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Glycoside Hydrolase Family 45

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Glycoside Hydrolase Family GH45
Clan none
Mechanism inverting
Active site residues known (but see discussion)
CAZy DB link
http://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
    [Schulein1998]
  2. Error fetching PMID 8377830: [Davies1993]
  3. Error fetching PMID 8519779: [Davies1995]
  4. Error fetching PMID 1390665: [Ludvigsen]
  5. Error fetching PMID 16984999: [Yennawar]
  6. Error fetching PMID 18971341: [Kerff]
  7. Error fetching PMID 18676702: [Igarashi]
  8. Error fetching PMID 8223652: [Schou93]
All Medline abstracts: PubMed | HubMed