GH20 and GH84 β-N-acetylglucosaminidases with different linkage specificities underpin mucin O-glycan breakdown capability of Bifidobacterium bifidum

Intestinal mucus layers mediate symbiosis and dysbiosis of host-microbe interactions. These interactions are influenced by the mucin O-glycan degrading ability of several gut microbes. The identities and prevalence of many glycoside hydrolyses (GHs) involved in microbial mucin O-glycan breakdown have been previously reported; however, the exact mechanisms and extent to which these GHs are dedicated to mucin O-glycan degradation pathways warrant further research. Here, using Bifidobacterium bifidum as a model mucinolytic bacterium, we revealed that two β-N-acetylglucosaminidases belonging to the GH20 (BbhI) and GH84 (BbhIV) families play important roles in mucin O-glycan degradation. Using substrate specificity analysis of natural oligosaccharides and O-glycomic analysis of porcine gastric mucin (PGM) incubated with purified enzymes or B. bifidum carrying bbhI and/or bbhIV mutations, we showed that BbhI and BbhIV are highly specific for β-(1→3)- and β-(1→6)-GlcNAc linkages of mucin core structures, respectively. Interestingly, we found that efficient hydrolysis of the β-(1→3)-linkage by BbhI of the mucin core 4 structure [GlcNAcβ1-3(GlcNAcβ1-6)GalNAcα-O-Thr] required prior removal of the β-(1→6)-GlcNAc linkage by BbhIV. Consistent with this, inactivation of bbhIV markedly decreased the ability of B. bifidum to release GlcNAc from PGM. When combined with a bbhI mutation, we observed that the growth of the strain on PGM was reduced. Finally, phylogenetic analysis suggests that GH84 members may have gained diversified functions through microbe-microbe and host-microbe horizontal gene transfer events. Taken together, these data strongly suggest GH84 family members in host glycan breakdown.

β-N-Acetylglucosaminidase MthNAG from Myceliophthora thermophila C1, a thermostable enzyme for production of N-acetylglucosamine from chitin

Thermostable enzymes are a promising alternative for chemical catalysts currently used for the production of N-acetylglucosamine (GlcNAc) from chitin. In this study, a novel thermostable β-N-acetylglucosaminidase MthNAG was cloned and purified from the thermophilic fungus Myceliophthora thermophila C1. MthNAG is a protein with a molecular weight of 71 kDa as determined with MALDI-TOF-MS. MthNAG has the highest activity at 50 °C and pH 4.5. The enzyme shows high thermostability above the optimum temperature: at 55 °C (144 h, 75% activity), 60 °C (48 h, 85% activity; half-life 82 h), and 70 °C (24 h, 33% activity; half-life 18 h). MthNAG releases GlcNAc from chitin oligosaccharides (GlcNAc)_2–5, p-nitrophenol derivatives of chitin oligosaccharides (GlcNAc)_1–3-pNP, and the polymeric substrates swollen chitin and soluble chitosan. The highest activity was detected towards (GlcNAc)₂. MthNAG released GlcNAc from the non-reducing end of the substrate. We found that MthNAG and Chitinase Chi1 from M. thermophila C1 synergistically degraded swollen chitin and released GlcNAc in concentration of approximately 130 times higher than when only MthNAG was used. Therefore, chitinase Chi1 and MthNAG have great potential in the industrial production of GlcNAc.

for application in N-acetyl-glucosamine production

The psychrotolerant Pseudoalteromonas issachenkonii PAMC 22718 was isolated for its high exo-acting chitinase activity in the Kara Sea, Arctic. An exo-acting chitinase (W-Chi22718) was homogeneously purified from the culture supernatant of PAMC 22718, the molecular weight of which was estimated to be approximately 112 kDa. Due to its β-N-acetylglucosaminidase activity, W-Chi22718 was able to produce N-acetyl-D-glucosamine (GlcNAc) monomers from chitin oligosaccharide substrates. W-Chi22718 displayed chitinase activity from 0 to 37°C (optimal temperature of 30°C) and maintained activity from pH 6.0 to 9.0 (optimal pH of 7.6). W-Chi22718 exhibited a relative activity of 13 and 35% of maximal activity at 0 and 10°C, respectively, which is comparable to the activities of previously characterized, cold-adapted bacterial chitinases. W-Chi22718 activity was enhanced by K⁺, Ca²⁺, and Fe²⁺, but completely inhibited by Cu²⁺ and SDS. We found that W-Chi22718 can produce much more (GlcNAcs) from colloidal chitin, working together with previously characterized cold-active endochitinase W-Chi21702. Genome sequencing revealed that the corresponding gene (chi22718_IV) was 2,856 bp encoding a 951 amino acid protein with a calculated molecular weight of approximately 102 kDa.

Enzymatic properties of β-N-acetylglucosaminidases

β-N-Acetylglucosaminidases (GlcNAcases) hydrolyse N-acetylglucosamine-containing oligosaccharides and proteins. These enzymes produce N-acetylglucosamine (GlcNAc) and have a wide range of promising applications in the food, energy, and pharmaceutical industries, such as synergistic degradation of chitin with endo-chitinases and using GlcNAc to produce sialic acid, bioethanol, single-cell proteins, and pharmaceutical therapeutics. GlcNAcases also play an important role in the dynamic balance of cellular O-linked GlcNAc levels, catabolism of ganglioside storage in Tay-Sachs disease, and bacterial cell wall recycling and flagellar assembly. In view of these important biological functions and the wide range of industrial applications of GlcNAcases, this review aims to provide a better understanding of various advances for these enzymes. It focuses on enzymatic properties of GlcNAcases, including substrate specificity, catalytic activity, pH optimum, temperature optimum, thermostability, the effects of various metal ions and organic reagents, and transglycosylation.

