Fig. 3: Structure of MttB-MttC complex.

Overall structure of MttB-MttC hetero-dodecamer viewed along (a) the threefold axis, and (b) the twofold axis. MttB subunits colored as in Fig. 1a and shown as surface, corrinoid cofactors shown as red stick and sit in the active site groove of MttB subunits. MttC subunits are shown in ribbon and colored by subunit. c Each MttC subunit interacts with a MttB dimer pair in the MttB-MttC complex. MttC colored by region: helical cap domain (residues 2–84) in red, core domain (residues 96–216) in pink, connecting loop (residues 85–95) in orange, and corrinoid cofactor shown as gray stick. One MttB subunit (cyan) interacts with the MttC core domain and corrinoid cofactor, while the second MttB subunit (blue) interacts with both the MttC cap and core. d View of active site of MttB-MttC highlighting important residues and their hydrogen-bonding interactions. The nitrogen of His107 is covalently bound to cobalt. The hydrogen-bonded triad (His107, Asp105, and Thr158) in MttC are shown in stick and labeled. The hydrogen bonds between cobamide (gray stick) and MttB (cyan ribbon), as well as between glycerol (yellow stick) and pyrrolysine (red stick) are shown as dashed black lines.