**Deducing Quaternary Structure via SDS-PAGE**SDS-PAGE is a convenient method for separating polypeptides solely based on size. Smaller polypeptides travel faster through the gel than larger ones; the rate of migration through the gel is inversely proportional to the logarithm of the molecular weight.The subunit structure of a multimeric protein can be deduced using this technique in conjunction with a protein cross-linking agent. Cross-linking agents react with amino acid residues of two polypeptides that are in contact, thereby linking them by covalent bonds. After limited treatment with the reagent, so that some but not all subunits become cross-linked to their neighbors, the protein is subjected to SDS-PAGE, and the molecular weights of the resulting proteins (bands) are estimated.The results of such experiments on two proteins (Protein A and Protein B) are shown below. What is the most likely subunit structure of each protein?**Graph Descriptions:**- **Protein A**: - Without cross-linking: Bands appear at 70,000 and 35,000 molecular weight (MW). - With cross-linking: A single band appears at 105,000 MW. - **Protein B**: - Without cross-linking: Bands appear at 40,000 and 15,000 MW. - With cross-linking: Bands appear at 55,000, 40,000, and 15,000 MW.**Interpretation:**For Protein A, the data suggests it likely consists of two subunits with molecular weights of 70,000 and 35,000 that, when cross-linked, form a single unit with a total molecular weight of 105,000. For Protein B, it appears to consist of subunits with molecular weights of 40,000 and 15,000. The partial cross-linking results in additional band formation at 55,000 MW, suggesting the formation of a dimer involving both subunits. This analysis helps deduce the quaternary structure of the proteins by identifying their subunit composition and the interactions between them.

SDS-PAGE is a chromatographic technique that is used to separate proteins based on their molecular weight. SDS is an anionic detergent that denatures the proteins and give them equal charge. In the SDS-PAGE, proteins with high molecular weight sieve slowly through the pores of the gel matrix, while proteins with low molecular weight sieve quickly through the pores of the gel matrix. In the case of protein A, the result of SDS-PAGE in the absence of crosslinking agent gives a single band, while in the presence of crosslinking agent there are three bands. In the presence of the crosslinking agent, the band that has separated at the bottom region has a molecular weight of 35,000 KDa. Since it is mentioned that the crosslinking agent will not cross link all the subunits and in the absence of crosslinking agent there is a single band for all the subunits, it can be assumed that, the molecular weight of a single subunit of protein A is 35,000KDa. The next band has a molecular weight of 70,000KDa, which is double the molecular weight of the first band. This indicates that the cross-linking agent has linked two subunits with equal molecular weights together. The next band has a molecular weight of 105,000KDa, which is the sum of molecular weights of first and second bands. That is, the third band is composed of three subunits with a molecular weight 35,000KDa. Thus, protein A is a homotrimer. That is, protein A is composed of 3 subunits with equal molecular weights. In the case of protein B, the result of SDS-PAGE in the absence of crosslinking agent gives two bands, while in the presence of crosslinking agent there are three bands. Since two bands with two different molecular weights are observed in the absence of crosslinking agent, it can be confirmed that there are two subunits with two different molecular weights. In the presence of the crosslinking agent, the initial two bands have the same molecular weight of the bands that are observed in the absence of crosslinking agent. This indicates that, there are two different subunits with two different molecular weights and the crosslinking agent did not link these subunits together at the region of first two bands. The third band in the presence of crosslinking agent has a molecular weight that is the sum of the molecular weights of first two bands. Thus, it can be confirmed that protein B is a heterodimer. Answer: Protein A is a homotrimer Protein B is a heterodimer