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![[Figure 4]](cb5108fig4mag.jpg)
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Figure 4
(a) Apo TtGH8 ribbon structure (coloured from blue to red) with the overall surface transparent, clearly showing the classical (α/α)6 fold. (b) TtGH8–xylotriose complex, showing xylotriose bound in sites −3 to −1, with the corresponding maximum-likelihood-weighted Fo − Fc `difference' electron density, calculated prior to any incorporation of ligands in refinement, at a contour level of 0.35 e Å−3 (approximately 2.5σ). The catalytic residues from the apo form (light blue) and X3 complex (pink) are shown to highlight the position of the scission site and the shift in the catalytic acid (Glu73). The nucleophilic water is shown as a blue sphere, with bonding distances to Asp281 and the catalytic cleavage site (were the substrate longer) depicted. (c) TtGH8 D281N–X6 complex shown as a surface (coloured by electrostatic potential), in which xylohexaose can be observed spanning the deep binding cleft. (d) TtGH8 D281N–X6 complex shown with xylohexaose spanning the six binding positions −3 to +3 with the associated electron density (contour level 0.35 e− Å−3; approximately 2.5σ). The catalytic acid and mutated base are shown in light blue. Here, it can be clearly seen that the position normally taken by the nucleophilic water (sitting below and in line with C1) is unavailable owing to the occupation of O2 caused by a ring flip. (e) TtGH8 D281N–X6 complex, showing the nearest neighbours of the X6 ligand (coloured by blending through model); waters are shown in blue, the cryoprotectant ethylene glycol (blue) is shown on the left and hydrogen bonds are depicted by dashed lines. (f) Close-up views of the sugar conformations found in the −1 position of the two TtGH8 complexes, showing the conformations, with C1 and C4 labelled, as 2,5B and 1C4 for xylotriose and xylohexaose, respectively. All images were produced using CCP4mg (McNicholas et al., 2011  ). |
![[Figure 4]](cb5108fig4.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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