Increased β‑Sheet Dynamics and D–E Loop Repositioning Are Necessary for Cu(II)-Induced Amyloid Formation by β‑2-Microglobulin

β-2-Microglobulin (β2m) forms amyloid fibrils in the joints of patients undergoing dialysis treatment as a result of kidney failure. One of the ways in which β2m can be induced to form amyloid fibrils in vitro is via incubation with stoichiometric amounts of Cu­(II). To better understand the structural changes caused by Cu­(II) binding that allow β2m to form amyloid fibrils, we compared the effect of Ni­(II) and Zn­(II) binding, which are two similarly sized divalent metal ions that do not induce β2m amyloid formation. Using hydrogen/deuterium exchange mass spectrometry (HDX/MS) and covalent labeling MS, we find that Ni­(II) has little effect on β2m structure, despite binding in the same region of the protein as Cu­(II). This observation indicates that subtle differences in the organization of residues around Cu­(II) cause distant changes that are necessary for oligomerization and eventual amyloid formation. One key difference that we find is that only Cu­(II), not Ni­(II) or Zn­(II), is able to cause the cis–trans isomerization of Pro32 that is an important conformational switch that initiates β2m amyloid formation. By comparing HDX/MS data from the three metal-β2m complexes, we also discover that increased dynamics in the β-sheet formed by the A, B, D, and E β strands of the protein and repositioning of residues in the D–E loop are necessary aspects of β2m forming an amyloid-competent dimer. Altogether, our results reveal new structural insights into the unique effect of Cu­(II) in the metal-induced amyloid formation of β2m.