Abstract
Backgroundβ-Mannans are abundant and diverse plant structural and storage polysaccharides. Certain human gut microbiota members including health-promoting Bifidobacterium spp. catabolize dietary mannans. Little insight is available on the enzymology of mannan deconstruction in the gut ecological niche. Here, we report the biochemical properties of the first family 5 subfamily 8 glycoside hydrolase (GH5_8) mannanase from the probiotic bacterium Bifidobacterium animalis subsp. lactis Bl-04 (BlMan5_8).ResultsBlMan5_8 possesses a novel low affinity carbohydrate binding module (CBM) specific for soluble mannan and displays the highest catalytic efficiency reported to date for a GH5 mannanase owing to a very high kcat (1828 ± 87 s-1) and a low Km (1.58 ± 0.23 g · L-1) using locust bean galactomannan as substrate. The novel CBM of BlMan5_8 mediates increased binding to soluble mannan based on affinity electrophoresis. Surface plasmon resonance analysis confirmed the binding of the CBM10 to manno-oligosaccharides, albeit with slightly lower affinity than the catalytic module of the enzyme. This is the first example of a low-affinity mannan-specific CBM, which forms a subfamily of CBM10 together with close homologs present only in mannanases. Members of this new subfamily lack an aromatic residue mediating binding to insoluble cellulose in canonical CBM10 members consistent with the observed low mannan affinity.ConclusionBlMan5_8 is evolved for efficient deconstruction of soluble mannans, which is reflected by an exceptionally low Km and the presence of an atypical low affinity CBM, which increases binding to specifically to soluble mannan while causing minimal decrease in catalytic efficiency as opposed to enzymes with canonical mannan binding modules. These features highlight fine tuning of catalytic and binding properties to support specialization towards a preferred substrate, which is likely to confer an advantage in the adaptation to competitive ecological niches.Electronic supplementary materialThe online version of this article (doi:10.1186/s12858-015-0055-4) contains supplementary material, which is available to authorized users.
Highlights
Background βMannans are abundant polysaccharides playing diverse roles in plants including energy storage in seed endosperms, e.g. carob and guar seeds, legumes, coconuts and coffee beans [1], or structural support in the hemicellulose cell wall matrix [2], where they can constitute up to 25 % of the dry mass in softwood
The fermentation of guar gum (GG) galactomannan has been demonstrated in the human gut [11] and intake of partial hydrolysates of this polysaccharide stimulated the proliferation of Bifidobacterium spp. in humans [12] and mice [13]
We show that the GH5 mannanase (BlMan5_8) from Bifidobacterium animalis subsp. lactis Bl-04, which is conserved within Bifidobacterium animalis subsp. lactis, displays the highest catalytic efficiency reported to date for a GH5 β-mannanase owing to a combination of very high kcat and low Km
Summary
Mannans (hereafter mannans) are abundant polysaccharides playing diverse roles in plants including energy storage in seed endosperms, e.g. carob and guar seeds, legumes, coconuts and coffee beans [1], or structural support in the hemicellulose cell wall matrix [2], where they can constitute up to 25 % of the dry mass in softwood. Mannans are present in human nutrition, either as cell wall components in cereal grains and some fruits and vegetables such as kiwi, apple and tomato [5,6,7,8] or as common hydrocolloid food additives used as thickeners or to adjust texture [9, 10]. Insight into the microbial strategies and enzymes mediating mannan degradation in the human gut lags behind
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