A surface-exposed GH26 β-mannanase from : Structure, role, and phylogenetic analysis of Man26B.

The galactomannan utilization locus (ManPUL) of the human gut bacterium encodes Man26B, a cell-surface-exposed endomannanase whose functional and structural features have been unclear. Our study now places Man26B in context with related enzymes and reveals the structural basis for its specificity. Man26B prefers longer substrates and is less restricted by galactose side-groups than the mannanase Man26A of the same locus. Using galactomannan, Man26B generated a mixture of (galactosyl) manno-oligosaccharides shorter than mannohexaose. Three defined manno-oligosaccharides had affinity for the SusD-like surface-exposed glycan-binding protein, predicted to be implicated in saccharide transport. Co-incubation of Man26B and the periplasmic α-galactosidase Gal36A increased the rate of galactose release by about 10-fold compared with the rate without Man26B. The results suggested that Man26B performs the initial attack on galactomannan, generating oligosaccharides that after transport to the periplasm are processed by Gal36A. A crystal structure of Man26B 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. Moreover, Lys-149 in the -4 subsite interacted with the galactosyl side-group of the ligand. A phylogenetic tree consisting of GH26 enzymes revealed four strictly conserved GH26 residues and disclosed that Man26A and Man26B reside on two distinct phylogenetic branches (A and B). The three other branches contain lichenases, xylanases, or enzymes with unknown activities. Lys-149 is conserved in a narrow part of branch B, and Trp-112 is conserved in a wider group within branch B.