The H+-induced dissociation of human plasma α2-macroglobulin

The dissociation of the tetrameric α2-macroglobulin molecule into two half-molecular fragments, which occurs at pH < 4.5, has been investigated using the small-angle neutron scattering method, and test of trypsin binding activity. Best fit with the relative forward scattering of neutrons is obtained for a model where the dissociation of the protein is driven by the uptake of H+ on altogether four acid-base groups, one per monomeric subunit of α2-macroglobulin. These groups are not (or only slightly) accessible in the native tetramer, but become exposed to the solvent after dissociation of the protein. The H+-binding constant obtained for these groups, after dissociation of the protein, log K1 in the range 4.2–4.5, suggests that they are most probably carboxylate groups. From the about 10% increase in the radius of gyration, which occurs when lowering the pH from 4.5 to 2.0, we can conclude that the dissociation is associated with a change in structure of the protein. Tests of trypsin binding show that there is also an irreversible loss in trypsin binding activity, which is directly related to the fraction of dissociated protein. Thus, at pH <4.5, there is a transition of α2-macroglobulin which results simultaneously in dissociation, disorganisation of the conformation of the subunits and loss in activity.