Analysis of a New Family of Widely Distributed Metal-independent α-Mannosidases Provides Unique Insight into the Processing of N-Linked Glycans

• Published in: The Journal of biological chemistry Vol. 286; no. 17; pp. 15586 - 15596
• Main Authors: Gregg, Katie J., Zandberg, Wesley F., Hehemann, Jan-Hendrik, Whitworth, Garrett E., Deng, Lehua, Vocadlo, David J., Boraston, Alisdair B.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 29.04.2011; American Society for Biochemistry and Molecular Biology
• Subjects: alpha-Mannosidase - chemistry; alpha-Mannosidase - metabolism; Bacteria; Carbohydrate Metabolism; Carbohydrate Processing; Catalysis; Catalytic Mechanism; Clostridium perfringens - enzymology; Crystal Structure; Glycobiology and Extracellular Matrices; Glycoside Hydrolase; Glycosylation; Metals; Polysaccharides - metabolism; Streptococcus pneumoniae - enzymology; Substrate Specificity; Crystal Structure; Carbohydrate Processing; Carbohydrate Metabolism; Bacteria; Glycosylation; Catalytic Mechanism; Glycoside Hydrolase
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M111.223172

The modification of N-glycans by α-mannosidases is a process that is relevant to a large number of biologically important processes, including infection by microbial pathogens and colonization by microbial symbionts. At present, the described mannosidases specific for α1,6-mannose linkages are very limited in number. Through structural and functional analysis of two sequence-related enzymes, one from Streptococcus pneumoniae (SpGH125) and one from Clostridium perfringens (CpGH125), a new glycoside hydrolase family, GH125, is identified and characterized. Analysis of SpGH125 and CpGH125 reveal them to have exo-α1,6-mannosidase activity consistent with specificity for N-linked glycans having their α1,3-mannose branches removed. The x-ray crystal structures of SpGH125 and CpGH125 obtained in apo-, inhibitor-bound, and substrate-bound forms provide both mechanistic and molecular insight into how these proteins, which adopt an (α/α)6-fold, recognize and hydrolyze the α1,6-mannosidic bond by an inverting, metal-independent catalytic mechanism. A phylogenetic analysis of GH125 proteins reveals this to be a relatively large and widespread family found frequently in bacterial pathogens, bacterial human gut symbionts, and a variety of fungi. Based on these studies we predict this family of enzymes will primarily comprise such exo-α1,6-mannosidases.