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

Many filamentous fungi produce beta-mannan-degrading beta-1,4-mannanases that belong to the glycoside hydrolase 5 (GH5) and GH26 families. Here we identified a novel beta-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 (M-2), mannotriose (M-3), and mannotetraose (M-4) but not mannopentaose (M-5) or higher manno-oligosaccharides when galactose-free beta-mannan was the substrate from the initial stage of the reaction, suggesting that Man134A preferentially reacts with beta-mannan via a unique catalytic mode. Man134A had high catalytic efficiency (k(cat)/K-m) toward mannohexaose (M-6) compared with the endo-beta-1,4-mannanase Man5C and notably converted M-6 to M-2, M-3, and M-4, with M-3 being the predominant reaction product. The action of Man5C toward beta-mannans was synergistic. The growth phenotype of a Man134A disruptant was poor when beta-mannans were the sole carbon source, indicating that Man134A is involved in beta-mannan degradation in vivo. These findings indicate a hitherto undiscovered mechanism of beta-mannan degradation that is enhanced by the novel beta-1,4-mannanase, Man134A, when combined with other mannanolytic enzymes including various endo-beta-1,4-mannanases.