Glycoside hydrolase from the GH76 family indicates that marine Salegentibacter sp. Hel_I_6 consumes alpha-mannan from fungi

Microbial glycan degradation is essential to global carbon cycling. The marine bacterium Salegentibacter sp. Hel_I_6 (Bacteroidota) isolated from seawater off Helgoland island (North Sea) contains an alpha-mannan inducible gene cluster with a GH76 family endo-alpha-1,6-mannanase (ShGH76). This cluster is related to genetic loci employed by human gut bacteria to digest fungal alpha-mannan. Metagenomes from the Hel_I_6 isolation site revealed increasing GH76 gene frequencies in free-living bacteria during microalgae blooms, suggesting degradation of alpha-1,6-mannans from fungi. Recombinant ShGH76 protein activity assays with yeast alpha-mannan and synthetic oligomannans showed endo-alpha-1,6-mannanase activity. Resolved structures of apo-ShGH76 (2.0 angstrom) and of mutants co-crystalized with fungal mannan-mimicking alpha-1,6-mannotetrose (1.90 angstrom) and alpha-1,6-mannotriose (1.47 angstrom) retained the canonical (alpha/alpha)(6) fold, despite low identities with sequences of known GH76 structures (GH76s from gut bacteria: <27%). The apo-form active site differed from those known from gut bacteria, and co-crystallizations revealed a kinked oligomannan conformation. Co-crystallizations also revealed precise molecular-scale interactions of ShGH76 with fungal mannan-mimicking oligomannans, indicating adaptation to this particular type of substrate. Our data hence suggest presence of yet unknown fungal alpha-1,6-mannans in marine ecosystems, in particular during microalgal blooms.

Automated summary

This study confirms the presence of a GH76 endo-alpha-1,6-mannanase in the marine bacterium Salegentibacter sp. Hel_I_6, which is similar to enzymes found in gut bacteria. The enzyme degrades fungal mannan oligosaccharides during microalgae blooms, and structural analysis shows adaptation to this type of substrate at the molecular level.