Glycoside Hydrolase Family 47

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• Author: ^^^Rohan Williams^^^
• Responsible Curator: ^^^Spencer Williams^^^

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Glycoside Hydrolase Family GHnn
Clan	none, (α/α)7 fold
Mechanism	inverting
Active site residues	debated
CAZy DB link
http://www.cazy.org/GH47.html

Substrate specificities

GH47 glycoside hydrolases are exo-acting α-1,2-mannosidases. Members from this family play important roles in the processing of N-glycans.

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Kinetics and Mechanism

GH47 mannosidases catalyze glycosidic cleavage with inversion of stereochemistry, as first determined using ¹H NMR spectroscopy with Saccharomyces cervisiae α-1,2-mannosidase using Man9GlcNAc as a substrate [1]. Classical inverting glycosidases operate through a single displacement mechanism, where a general base residue acts to deprotonate a water molecule, facilitating nucleophilic attack at the anomeric position. This is assisted by concurrent activation of the glycosidic linkage through protonation by a general acid residue.

GH47 enzymes are Ca²⁺-dependent, as demonstrated by loss of activity upon addition of the metal binding ligand EDTA, and restoration of activity through subsequent addition of Ca²⁺ [2]. Exo-α-mannosidases from GH38 and GH92 also require a metal ion for catalysis.

GH47 mannosidases operate through an unusual ^3,OB/³S₁→³H₄→¹C₄ conformational itinerary, supported by both computational [3, 4, 5] and structural studies {list which complexes are informative] [5, 6, 7]. Quantum mechanical/molecular modelling calculations have found that the free energy landscape of α-D-mannopyranose is perturbed on-enzyme such that the accessible conformations of the ligand are altered to those that correlate well with a ^3,OB/³S₁→³H₄→¹C₄ conformational itinerary [5].

Catalytic Residues

Unequivocal assignment of catalytic residues for GH47 α-mannosidases is complicated by the presence of 3 carboxylate-containing residues in the active site each of which could plausibly fulfill roles as catalytic residues [8]. Furthermore, all of the plausible catalytic residues complex water, as would be expected of the general base residue. Thus, it appears that the general acid residue transmits a proton to the glycosidic oxygen atom through a water molecule. Crystal structures of human ER α-mannosidase I in complex with kifunensine and 1-deoxynojirimycin found that an inverting mechanism was only compatible with Glu599 or Asp463 (Glu435 and Asp275 in Saccharomyces, respectively) acting as the general base [6]. A computational docking study found Glu599 to be the most likely general base, with Ca²⁺ also coordinated to the nucelophilic water molecule [9]. Based upon its position on the opposite face of the glycan ring to the potential general base residues in human ER a-mannosidase I, Glu330 (Glu132 in Saccharomyces) is widely believed to act as the general acid [6]. However, a computational docking study found Asp463 (Asp275 in Saccharomyces) to be the most likely general acid, based upon the assumption that GH47 mannosidases are anti-protonators [10].

Three-dimensional structures

GH47 enzymes adopt a (α/α)₇ barrel fold with a Ca²⁺ ion coordinated at the base of the barrel that is plugged by a β-hairpin at the C-terminus [8]. The –1 subsite lies in the core of the barrel with Ca²⁺ coordinating to the 2-OH and 3-OH groups of a ligand (inhibitor or substrate analogue), whose glycan ring is parallel to the barrel upon complexation [6].

The structural basis for differences in N-glycan branch specificity between ER and Golgi GH47 α-mannosidases has been examined through crystallographic studies comparing their binding to N-glycans [11]. The presumed enzyme-product complexes differed in their oligosaccharide conformation such that different oligosaccharide branches, corresponding to those readily cleaved by the respective enzymes, were projected into the active site.

Family Firsts

First sterochemistry determination

Saccharomyces cerevisiae α-1,2-mannosidase was shown to be inverting by ¹H NMR [1].

First general base identification

Unambiguous identification hindered by presence of 3 carboxylate-containing residues in the active site that coordinate ligands through water molecules [8]. Widely believed to be Glu559 in human ER α-mannosidase I (Glu435 in S. cerevisiae) [9].

