Reaction: (1) Man9GlcNAc2-[protein] + 4 H2O = Man5GlcNAc2-[protein] + 4 β-D-mannopyranose (overall reaction)
(1a) Man9GlcNAc2-[protein] + H2O = Man8GlcNAc2-[protein] (isomer 8A1,2,3B1,2) + β-D-mannopyranose
(1b) Man8GlcNAc2-[protein] (isomer 8A1,2,3B1,2) + H2O = Man7GlcNAc2-[protein] (isomer 7A1,2,3B2) + β-D-mannopyranose
(1c) Man7GlcNAc2-[protein] (isomer 7A1,2,3B2) + H2O = Man6GlcNAc2-[protein] (isomer 6A1,2B2) + β-D-mannopyranose
(1d) Man6GlcNAc2-[protein] (isomer 6A1,2B2) + H2O = Man5GlcNAc2-[protein] + β-D-mannopyranose
(2) Man8GlcNAc2-[protein] (isomer 8A1,2,3B1,3) + 3 H2O = Man5GlcNAc2-[protein] + 3 β-D-mannopyranose (overall reaction)
(2a) Man8GlcNAc2-[protein] (isomer 8A1,2,3B1,3) + H2O = Man7GlcNAc2-[protein] (isomer 7A1,2,3B1) + β-D-mannopyranose
(2b) Man7GlcNAc2-[protein] (isomer 7A1,2,3B1) + H2O = Man6GlcNAc2-[protein] (isomer 6A1,2,3) + β-D-mannopyranose
(2c) Man6GlcNAc2-[protein] (isomer 6A1,2,3) + H2O = Man5GlcNAc2-[protein] + β-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)-β-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)-β-D-GlcNAc]-N-Asn-[protein]
Man5GlcNAc2-[protein] = [α-D-Man-(1→3)-{α-D-Man-(1→3)-[α-D-Man-(1→6)]-α-D-Man-(1→6)}-β-D-Man-(1→4)-β-D-GlcNAc-(1→4)-β-D-GlcNAc]-N-Asn-[protein]
Other name(s): mannosidase 1A; mannosidase 1B; 1,2-α-mannosidase; exo-α-1,2-mannanase; mannose-9 processing α-mannosidase; glycoprotein processing mannosidase I; mannosidase I; Man9-mannosidase; ManI; 1,2-α-mannosyl-oligosaccharide α-D-mannohydrolase; MAN1A1 (gene name); MAN1A2 (gene name); MAN1C1 (gene name); 2-α-mannosyl-oligosaccharide α-D-mannohydrolase
Systematic name: Man9GlcNAc2-[protein] α-2-mannohydrolase (configuration-inverting)
Comments: This family of mammalian enzymes, located in the Golgi system, participates in the maturation process of N-glycans that leads to formation of hybrid and complex structures. The enzymes catalyse the hydrolysis of the four (1→2)-linked α-D-mannose residues from the Man9GlcNAc2 oligosaccharide attached to target proteins as described in reaction (1). Alternatively, the enzymes act on the Man8GlcNAc2 isomer formed by EC 3.2.1.209, endoplasmic reticulum Man9GlcNAc2 1,2-α-mannosidase, as described in reaction (2). The enzymes are type II membrane proteins, require Ca2+, and use an inverting mechanism. While all three human enzymes can catalyse the reactions listed here, some of the enzymes can additionally catalyse hydrolysis in an alternative order, generating additional isomeric intermediates, although the final product is the same. The names of the isomers listed here are based on a nomenclature system proposed by Prien et al. [7].
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 9068-25-1
References:
1. Tabas, I. and Kornfeld, S. Purification and characterization of a rat liver Golgi α-mannosidase capable of processing asparagine-linked oligosaccharides. J. Biol. Chem. 254 (1979) 11655-11663. [PMID: 500665]
2. Tulsiani, D.R.P., Hubbard, S.C., Robbins, P.W. and Touster, O. α-D-Mannosidases of rat liver Golgi membranes. Mannosidase II is the GlcNAcMAN5-cleaving enzyme in glycoprotein biosynthesis and mannosidases IA and IB are the enzymes converting Man9 precursors to Man5 intermediates. J. Biol. Chem. 257 (1982) 3660-3668. [PMID: 7061502]
3. Bieberich, E. and Bause, E. Man9-mannosidase from human kidney is expressed in COS cells as a Golgi-resident type II transmembrane N-glycoprotein. Eur. J. Biochem. 233 (1995) 644-649. [PMID: 7588811]
4. Tremblay, L.O., Campbell Dyke, N. and Herscovics, A. Molecular cloning, chromosomal mapping and tissue-specific expression of a novel human α1,2-mannosidase gene involved in N-glycan maturation. Glycobiology 8 (1998) 585-595. [PMID: 9592125]
5. Lal, A., Pang, P., Kalelkar, S., Romero, P.A., Herscovics, A. and Moremen, K.W. Substrate specificities of recombinant murine Golgi α1,2-mannosidases IA and IB and comparison with endoplasmic reticulum and Golgi processing α1,2-mannosidases. Glycobiology 8 (1998) 981-995. [PMID: 9719679]
6. Tremblay, L.O. and Herscovics, A. Characterization of a cDNA encoding a novel human Golgi α 1, 2-mannosidase (IC) involved in N-glycan biosynthesis. J. Biol. Chem. 275 (2000) 31655-31660. [PMID: 10915796]
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]