Mechanism of high-mannose N-glycan breakdown and metabolism by Bifidobacterium longum.

Bifidobacteria are early colonizers of the human gut and play central roles in human health and metabolism. To thrive in this competitive niche, these bacteria evolved the capacity to use complex carbohydrates, including mammalian N-glycans. Herein, we elucidated pivotal biochemical steps involved in high-mannose N-glycan utilization by Bifidobacterium longum. After N-glycan release by an endo-β-N-acetylglucosaminidase, the mannosyl arms are trimmed by the cooperative action of three functionally distinct glycoside hydrolase 38 (GH38) α-mannosidases and a specific GH125 α-1,6-mannosidase. High-resolution cryo-electron microscopy structures revealed that bifidobacterial GH38 α-mannosidases form homotetramers, with the N-terminal jelly roll domain contributing to substrate selectivity. Additionally, an α-glucosidase enables the processing of monoglucosylated N-glycans. Notably, the main degradation product, mannose, is isomerized into fructose before phosphorylation, an unconventional metabolic route connecting it to the bifid shunt pathway. These findings shed light on key molecular mechanisms used by bifidobacteria to use high-mannose N-glycans, a perennial carbon and energy source in the intestinal lumen.