First general acid identification

Unambiguous identification hindered by presence of 3 carboxylate-containing residues in the active site that coordinate ligands through water molecules [8]. Widely believed to be Glu330 in human ER α-mannosidase I (Glu132 in S. cerevisiae) [7], however, a computational study has concluded that Asp463 acts as the general acid in human ER α-mannosidase I (Asp275 in S. cerevisiae) [10].

First 3-D structure

Saccharomyces cerevisiae α-1,2-mannosidase [8].

References

1. Lipari F, Gour-Salin BJ, and Herscovics A. (1995). The Saccharomyces cerevisiae processing alpha 1,2-mannosidase is an inverting glycosidase. Biochem Biophys Res Commun. 1995;209(1):322-6. DOI:10.1006/bbrc.1995.1506 |
2. Jelinek-Kelly S and Herscovics A. (1988). Glycoprotein biosynthesis in Saccharomyces cerevisiae. Purification of the alpha-mannosidase which removes one specific mannose residue from Man9GlcNAc. J Biol Chem. 1988;263(29):14757-63. | Google Books | Open Library
3. Mulakala C, Nerinckx W, and Reilly PJ. (2006). Docking studies on glycoside hydrolase Family 47 endoplasmic reticulum alpha-(1-->2)-mannosidase I to elucidate the pathway to the substrate transition state. Carbohydr Res. 2006;341(13):2233-45. DOI:10.1016/j.carres.2006.05.011 |
4. Mulakala C, Nerinckx W, and Reilly PJ. (2007). The fate of beta-D-mannopyranose after its formation by endoplasmic reticulum alpha-(1-->2)-mannosidase I catalysis. Carbohydr Res. 2007;342(2):163-9. DOI:10.1016/j.carres.2006.11.012 |
5. Thompson AJ, Dabin J, Iglesias-Fernández J, Ardèvol A, Dinev Z, Williams SJ, Bande O, Siriwardena A, Moreland C, Hu TC, Smith DK, Gilbert HJ, Rovira C, and Davies GJ. (2012). The reaction coordinate of a bacterial GH47 α-mannosidase: a combined quantum mechanical and structural approach. Angew Chem Int Ed Engl. 2012;51(44):10997-1001. DOI:10.1002/anie.201205338 |
6. Vallee F, Karaveg K, Herscovics A, Moremen KW, and Howell PL. (2000). Structural basis for catalysis and inhibition of N-glycan processing class I alpha 1,2-mannosidases. J Biol Chem. 2000;275(52):41287-98. DOI:10.1074/jbc.M006927200 |
7. Karaveg K, Siriwardena A, Tempel W, Liu ZJ, Glushka J, Wang BC, and Moremen KW. (2005). Mechanism of class 1 (glycosylhydrolase family 47) {alpha}-mannosidases involved in N-glycan processing and endoplasmic reticulum quality control. J Biol Chem. 2005;280(16):16197-207. DOI:10.1074/jbc.M500119200 |
8. Vallée F, Lipari F, Yip P, Sleno B, Herscovics A, and Howell PL. (2000). Crystal structure of a class I alpha1,2-mannosidase involved in N-glycan processing and endoplasmic reticulum quality control. EMBO J. 2000;19(4):581-8. DOI:10.1093/emboj/19.4.581 |
9. Mulakala C and Reilly PJ. (2002). Understanding protein structure-function relationships in Family 47 alpha-1,2-mannosidases through computational docking of ligands. Proteins. 2002;49(1):125-34. DOI:10.1002/prot.10206 |
10. Cantú D, Nerinckx W, and Reilly PJ. (2008). Theory and computation show that Asp463 is the catalytic proton donor in human endoplasmic reticulum alpha-(1-->2)-mannosidase I. Carbohydr Res. 2008;343(13):2235-42. DOI:10.1016/j.carres.2008.05.026 |
11. Tempel W, Karaveg K, Liu ZJ, Rose J, Wang BC, and Moremen KW. (2004). Structure of mouse Golgi alpha-mannosidase IA reveals the molecular basis for substrate specificity among class 1 (family 47 glycosylhydrolase) alpha1,2-mannosidases. J Biol Chem. 2004;279(28):29774-86. DOI:10.1074/jbc.M403065200 |