A 160 kDa complex of four protein molecules, consists of a dimer formed by a 25 kDa protein connected by two disulfide bonds, and three other proteins of 10, 30 and 70 kDa, respectively. It was isolated and analyzed on an SDS-PAGE gel without DTT in the gel loading dye. Which one of the following options would represent the SDS-PAGE profile?

Correct option is A

Explanation:

• The dimer formed by the 25 kDa protein will appear as a 50 kDa band on the SDS-PAGE gel, as two 25 kDa proteins are connected by disulfide bonds.
• The other proteins (10 kDa, 30 kDa, and 70 kDa) will each appear as separate bands at 10 kDa, 30 kDa, and 70 kDa, respectively.
• Therefore, there will be four bands in total: 10 kDa, 30 kDa, 50 kDa, and 70 kDa.

Information Booster:

1. The 50 kDa band represents the dimer of the 25 kDa protein.
2. The proteins with molecular weights of 10 kDa, 30 kDa, and 70 kDa will appear as distinct bands.
3. No DTT (dithiothreitol) was used in this case, meaning the disulfide bonds in the 25 kDa protein dimer remain intact, preserving the dimer's molecular weight of 50 kDa.
4. SDS-PAGE allows proteins to be separated based on their size, where larger proteins migrate more slowly through the gel.
5. This profile is typical of non-reducing SDS-PAGE, where disulfide bonds are not broken, and protein subunits held together by these bonds remain intact.

Additional Information:

Option 2: Four bands corresponding to 10, 25, 30, and 70 kDa

• Incorrect because the 25 kDa protein forms a 50 kDa dimer due to disulfide bonds, not a 25 kDa band. So, the 25 kDa band won’t appear.

Option 3: One band corresponding to 160 kDa

• Incorrect because the proteins will be separated based on their individual molecular weights (10 kDa, 30 kDa, 50 kDa, 70 kDa), not as a single 160 kDa band.

Option 4: Two bands corresponding to 50 kDa and 110 kDa

• Incorrect because there is no 110 kDa protein in the complex. The correct bands are for 10 kDa, 30 kDa, and 70 kDa proteins.