Glycoside Hydrolase Family 38

• Author: David Rose
• Responsible Curator: David Rose

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Glycoside Hydrolase Family GH38
Clan	GH-x
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/fam/GH38.html

Substrate specificities

GH38 enzymes are Class II α-mannosidases. They range in breadth of specificity from the Golgi α-mannosidase (2A1), which has a dual specificity for α1,6 and α1,3-linked mannoses, to the lysosomal mannosidases, which have either broad (2B1 cleaves α1,2, α1,3 and α1,6 linkages) or narrow specificities (2B2 is specific for α1,6). GH38 active sites can be quite long and open, and some are sensitive to the polysaccharide substrate structure. For example, Golgi α-mannosidase II requires the presence of a GlcNAc residue some five residues away from the cleavage site, while lysosomal mannosidases do not have that requirement [1][2].

Kinetics and Mechanism

GH38 enzymes operate by the conventional Koshland double-displacement retaining mechanism. There have been GH38 mannosidases identified in a number of different localizations, classed into subfamilies with different substrate specificities and biochemical properties, and, presumably, different physiological roles. The Golgi enzyme is identified as 2A1 (Class 2, A for Golgi, enzyme 1). Lysosomal GH38 mannosidases are indicated by 'B' (2B1, 2B2) and those likely existing in the cytoplasm by 'C'. Physiological roles have been identified for the Golgi enzyme in the protein N-glycosylation pathway and lysosomal mannosidases in general are likely to be involved in scavenging of degraded glycoproteins. Roles for the cytoplasmic subclass have not been identified definitively, but they play a role in protein recognition or signalling.

Catalytic Residues

Both catalytic side chains are Asp residues. The nucleophile Asp204 (Golgi α-mannosidase II numbering) was inferred from previous studies with Jack Bean mannosidase [3] and confirmed in the crystal structures of covalent intermediates [4]. Mutagenesis studies implicated Asp341 as the likely acid-base catalyst.

Three-dimensional structures

The crystal structure of the GH38 domain of Drosophila Golgi α-mannosidase II [1] has been determined in complex with a large number of inhibitors and catalytic intermediate mimics. Many of these are at very high resolution: 1.2-1.6Å ([5][6][7][8][9] for examples). An enzyme-substrate complex has also been determined [10] The crystal structure of bovine lysosomal α-mannosidase II was determined to 2.7Å resolution indicated interesting low-pH activation effects [11] .

Family Firsts

First sterochemistry determination

First catalytic nucleophile identification

Jack Bean mannosidase [3]

First general acid/base residue identification

Golgi α-mannosidase II covalent intermediate stabilization [4]

First 3-D structure

Golgi α-mannosidase II [1]

References

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1. van den Elsen JM, Kuntz DA, and Rose DR. (2001). Structure of Golgi alpha-mannosidase II: a target for inhibition of growth and metastasis of cancer cells. EMBO J. 2001;20(12):3008-17. DOI:10.1093/emboj/20.12.3008 |

[[Category:Glycoside Hydrolase Families]]