A scientist runs an SDS-PAGE experiment two separate times on a protein with a m

Question

A scientist runs an SDS-PAGE experiment two separate times on a protein with a molecular weight of 200 kD. For the first experiment he treats the protein with SDS only and for the second experiment he treats the protein with both SDS and a reducing agent. If the first gel yields one band showing a molecular weight of 200 kD and the second run yields one band showing a molecular weight of 25 kD, the researcher should conclude that:

Select one:
a. The consists of 2 subunits of equal size connected by a disulfide linkage.
b. The protein has only 1 subunit because both runs yield a single band.
c. The consists of 8 subunits of equal size connected by a disulfide linkage. Correct
d. The protein contains no disulfide linkages.
e. The protein contains only covalent bonds and no non-covalent interactions.

Please explain the correct answer! (C)

Explanation / Answer

The correct option is C. The protein consists of 8 subunits of equal size connected by a disulfide linkage.

SDS is a detergent which when treated with protein disrupts the hydrophobic interactions among the amino acids and masks the protein with its negative charge.It changes the native structure of the protein into a linearized form.But it does not break any covalent bonds.So the protein when treated with SDS we get only one band of 200kD as the primary structure remains intact.

In the 2nd experiment, he treats the protein with both SDS and a reducing agent.SDS works in the same above way but the reducing agent cleaves the disulfide bonds present in the protein and as a result cleaves the covalent bond(disulfide) which is responsible for holding all the 8 subunits of protein together.So when treated with the reducing agent, it produces fragments each of 25 kD.As the total molecular weight of the protein is 200 kD and we get only one band of 25kD so there must be 8 subunits of equal size which were held by disulfide bonds.

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