An unusual Co-S bond links B

Chaperones in trafficking pathways ensure specificity of transition metal loading and protection against adventitious side reactions. In B12, an essential cofactor for humans, a cobalt ion is coordinated to a >1.3 kDa tetrapyrrolic scaffold, posing logistical challenges for its translocation. In this study, we report the 3.4 Å crystal structure of the human MMACHC and MMADHC B12 chaperones, tethered via a rarely seen covalent cobalt-sulfur bond. B12 is bound in the base-off state to MMACHC, with Cys-261 on MMADHC serving as the upper axial ligand; the lower-axial position is vacant. The propensity of thiolato-cobalamin derivatives bound to MMACHC to undergo spontaneous decomposition via general acid catalysis or reduction/oxidation chemistry is averted in the interprotein complex with MMADHC. An exposed face in the complex suggests an exit route for B12. No known clinical variants localize to the interprotein interface, consistent with the cobalt-sulfur bond being key to forming the high-affinity complex.