X-ray Structural Characterization of Imidazolylcobalamin and Histidinylcobalamin: Cobalamin Models for Aquacobalamin Bound to the B₁₂ Transporter Protein Transcobalamin

The X-ray structures of imidazolylcobalamin (ImCbl) and histidinylcobalamin (HisCbl) are reported. These structures are of interest given that the recent structures of human and bovine transcobalamin prepared in their holo forms from aquacobalamin show a histidine residue of the metalloprotein bound at the β-axial site of the cobalamin (Wuerges, J. et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 4386−4391). The β-axial Co−N bond distances for ImCbl and HisCbl are 1.94(1) and 1.951(7) Å, respectively. The α-axial Co−N bond distances to the 5,6-dimethylbenzimidazole are 2.01(1) and 1.979(8) Å for ImCbl and HisCbl, respectively, and are typical for cobalamins with weak σ-donor ligands at the β-axial site. The corrin fold angles of 11.8(3)° (ImCbl) and 12.0(3)° (HisCbl) are smaller than those typically observed for cobalamins.