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Figure 3
The crystal structure of FhuE in complex with coprogen reveals a substrate-binding site selective for planar hydroxamate siderophores. (a) Structure of FhuE coloured as a Jones' rainbow from the N-terminus (blue) to the C-­terminus (red). The transmembrane region is indicated as being within the outer membrane (OM), and extracellular loops are labelled from L1 to L11. Transposed by 90°, the lower panel shows a top-down view of FhuE. (b) Fe-coprogen co-crystallized with FhuE is shown bound to the substrate-binding pocket as a red stick model, with FhuE as a blue cartoon (top), key interacting residues as blue sticks (middle) and a white surface representation (bottom). Models of Fe-rhodotorulic acid (c) and ferrioxamine B (d) in complex with FhuE derived from in silico docking are shown with FhuE as a white surface representation. (e) The location of the coprogen trans-anhydromevalonyl group in the substrate-binding tunnel of FhuE and the dangling group of ferrioxamine B docked in an analogous position. (f) The role of Arg142 in restricting the FhuE binding pocket to planar hydroxamate siderophores. Planar Fe-coprogen is accommodated in the FhuE binding site (left), while the nonplanar sidero­phore ferrichrome is sterically excluded by Arg142 (right).

IUCrJ
Volume 6| Part 3| May 2019| Pages 401-411
ISSN: 2052-2525
https://doi.org/10.1107/S2052252519002926