Characterization of a B12 Trafficking Chaperone Protein From Caenorhabditis elegans

The human B12 trafficking chaperone protein hCblC is responsible for escorted delivery and early processing of B12 in intracellular B12 metabolism. In this study, we characterized a putative B12 trafficking chaperone of Caenorhabditis elegans (cCblC), which shows 26% amino acid sequence identity with hCblC. cCblC was shown to bind B12 with a broad specificity for the upper axial ligand, as previously observed with other homologous proteins. In addition, cCblC catalyzed glutathione (GSH)-dependent elimination of alkyl and GSH upper axial ligands from alkylcobalamins and glutathionylcobalamin (GSCbl), respectively. Dealkylation of methylcobalamin (MeCbl) generated cob(II)alamin with S-methylglutathione. Cob(I)alamin was detected as the intermediate for cob(II)alamin generation, indicating that the reaction is a nucleophilic displacement using the thiolate of GSH. Deglutathionylation of GSCbl also generated cob(II)alamin, via cob(I)alamin intermediate, with glutathione disulfide, indicating the reaction is chemically analogous with dealkylation. Cob(II)alamin generated by dealkylation and deglutathionylation was bound to cCblC in the base-off state and stable under aerobic conditions, which would be favorable for subsequent enzyme cofactor synthesis. These results demonstrate that cCblC is a B12 trafficking chaperone of C. elegans catalyzing dealkylation and deglutathionylation via a nucleophilic displacement using the thiolate of GSH.