Fig. 6.

A cavity adjacent to − 1 subsite accommodates the second decoration of di-substituted AXOS. a Structural superposition of XacAbf51 structure (violet surface) with TxAbfD3 structure in complex with 3²-α-l-arabinofuranosyl-xylotriose (XA³X; blue C atoms). Subsites are labeled according to the nomenclature used by McKee and coworkers [14]. NR non-reducing end, R  reducing end. b Comparison between XacAbf51 crystal structure and the modeled XacAbf51–XA²⁺³X complex after 100 ns of molecular dynamics simulation. According to this simulation, the xylan backbone bends at the β-1,4 linkage involving the reducing end of substrate to better accommodate the di-substitution in the cavity adjacent to the − 1 subsite of XacAbf51. c Selected inter-atomic distances between enzyme and substrate indicate favorable interactions over the simulation. Colored circles refer to the selected substrate atoms highlighted in b (open circles). d Structural comparison of XacAbf51 and TxAbfD3 crystal structures highlighting the divergent loops β5–α5 and β6–α6 that delineates the +1 and +2R subsites. The substrate XA³X bound to TxAbfD3, as well as the different positioning of W254 compared to W248 and the side chains of SDD and NTA motifs are shown in sticks and color-coded according to the respective structure. Note the hydrogen bond between N216 and the O2 of +2R Xylp residue that is absent in XacAbf51