Cobalamin-binding site in CobDH. The cobalamin and the conserved residues Asp100, His102 and Ser147 are shown as sticks (C atoms coloured grey) and are labelled using the single-letter amino-acid codes for clarity. Asp100 hydrogen-bonds to His102 (2.7 Å distance), which in turn is coordinated to the Co atom of the cofactor from the lower axial site (2.5 Å distance). The side chain of Ser147 forms a hydrogen bond to an N atom of the dimethylbenzimidazole (DMB) of the cobalamin (2.7 Å distance). The interactions described are indicated with green dotted lines. The blue mesh corresponds to a 2Fo − Fc map with the cobalamin and the key residues omitted from map calculations. The map was contoured at 2.0σ.