Design of a protein scaffold with a selective, Bi-containing heterodinuclear metal coordination motif

The design of proteins with selective metal-binding properties to accommodate distinct metal ions remains a challenge in the field of artificial metalloprotein design. Here, we report our design approach to incorporate heterodinuclear metal coordination motifs along the interface of a trimeric protein assembly, termed ^H77CTriCyt2, that enables the selective binding of the post-transition metal ion Bi(III) and a first-row transition metal ion (M(II)). The coordination sites are composed of soft tris-Cys donors and intermediate/hard tris-His centers that are placed proximally to one another within the highly stable, preorganized trimeric architecture of ^H77CTriCyt2 to allow for the formation of a heterodinuclear center. We obtained crystal structures (at 2.5 Å resolution or better) of the ^H77CTriCyt2 trimer complexed with a first-row transition metal (Mn(II), Co(II), Ni(II), Cu(II), or Zn(II)) that binds at the tris-His site, and a ^H77CTriCyt2 structure complexed with Bi(III) at the tris-Cys site, thereby confirming the selectivity of metal binding motifs as designed. Additionally, we obtained the crystal structures of BiCo, BiNi, and BiZn heterodinuclear proteins, and demonstrated through solution experiments that the trimeric state of ^H77CTriCyt2 is maintained upon the concomitant addition of Bi(III) and various M(II) species. Structural analyses of the mononuclear and heterodinuclear ^H77CTriCyt2 structures further revealed different Bi(III) binding geometries in each case, which indicated that the preferential coordination geometries of the transition metal ions can influence the Bi(III) coordination geometry and that subtle structural changes within the protein structure may promote or disfavor Bi(III) binding.