Fig. 6.

Structures of BtuB in three different configurations. (a) The apo form of BtuB (PDB ID code 1NQE), where the Ton box is shown in red and the N-terminal fold is shown in yellow. (b) A structure based on the substrate-bound crystal structure (PDB ID code 1NQH), with the Ton box configuration adjusted to fit EPR-derived distance constraints (15). The substrate, vitamin B₁₂, is shown in a Corey–Pauling–Koltun representation. (c) Structure of BtuB bound to a C-terminal fragment of TonB (PDB ID code 2GSK), where the TonB fragment is shown in light gray (20).