Structural and mechanistic insight into N-glycan processing by endo-α-mannosidase

Proc Natl Acad Sci U S A. 2012 Jan 17;109(3):781-6. doi: 10.1073/pnas.1111482109. Epub 2012 Jan 4.


N-linked glycans play key roles in protein folding, stability, and function. Biosynthetic modification of N-linked glycans, within the endoplasmic reticulum, features sequential trimming and readornment steps. One unusual enzyme, endo-α-mannosidase, cleaves mannoside linkages internally within an N-linked glycan chain, short circuiting the classical N-glycan biosynthetic pathway. Here, using two bacterial orthologs, we present the first structural and mechanistic dissection of endo-α-mannosidase. Structures solved at resolutions 1.7-2.1 Å reveal a (β/α)(8) barrel fold in which the catalytic center is present in a long substrate-binding groove, consistent with cleavage within the N-glycan chain. Enzymatic cleavage of authentic Glc(1/3)Man(9)GlcNAc(2) yields Glc(1/3)-Man. Using the bespoke substrate α-Glc-1,3-α-Man fluoride, the enzyme was shown to act with retention of anomeric configuration. Complexes with the established endo-α-mannosidase inhibitor α-Glc-1,3-deoxymannonojirimycin and a newly developed inhibitor, α-Glc-1,3-isofagomine, and with the reducing-end product α-1,2-mannobiose structurally define the -2 to +2 subsites of the enzyme. These structural and mechanistic data provide a foundation upon which to develop new enzyme inhibitors targeting the hijacking of N-glycan synthesis in viral disease and cancer.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacteroides / enzymology*
  • Biocatalysis
  • Carbohydrate Conformation
  • Catalytic Domain
  • Conserved Sequence
  • Humans
  • Kinetics
  • Ligands
  • Models, Molecular
  • Polysaccharides / chemistry*
  • Polysaccharides / metabolism*
  • Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
  • Static Electricity
  • alpha-Mannosidase / antagonists & inhibitors
  • alpha-Mannosidase / metabolism*


  • Ligands
  • Polysaccharides
  • alpha-Mannosidase

Associated data

  • PDB/4ACY
  • PDB/4ACZ
  • PDB/4AD0
  • PDB/4AD1
  • PDB/4AD2
  • PDB/4AD3
  • PDB/4AD4
  • PDB/4AD5