Figure 4. Crystal structures of Tgt(Glu²³⁵Gln).

(a) Superposition of Tgt(Glu²³⁵Gln) in its apo form (crystallised at pH 5.5; the structure obtained at pH 8.5 looks identical) and in complex with preQ₀ reveals equal binding pocket geometries. (b) 2 |F_o| − |F_c| electron density map of Tgt(Glu²³⁵Gln) in complex with guanine contoured at 1.5 σ (blue). The green density represents an |F_o| − |F_c| map with the ligand omitted from the calculation contoured at 3.0 σ. (c) 2 |F_o| − |F_c| electron density map of Tgt(Glu²³⁵Gln) in complex with preQ₁ contoured at 1.5σ (blue). The green density represents an |F_o| − |F_c| map with the ligand omitted from the calculation contoured at 3.0 σ. The electron density reveals a split conformation for the Asp¹⁰² side chain indicating an uncomplete occupancy of the binding pocket with the substrate base. The Leu²³¹/Ala²³² peptide bond is present in two different conformations. The occupancy of the ligand and of the predominant Leu²³¹/Ala²³² and Asp¹⁰² conformers refine to 0.7.