Glycoside Hydrolase Family 45

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• Author: Gideon Davies
• Responsible Curator: Gideon Davies

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Glycoside Hydrolase Family GH45
Clan	none
Mechanism	inverting
Active site residues	known (but see discussion)
CAZy DB link
https://www.cazy.org/GH45.html

Substrate specificities

Glycoside hydrolases of GH45 are endoglucanases (EC 3.2.1.4); mainly the hydrolysis of soluble β-1,4 glucans. GH45 enzymes are perhaps best known for their uses in the textile / detergent industries (see for example [1]).

Kinetics and Mechanism

The enzymes, formally known as cellulase family "K" in some historic literature, act with inversion of anomeric configuration to generate the α-D anomer of the oligosaccharide as product.

Catalytic Residues

Based upon the structure of the Humicola insolens endoglucanase V (now known as Cel45) [2, 3] it was concluded that Asp121 (in an HxD motif) acted as the general acid (implied by its hydrogen bonding to the glycosidic oxygen of a ligand in the +1 subsite) and that the most likely general base is Asp10 (in a YxD motif), appropriately positioned "below" the sugar plane. As with many inverting enzymes the general base assignment is less secure than that of the acid.

Three-dimensional structures

The 3-D structure of canonical GH45 enzymes is a six-stranded β-barrel to which a seventh strand is appended. The structure differs from classical β-barrels in containing both parallel and anti-parallel β-strands. At the time of the first structure solution the fold had ony previously been observed in "Barwin" [4]; a plant defense protein of unknown function. As is now expected for endo-enzymes, the active centre is located in an open substrate-binding groove. The original native (uncomplexed) structure had an disordered loop above the active centre and this was only seen to become ordered subsequently upon the binding of cello-oligosaccharides [3].

Family GH45 enzymes are structurally related to plant [5] and bacterial [6] expansins. Indeed they even display some of the catalytic centre motifs such as the catalytic acid. The putative catalytic base is absent in plant and bacterial expansins. There are also a few fungal GH45 members, exemplified by Phanerochaete chrysosporium Cel45 which also appear to lack the putative base [7].

Family Firsts

First sterochemistry determination

As part of an analysis of many families reported in [8].

First general acid identification

Catalytic residue proposals have been made solely on the basis of 3-D structure [2, 3].

First general base identification

Catalytic residue proposals have been made solely on the basis of 3-D structure [2, 3].

First 3-D structure

The Humicola insolens EGV (now Cel45) by the Davies group [2].

References

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1. Schülein M, Kauppinen M, Lange L, Lassen S, Andersen L, Klysner S, and Nielsen, J (1998) Characterization of fungal cellulases for fiber modification. ACS Symposium Series, 687 (Enzyme Applications in Fiber Processing): 66-74. DOI: 10.1021/bk-1998-0687.ch006
5. Yennawar NH, Li LC, Dudzinski DM, Tabuchi A, and Cosgrove DJ. Crystal structure and activities of EXPB1 (Zea m 1), a beta-expansin and group-1 pollen allergen from maize. Proc Natl Acad Sci U S A. 2006 Oct 3;103(40):14664-71. DOI:10.1073/pnas.0605979103 |
6. Kerff F, Amoroso A, Herman R, Sauvage E, Petrella S, Filée P, Charlier P, Joris B, Tabuchi A, Nikolaidis N, and Cosgrove DJ. Crystal structure and activity of Bacillus subtilis YoaJ (EXLX1), a bacterial expansin that promotes root colonization. Proc Natl Acad Sci U S A. 2008 Nov 4;105(44):16876-81. DOI:10.1073/pnas.0809382105 |