Probing the stability of native and activated forms of α2-macroglobulin

α2-Macroglobulin (α2M) is a 718 kDa homotetrameric proteinase inhibitor which undergoes a large conformational change upon activation. This conformational change can occur either by proteolytic attack on an ∼40 amino acid stretch, the bait region, which results in the rupture of the four thioester bonds in α2M, or by direct nucleophilic attack on these thioesters by primary amines. Amine activation circumvents both bait region cleavage and protein incorporation, which occurs by proteolytic activation. These different activation methods allow for examination of the roles bait region cleavage and thioester rupture play in α2M stability. Differential scanning calorimetry and urea gel electrophoresis demonstrate that both bait region cleavage and covalent incorporation of protein ligands in the thioester pocket play critical roles in the stability of α2M complexes.