Thank you! 4. You are studying an enzyme, USFase. Your data shows that active an
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Thank you! 4. You are studying an enzyme, USFase. Your data shows that active and pure USFase has a molecular weight of 80 kDa. Below are the SDS-polyacrylamide gels you obtained for pure USFase. Which of the following is the best explanation for these data? Gel 1 No BME Gel 2 with ??? 100kDa |- 80 kDa 60 kDa 100 kDa 40 kDa 25 kDa 60kDa |- 40 kDa 25 kDa Note: BME stands for B-mercaptoethanol. A. USFase is a dimer with subunits that are completely identical B. USFase is a dimer with subunits that are different. C. USFase is a dimer with subunits that are held together by an ester linkage between an aspartate in one subunit attached to a serine in the other subunit. D. USFase is a dimer with subunits that are held together by a disulfide linkage between cysteines on each, respective subunit. E. USFase is a dimer with subunits that are held together by an ionie pair, a lysine in one subunit attached to an aspartate in the other subunit.Explanation / Answer
2-mercaptoethanol is a chemical compound used in biological studies to primarily reduce disulfide linkages to give back the cysteine residues. From the first gel without beta-ME it is observed that the molecular mass of USFase is 80kDa. In the next SDS-PAGE, when a compound that selectively reduces disulfide linkages is used, the resulting USFase lane gives a band at 40kDa instead of 80.
Since we know that USFase is a dimer, which itself implies that the two subunits are nearly identical in their molecular masses and the mass difference of exactly half relative to the first gel, the options A and B are nonsense. Now armed with the knowledge that it is 2-mercaptoethanol that has caused this cleavage of dimers and the fact that it selectively reduces disulfide bonds, one can find that options C and E are also false. Thus the best explanation is given by option D which states the precise inference that the dimers of USFase are held by disulfide bonds between subunits.
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