Distinctive molecular and biochemical characteristics of a glycoside hydrolase family 20 β-N-acetylglucosaminidase and salt tolerance

Background

Enzymatic degradation of chitin has attracted substantial attention because chitin is an abundant renewable natural resource, second only to lignocellulose, and because of the promising applications of N-acetylglucosamine in the bioethanol, food and pharmaceutical industries. However, the low activity and poor tolerance to salts and N-acetylglucosamine of most reported β-N-acetylglucosaminidases limit their applications. Mining for novel enzymes from new microorganisms is one way to address this problem.

Results

A glycoside hydrolase family 20 (GH 20) β-N-acetylglucosaminidase (GlcNAcase) was identified from Microbacterium sp. HJ5 harboured in the saline soil of an abandoned salt mine and was expressed in Escherichia coli. The purified recombinant enzyme showed specific activities of 1773.1 ± 1.1 and 481.4 ± 2.3 μmol min⁻¹ mg⁻¹ towards p-nitrophenyl β-N-acetylglucosaminide and N,N'-diacetyl chitobiose, respectively, a V_max of 3097 ± 124 μmol min⁻¹ mg⁻¹ towards p-nitrophenyl β-N-acetylglucosaminide and a K_i of 14.59 mM for N-acetylglucosamine inhibition. Most metal ions and chemical reagents at final concentrations of 1.0 and 10.0 mM or 0.5 and 1.0% (v/v) had little or no effect (retaining 84.5 − 131.5% activity) on the enzyme activity. The enzyme can retain more than 53.6% activity and good stability in 3.0–20.0% (w/v) NaCl. Compared with most GlcNAcases, the activity of the enzyme is considerably higher and the tolerance to salts and N-acetylglucosamine is much better. Furthermore, the enzyme had higher proportions of aspartic acid, glutamic acid, alanine, glycine, random coils and negatively charged surfaces but lower proportions of cysteine, lysine, α-helices and positively charged surfaces than its homologs. These molecular characteristics were hypothesised as potential factors in the adaptation for salt tolerance and high activity of the GH 20 GlcNAcase.

Conclusions

Biochemical characterization revealed that the GlcNAcase had novel salt–GlcNAc tolerance and high activity. These characteristics suggest that the enzyme has versatile potential in biotechnological applications, such as bioconversion of chitin waste and the processing of marine materials and saline foods. Molecular characterization provided an understanding of the molecular–function relationships for the salt tolerance and high activity of the GH 20 GlcNAcase.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0358-1) contains supplementary material, which is available to authorized users.

Improvement of glycosylation structure by suppression of β-N-acetylglucosaminidases in silkworm

The baculovirus-silkworm recombinant protein expression system is an excellent method for achieving high-level expression and post-translational modifications, especially glycosylation. However, the presence of paucimannosidic-type N-glycan in glycoproteins restricts their clinical use. Paucimannosidic-type N-glycan is produced by insect-specific membrane-binding-type β-N-acetylglucosaminidase (GlcNAcase). In the silkworm, BmGlcNAcase1, BmGlcNAcase2, and BmFDL are membrane-binding-type GlcNAcases. We investigated the localization of these GlcNAcases and found that BmFDL and BmGlcNAcase2 were mainly located in the fat body and hemolymph, respectively. The fat body is the main tissue of recombinant protein expression by baculovirus, and many glycoproteins are secreted into the hemolymph. These results suggest that inhibition of BmFDL and BmGlcNAcase2 could increase GlcNAc-type N-glycan levels. We therefore injected a GlcNAcase inhibitor into silkworms to investigate changes in the N-glycan structure of the glycoprotein expressed by baculovirus; modest levels of GlcNAc-type N-glycan were observed (0.8% of total N-glycan). Next, we generated a transgenic silkworm in which RNA interference (RNAi) reduced the BmFDL transcript level and enzyme activity to 25% and 50%, respectively, of that of the control silkworm. The proportion of GlcNAc-type N-glycan increased to 4.3% in the RNAi-transgenic silkworm. We conclude that the structure of N-glycan can be changed by inhibiting the GlcNAcases in silkworm.

E. coli sabotages the in vivo production of O-linked β-N-acetylglucosamine-modified proteins

The O-linked β-N-acetylglucosamine (O-GlcNAc) post-translational modification is an important, regulatory modification of cytosolic and nuclear enzymes. To date, no 3-dimensional structures of O-GlcNAc-modified proteins exist due to difficulties in producing sufficient quantities with either in vitro or in vivo techniques. Recombinant co-expression of substrate protein and O-GlcNAc transferase in Escherichia coli was used to produce O-GlcNAc-modified domains of human cAMP responsive element-binding protein (CREB1) and Abelson tyrosine-kinase 2 (ABL2). Recombinant expression in E. coli is an advantageous approach, but only small quantities of insoluble O-GlcNAc-modified protein were produced. Adding β-N-acetylglucosaminidase inhibitor, O-(2-acetamido-2-dexoy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc), to the culture media provided the first evidence that an E. coli enzyme cleaves O-GlcNAc from proteins in vivo. With the inhibitor present, the yields of O-GlcNAc-modified protein increased. The E. coli β-N-acetylglucosaminidase was isolated and shown to cleave O-GlcNAc from a synthetic O-GlcNAc-peptide in vitro. The identity of the interfering β-N-acetylglucosaminidase was confirmed by testing a nagZ knockout strain. In E. coli, NagZ natively cleaves the GlcNAc-β1,4-N-acetylmuramic acid linkage to recycle peptidoglycan in the cytoplasm and cleaves the GlcNAc-β-O-linkage of foreign O-GlcNAc-modified proteins in vivo, sabotaging the recombinant co-expression system.