Overall structure of CobDH (cartoon representation) with bound methylcobalamin. (a, b) The N-terminal helix-bundle domain is shown in red and the C-terminal Rossmann-fold domain is shown in orange. The central Co atom of the cofactor (shown as sticks; C atoms coloured grey) is coordinated to His102 as the lower axial ligand. The black dotted line connects the domains to each other and represents the missing linker region for which no electron density was observed (residues 83–89). (c) Domain interactions between the helix-bundle domain (red) and the cobalamin-binding domain (orange). Asp54 and Asn198 form a hydrogen bond; Met46, Leu70 and Met112 are involved in hydrophobic interactions.