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. Author manuscript; available in PMC: 2017 Nov 8.
Published in final edited form as: Nat Chem Biol. 2017 May 8;13(7):730–736. doi: 10.1038/nchembio.2376

Figure 2. Crystal structures of Tet(50), Tet(51), Tet(55), and Tet(56) reveal a conserved architecture, structural changes that enable substrate loading channel accessibility, and two conformations of the FAD cofactor.

Figure 2

(a) Overlay of the Tet(50) monomer A, Tet(50) monomer B, Tet(51), Tet(55), and Tet(56) crystal structures. The FAD-binding domain (salmon), the tetracycline-binding domain (pale green), the first (cyan) and second (deep teal) C-terminal α-helixes, and FAD molecules (orange) are shown. b-d Surface representation of (b) Tet(50) monomer A with the substrate-loading channel closed, (c) Tet(50) monomer B with the substrate-loading channel open, and (d) a previously published structure of Tet(X) with chlortetracycline (yellow) bound – PDB ID 2Y6R. e-g Zoomed in view of (e) the closed substrate-loading channel in Tet(50) monomer A (f) the open substrate-loading channel in Tet(50) monomer B, and (g) the wide open substrate-binding site in Tet(X). (h) The FAD cofactor adopts the IN conformation in Tet(50) monomer A, characterized by a 12.3 Å distance between the C8M and C2B atoms of the FAD molecule (i) The FAD cofactor adopts the OUT conformation in Tet(50) monomer B, characterized by a 5.2 Å distance between the C8M and C2B atoms of the FAD molecule. (j) The IN conformation of FAD allows for substrate catalysis. The OUT conformation of FAD allows for regeneration of the reduced FAD for the next round of catalysis. The green area indicates the substrate-binding site. The pink area indicates the putative NADPH binding site.