Practice question

Beta-mercaptoethanol is a thiol-based reducing agent present in Laemmli sample buffer to cleave both interchain and intrachain disulfide bridges by reducing cystine to cysteine sulfhydryl groups. Disulfide bonds maintain tertiary and quaternary structure, preventing complete unfolding even after SDS binding and heating. Their reduction ensures proteins become fully linearized extended polypeptides uniformly coated with SDS, allowing accurate size estimation and preventing aggregation. It does not initiate acrylamide polymerization, impart charge to proteins, or enhance Coomassie staining, which depend on APS/TEMED, SDS, and dye chemistry respectively.