Protein X comprises four equal-sized subunits: alpha, beta, delta, and gamma, bound by noncovalent interactions or disulfide bridges. Only beta and delta subunits are linked by disulfide bridges. In an SDS-PAGE experiment using the reducing agent beta-mercaptoethanol, how many bands would you expect to see? a. 1 b. 4 c. None d. 2 e. 3

 In an SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis) experiment which includes beta-mercaptoethanol (a reducing agent), you would expect to see 4 bands. Beta-mercaptoethanol reduces disulfide bridges (S-S bonds) between cysteine residues, which would separate the beta and delta subunits that are linked by these disulfide bridges. Since Protein X is composed of four different subunits of equal size and since the noncovalent interactions are also disrupted by SDS (a detergent included in SDS-PAGE to denature proteins and give them negative charge), each of these subunits would separate into individual polypeptides and migrate through the gel independently, forming a band for each subunit. This means that one would observe a single band for the alpha, beta, gamma, and delta subunits, making a total of 4 bands.