In the absence of betamercaptoethanol nonreduced conditions

In the absence of beta-mercaptoethanol (non-reduced conditions) protein migrate on SDS-PAGE with molecular weight approx. 90 kDa. In the presence of beta-mercaptoethanol (reduced conditions) on the gel appeared only one band in molecular weight resembled to 15 kDa. Which sentence described the best structure of this protein? This could be only homohexamer This could be only heterohexamer This could be only homoheptamer This could be homohexamer or heterohexamer Explain:

Solution

Mercaptoethanol destroys RNases. Mercaptoethanol breaks disulfide bonds. Due to addition of BME in a sample buffer within SDS gel leads to separate the subunits of protein. homo refers about similar and hetero refers about different. 90KDa molecule upon BME denaturation produced a single 19KDa subunit. Homohexamer shows 6 similar monomers and heterohexamer shows 6 dissimilar monomers. This shows, the molecule has been a heterohexamer where only a single subunit has been observed as the monomers are dissimilar and the remaining monomers could be observed in a different lane within the electrophoretic unit.