Unique Microbial Catabolic Pathway for the Human Core N -Glycan Constituent Fucosyl-α-1,6- N -Acetylglucosamine-Asparagine.

The survival of commensal bacteria in the human gut partially depends on their ability to metabolize host-derived molecules. The use of the glycosidic moiety of N -glycoproteins by bacteria has been reported, but the role of N -glycopeptides or glycoamino acids as the substrates for bacterial growth has not been evaluated. We have identified in Lactobacillus casei strain BL23 a gene cluster ( alf-2 ) involved in the catabolism of the glycoamino acid fucosyl-α-1,6- N -GlcNAc-Asn (6'FN-Asn), a constituent of the core-fucosylated structures of mammalian N -glycoproteins. The cluster consists of the genes alfHC , encoding a major facilitator superfamily (MFS) permease and the α-l-fucosidase AlfC, and the divergently oriented asdA (aspartate 4-decarboxylase), alfR2 (transcriptional regulator), pepV (peptidase), asnA2 (glycosyl-asparaginase), and sugK (sugar kinase) genes. Knockout mutants showed that alfH , alfC , asdA , asnA2 , and sugK are necessary for efficient 6'FN-Asn utilization. The alf-2 genes are induced by 6'FN-Asn, but not by its glycan moiety, via the AlfR2 regulator. The constitutive expression of alf-2 genes in an alfR2 strain allowed the metabolism of a variety of 6'-fucosyl-glycans. However, GlcNAc-Asn did not support growth in this mutant background, indicating that the presence of a 6'-fucose moiety is crucial for substrate transport via AlfH. Within bacteria, 6'FN-Asn is defucosylated by AlfC, generating GlcNAc-Asn. This glycoamino acid is processed by the glycosylasparaginase AsnA2. GlcNAc-Asn hydrolysis generates aspartate and GlcNAc, which is used as a fermentable source by L. casei These data establish the existence in a commensal bacterial species of an exclusive metabolic pathway likely to scavenge human milk and mucosal fucosylated N -glycopeptides in the gastrointestinal tract. IMPORTANCE The gastrointestinal tract accommodates more than 10 ¹⁴ microorganisms that have an enormous impact on human health. The mechanisms enabling commensal bacteria and administered probiotics to colonize the gut remain largely unknown. The ability to utilize host-derived carbon and energy resources available at the mucosal surfaces may provide these bacteria with a competitive advantage in the gut. Here, we have identified in the commensal species Lactobacillus casei a novel metabolic pathway for the utilization of the glycoamino acid fucosyl-α-1,6- N -GlcNAc-Asn, which is present in the core-fucosylated N -glycoproteins from mammalians. These results give insight into the molecular interactions between the host and commensal/probiotic bacteria and may help to devise new strategies to restore gut microbiota homeostasis in diseases associated with dysbiotic microbiota.
(Copyright © 2020 Becerra et al.)