IUBMB Enzyme Nomenclature


Accepted name: endoplasmic reticulum Man9GlcNAc2 1,2-α-mannosidase

Reaction: Man9GlcNAc2-[protein] + H2O = Man8GlcNAc2-[protein] (isomer 8A1,2,3B1,3) + D-mannopyranose

Glossary: Man9GlcNAc2-[protein] = {α-D-Man-(1→2)-α-D-Man-(1→2)-α-D-Man-(1→3)-[α-D-Man-(1→2)-α-D-Man-(1→3)-[α-D-Man-(1→2)-α-D-Man-(1→6)]-α-D-Man-(1→6)]-β-D-Man-(1→4)-β-D-GlcNAc-(1→4)-α-D-GlcNAc}-N-Asn-[protein]
Man8GlcNAc2-[protein] (isomer 8A1,2,3B1,3) = {α-D-Man-(1→2)-α-D-Man-(1→2)-α-D-Man-(1→3)-[α-D-Man-(1→3)-[α-D-Man-(1→2)-α-D-Man-(1→6)]-α-D-Man-(1→6)]-β-D-Man-(1→4)-β-D-GlcNAc-(1→4)-α-D-GlcNAc}-N-Asn-[protein]

Other name(s): MAN1B1 (gene name); MNS1 (gene name); MNS3 (gene name)

Systematic name: Man9GlcNAc2-[protein]2-α-mannohydrolase (configuration-inverting)

Comments: The enzyme, located in the endoplasmic reticulum, primarily trims a single α-1,2-linked mannose residue from Man9GlcNAc2 to produce Man8GlcNAc2 isomer 8A1,2,3B1,3 (the names of the isomers listed here are based on a nomenclature system proposed by Prien et al [7]). The removal of the single mannosyl residue occurs in all eukaryotes as part of the processing of N-glycosylated proteins, and is absolutely essential for further elongation of the outer chain of properly-folded N-glycosylated proteins in yeast. In addition, the enzyme is involved in glycoprotein quality control at the ER quality control compartment (ERQC), helping to target misfolded glycoproteins for degradation. When present at very high concentrations in the ERQC, the enzyme can trim the carbohydrate chain further to Man(5-6)GlcNAc2.

Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number:


1. Jelinek-Kelly, S. and Herscovics, A. Glycoprotein biosynthesis in Saccharomyces cerevisiae. Purification of the α-mannosidase which removes one specific mannose residue from Man9GlcNAc. J. Biol. Chem. 263 (1988) 14757-14763. [PMID: 3049586]

2. Ziegler, F.D. and Trimble, R.B. Glycoprotein biosynthesis in yeast: purification and characterization of the endoplasmic reticulum Man9 processing α-mannosidase. Glycobiology 1 (1991) 605-614. [PMID: 1822240]

3. Gonzalez, D.S., Karaveg, K., Vandersall-Nairn, A.S., Lal, A. and Moremen, K.W. Identification, expression, and characterization of a cDNA encoding human endoplasmic reticulum mannosidase I, the enzyme that catalyzes the first mannose trimming step in mammalian Asn-linked oligosaccharide biosynthesis. J. Biol. Chem. 274 (1999) 21375-21386. [PMID: 10409699]

4. Herscovics, A., Romero, P.A. and Tremblay, L.O. The specificity of the yeast and human class I ER α 1,2-mannosidases involved in ER quality control is not as strict previously reported. Glycobiology 12 (2002) 14G-15G. [PMID: 12090241]

5. Avezov, E., Frenkel, Z., Ehrlich, M., Herscovics, A. and Lederkremer, G.Z. Endoplasmic reticulum (ER) mannosidase I is compartmentalized and required for N-glycan trimming to Man5-6GlcNAc2 in glycoprotein ER-associated degradation. Mol. Biol. Cell 19 (2008) 216-225. [PMID: 18003979]

6. Liebminger, E., Huttner, S., Vavra, U., Fischl, R., Schoberer, J., Grass, J., Blaukopf, C., Seifert, G.J., Altmann, F., Mach, L. and Strasser, R. Class I α-mannosidases are required for N-glycan processing and root development in Arabidopsis thaliana. Plant Cell 21 (2009) 3850-3867. [PMID: 20023195]

7. Prien, J.M., Ashline, D.J., Lapadula, A.J., Zhang, H. and Reinhold, V.N. The high mannose glycans from bovine ribonuclease B isomer characterization by ion trap MS. J. Am. Soc. Mass Spectrom. 20 (2009) 539-556. [PMID: 19181540]

[EC created 2019]

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