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g A graduate student ran a gel filtration column and determined that a specific protein has a molecular mass of 400 kDa. The student then ran an SDS-PAGE gel of the protein under non-reducing conditions and found the presence of three bands: 180, 160, and 60 kDa. The student then ran SDS-PAGE under reducing conditions and still found three bands but the sizes were: 160, 90, and 60 kDa. Explain (1) the subunit composition of the protein, (2) the number of unique and identical polypeptides present in a single protein, (3) and why the discrepancies occur in the molecular weights.

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Answer:

1) The protein is a tetramer, i.e., it has 4 subunits

2) The protein has two identical subunits with a molecular weight of 90 kDa

3) Because this protein contains two subunits with identical molecular weight (90 + 90= 180)

Explanation:

160 kDa + 90 kDa + 60 kDa = 310 kDa >>

Subunit X = 400 kDa - 310 kDa = 90 kDa >> Two subunits with a weight of 90 kDa likely linked by disulfide bonds (i.e., non-reducing conditions can break all bonds except sulfide interactions)

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