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A phylogenetic tree consisting of GH26 enzymes revealed four strictly conserved GH26 residues and disclosed that BoMan26A and BoMan26B reside on two distinct phylogenetic branches (A and B). Moreover, Lys-149 in the −4 subsite interacted with the galactosyl side-group of the ligand. A crystal structure of BoMan26B with galactosyl-mannotetraose bound in subsites −5 to −2 revealed an open and long active-site cleft with Trp-112 in subsite −5 concluded to be involved in mannosyl interaction. The results suggested that BoMan26B performs the initial attack on galactomannan, generating oligosaccharides that after transport to the periplasm are processed by BoGal36A.
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Co-incubation of BoMan26B and the periplasmic α-galactosidase BoGal36A increased the rate of galactose release by about 10-fold compared with the rate without BoMan26B. Three defined manno-oligosaccharides had affinity for the SusD-like surface–exposed glycan-binding protein, predicted to be implicated in saccharide transport. Using galactomannan, BoMan26B generated a mixture of (galactosyl) manno-oligosaccharides shorter than mannohexaose. BoMan26B prefers longer substrates and is less restricted by galactose side-groups than the mannanase BoMan26A of the same locus. Our study now places BoMan26B in context with related enzymes and reveals the structural basis for its specificity. The galactomannan utilization locus ( BoManPUL) of the human gut bacterium Bacteroides ovatus encodes BoMan26B, a cell-surface–exposed endomannanase whose functional and structural features have been unclear.