A surface-exposed GH26 ß-mannanase from Bacteroides ovatus: Structure, role, and phylogenetic analysis of BoMan26B.

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. Our study now places BoMan26B in context with related enzymes and reveals the structural basis for its specificity. BoMan26B prefers longer substrates and is less restricted by galactose side-groups than the mannanase BoMan26A of the same locus. Using galactomannan, BoMan26B 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 BoMan26B and the periplasmic α-galactosidase BoGal36A increased the rate of galactose release by about 10-fold compared with the rate without BoMan26B. The results suggested that BoMan26B performs the initial attack on galactomannan, generating oligosaccharides that after transport to the periplasm are processed by BoGal36A. 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. 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 BoMan26A and BoMan26B 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.

Biochemical characterization of thermostable ß-1,4-mannanase belonging to the glycoside hydrolase family 134 from Aspergillus oryzae.

A ß-1,4-mannanase, termed AoMan134A, that belongs to the GH 134 family was identified in the filamentous fungus Aspergillus oryzae. Recombinant AoMan134A was expressed in Pichia pastoris, and the purified enzyme produced mannobiose, mannotriose, mannotetraose, and mannopentaose from galactose-free ß-mannan, with mannotriose being the predominant reaction product. The catalytic efficiency (k cat/K m ) of AoMan134A was 6.8-fold higher toward galactomannan from locust bean gum, than toward galactomannan from guar gum, but similar toward galactomannan from locust bean gum and glucomannan from konjac flour. After incubation at 70°C for 120 min, the activity of AoMan134A toward glucomannan decreased to 50% of the maximal activity at 30°C. AoMan134A retained 50% of its ß-1,4-mannanase activity after heating at 90°C for 30 min, indicating that AoMan134A is thermostable. Furthermore, AoMan134A was stable within a neutral-to-alkaline pH range, as well as exhibiting stability in the presence of a range of organic solvents, detergents, and metal ions. These findings suggest that AoMan134A could be useful in a diverse range of industries where conversion of ß-mannans is of prime importance.

Novel ß-1,4-Mannanase Belonging to a New Glycoside Hydrolase Family in Aspergillus nidulans.

Many filamentous fungi produce ß-mannan-degrading ß-1,4-mannanases that belong to the glycoside hydrolase 5 (GH5) and GH26 families. Here we identified a novel ß-1,4-mannanase (Man134A) that belongs to a new glycoside hydrolase (GH) family (GH134) in Aspergillus nidulans. Blast analysis of the amino acid sequence using the NCBI protein database revealed that this enzyme had no similarity to any sequences and no putative conserved domains. Protein homologs of the enzyme were distributed to limited fungal and bacterial species. Man134A released mannobiose (M2), mannotriose (M3), and mannotetraose (M4) but not mannopentaose (M5) or higher manno-oligosaccharides when galactose-free ß-mannan was the substrate from the initial stage of the reaction, suggesting that Man134A preferentially reacts with ß-mannan via a unique catalytic mode. Man134A had high catalytic efficiency (kcat/Km) toward mannohexaose (M6) compared with the endo-ß-1,4-mannanase Man5C and notably converted M6 to M2, M3, and M4, with M3 being the predominant reaction product. The action of Man5C toward ß-mannans was synergistic. The growth phenotype of a Man134A disruptant was poor when ß-mannans were the sole carbon source, indicating that Man134A is involved in ß-mannan degradation in vivo. These findings indicate a hitherto undiscovered mechanism of ß-mannan degradation that is enhanced by the novel ß-1,4-mannanase, Man134A, when combined with other mannanolytic enzymes including various endo-ß-1,4-mannanases.

Arabidopsis thaliana bZIP44: a transcription factor affecting seed germination and expression of the mannanase-encoding gene AtMAN7.

Endo-ß-mannanases (MAN; EC. 3.2.1.78) catalyze the cleavage of ß1→4 bonds in mannan polymers and have been associated with the process of weakening the tissues surrounding the embryo during seed germination. In germinating Arabidopsis thaliana seeds, the most highly expressed MAN gene is AtMAN7 and its transcripts are restricted to the micropylar endosperm and to the radicle tip just before radicle emergence. Mutants with a T-DNA insertion in AtMAN7 have a slower germination than the wild type. To gain insight into the transcriptional regulation of the AtMAN7 gene, a bioinformatic search for conserved non-coding cis-elements (phylogenetic shadowing) within the Brassicaceae MAN7 gene promoters has been done, and these conserved motifs have been used as bait to look for their interacting transcription factors (TFs), using as a prey an arrayed yeast library from A. thaliana. The basic-leucine zipper TF AtbZIP44, but not the closely related AtbZIP11, has thus been identified and its transcriptional activation upon AtMAN7 has been validated at the molecular level. In the knock-out lines of AtbZIP44, not only is the expression of the AtMAN7 gene drastically reduced, but these mutants have a significantly slower germination than the wild type, being affected in the two phases of the germination process, both in the rupture of the seed coat and in the breakage of the micropylar endosperm cell walls. In the over-expression lines the opposite phenotype is observed.

Re-interpreting the role of endo-beta-mannanases as mannan endotransglycosylase/hydrolases in the plant cell wall.

BACKGROUND: Mannans are hemicellulosic polysaccharides in the plant primary cell wall with two major physiological roles: as storage polysaccharides that provide energy for the growing seedling; and as structural components of the hemicellulose-cellulose network with a similar function to xyloglucans. Endo-beta-mannanases are hydrolytic enzymes that cleave the mannan backbone. They are active during seed germination and during processes of growth or senescence. The recent discovery that endo-beta-mannanase LeMAN4a from ripe tomato fruit also has mannan transglycosylase activity requires the role of endo-beta-mannanases to be reinterpreted. AIMS: In this review, the role of endo-beta-mannanases as mannan endotransglycosylase/hydrolases (MTHs) in remodelling the plant cell wall is considered by analogy to the role of xyloglucan endotransglucosylase/hydrolases (XTHs). The current understanding of the reaction mechanism of these enzymes, their three-dimensional protein structure, their substrates and their genes are reported. FUTURE OUTLOOK: There are likely to be more endohydrolases within the plant cell wall that can carry out hydrolysis and transglycosylation reactions. The challenge will be to demonstrate that the transglycosylation activities shown in vitro also exist in vivo and to validate a role for transglycosylation reactions during the growth and development of the plant cell wall.