Glycoside Hydrolase Family 99

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• Author: ^^^Spencer Williams^^^
• Responsible Curator: ^^^Gideon Davies^^^

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Glycoside Hydrolase Family GH99
Clan	not assigned
Mechanism	retaining/inverting
Active site residues	known/not known
CAZy DB link
https://www.cazy.org/GH99.html

Substrate specificities

Glycoside hydrolases of family GH99 are endo-acting α-mannosidases that cleave glucose-substituted mannose within immature N-linked glycans of the general formula Glc_1-3Man₉GlcNAc₂, with maximal activity on the monoglucosylated forms [1]. This family was originally created from mammalian enzyme, cloned by Spiro and co-workers [2]. Mammalian GH99 enzymes are localized to the Golgi apparatus [3] and appear to play a role in the rescue of glucosylated N-linked glycans that have evaded the action of the endoplasmic reticulum exo-glucosidases I and II [4]. Mammalian endo-α-mannosidases has increased activity on glucosylated N-linked glycans that have been trimmed in the non-glucose-substituted branches [2]. There is evidence that mammalian endo-α-mannosidases act on dolichol-bound N-glycan precursors [5], as well as free oligosaccharides released from N-glycoproteins and which undergo retrograde transport through the secretory pathway [6].

Kinetics and Mechanism

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Catalytic Residues

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Three-dimensional structures

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Sample structures

Three-dimensional structure of GH99 endo-α-mannosidase from Bacteroides xylanisolvens, PDB code [1] [7].	Three-dimensional structure of GH99 endo-α-mannosidase from Bacteroides xylanisolvens bound to glucose-1,3-isofagomine and α-1,2- mannobiose, PDB code [2] [7].
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Family Firsts

First stereochemistry determination

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First catalytic nucleophile identification

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First general acid/base residue identification

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First 3-D structure

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References

1. Roth J, Ziak M, and Zuber C. (2003). The role of glucosidase II and endomannosidase in glucose trimming of asparagine-linked oligosaccharides. Biochimie. 2003;85(3-4):287-94. DOI:10.1016/s0300-9084(03)00049-x |
2. Spiro MJ, Bhoyroo VD, and Spiro RG. (1997). Molecular cloning and expression of rat liver endo-alpha-mannosidase, an N-linked oligosaccharide processing enzyme. J Biol Chem. 1997;272(46):29356-63. DOI:10.1074/jbc.272.46.29356 |
3. Zuber C, Spiro MJ, Guhl B, Spiro RG, and Roth J. (2000). Golgi apparatus immunolocalization of endomannosidase suggests post-endoplasmic reticulum glucose trimming: implications for quality control. Mol Biol Cell. 2000;11(12):4227-40. DOI:10.1091/mbc.11.12.4227 |
4. Dale MP, Kopfler WP, Chait I, and Byers LD. (1986). Beta-glucosidase: substrate, solvent, and viscosity variation as probes of the rate-limiting steps. Biochemistry. 1986;25(9):2522-9. DOI:10.1021/bi00357a036 |
5. Spiro MJ and Spiro RG. (2000). Use of recombinant endomannosidase for evaluation of the processing of N-linked oligosaccharides of glycoproteins and their oligosaccharide-lipid precursors. Glycobiology. 2000;10(5):521-9. DOI:10.1093/glycob/10.5.521 |
6. Kukushkin NV, Alonzi DS, Dwek RA, and Butters TD. (2011). Demonstration that endoplasmic reticulum-associated degradation of glycoproteins can occur downstream of processing by endomannosidase. Biochem J. 2011;438(1):133-42. DOI:10.1042/BJ20110186 |