Question

Sample problem: Protein structure • A protein gives two bands of 50 and 80 kD on SDS PAGE. Treatment with 2-mercaptoethanol gives two bands of 50kD and 40kD. Give detailed information about this protein including its molecular weight.

Answer 1

PAGE (Polyacrylamide Gel Electrophoresis), is an analytical method used to separate components of a protein mixture based on their size. The technique is based upon the principle that a charged molecule will migrate in an electric field towards an electrode with opposite sign.The general electrophoresis techniques cannot be used to determine the molecular weight of biological molecules because the mobility of a substance in the gel depends on both charge and size. To overcome this, the biological samples needs to be treated so that they acquire uniform charge, then the electrophoretic mobility depends primarily on size. For this different protein molecules with different shapes and sizes, needs to be denatured(done with the aid of SDS) so that the proteins lost their secondary, tertiary or quaternary structure .The proteins being covered by SDS are negatively charged and when loaded onto a gel and placed in an electric field, it will migrate towards the anode (positively charged electrode) are separated by a molecular sieving effect based on size. After the visualization by a staining (protein-specific) technique, the size of a protein can be calculated by comparing its migration distance with that of a known molecular weight ladder(marker).

β-mercaptoethanol  is suitable for reducing protein disulfide bonds prior to polyacrylamide gel electrophoresis and is usually included in a sample buffer for SDS-PAGE at a concentration of 5%. Cleaving intermolecular (between subunits) disulfide bonds allows the subunits of a protein to separate independently on SDS-PAGE.

First, there two different bands(50 kD and 80 kD) obtained. Which indicates two subunits in the protein.

After treatment with β-mercaptoethanol gives two different bands but this time 50 kD and 40kD. Which indicates 80 kD subunit containing a disulfide bond at the center, so mercaptoethanol cleaves the disulfide bond and gives two equal 40 kD subunits.