How Coordination Chemistry Controls Reactivity in the B-12 Trafficking Protein CbIC

CblC is involved in an early step in cobalamin (or B 12 ) processing following entry of the cofactor into the cytoplasm. CblC catalyzes glutathione‐dependent dealkylation of alkylcobalamin and reductive decyanation of cyanocobalamin (CNCbl). Recently, we found that Caenorhabdiitis elegans CblC ( ce CblC) exhibits a robust thiol oxidase activity, converting GSH to GSSG accompanied by O 2 scrubbing. This futile oxidative cycle is suppressed in wild‐type human CblC (hCblC) but significant in two of the pathogenic mutants, R161G and R161Q. Based on our biochemical and structural analyses, we have proposed GS‐Co(II) is a key intermediate formed in the catalytic cycle of the thiol oxidase activity of ce CblC. In this study, we aimed to capture GS‐Co(II) intermediate using biophysical tools. But instead, both EPR and MCD experiments revealed Cl‐Co(II) intermediate. Further biochemical assays suggest thiol‐oxidase activity of ce CblC is dependent on Cl − or Br − that is present in the reaction buffer. Based on our new findings, we propose that coordination of the cobalt atom in B 12 controls the thiol oxidase activity of CblC, and ligand relay (H 2 O to Cl − to GS − ) on Co(II) is a key step in the mechanism. Support or Funding Information This work was supported in part by the National Institutes of Health (DK45776) This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .