SH and R2-SH, two separate proteins. Together with the SDS page that fully denatures and separates all the subunits of your protein.