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
http://www.cazy.org/fam/GH45.html

Substrate specificities

Glycoside hydrolases of GH45 are endoglucanases (EC 3.2.1.4); mainly the hydrolysis of soluble beta 1,4 glucans.

Kinetics and Mechanism

The enzymes, formally known as cellulase family "K" in some historic literature, act with inversion of anomeric configuration to generate the alpha-D glucoside as product. Based upon the structure of the Humicola insolens endoglucanase V (now known as Cel45)[1][2] it was concluded that Asp121 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, appropriately positioned "below" the sugar plane. As with many inverting enzymes the base assignment is less secure than that of the acid.

Catalytic Residues

"Classical" GH45 enzymes likely use twin carboxylates corresponding to Asp10 and 121 of the Humicola insolens endoglucanase V.

Three-dimensional structures

The 3-D structure of canonical GH45 enzymes is a six-stranded b-barrel to which a seventh strand is appended. The structure differs from classical b-barrels in containing both paralle and anti-parallel b-strands. At the time of the first structure solution the fold had ony previously been observed in "Barwin"; 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 uncomplexes native structure had an disordered loop above the active centre and this was only seen to become ordered subsequently upon the binding of cello-oligosaccharides [2].

Family GH45 enzymes are structurally related to plant expansins. Indeed they even display some of the catalytic centre motifs such as the catalytic acid

Family Firsts

First sterochemistry determination

As part o an analysis of many families reported in [3].

First general acid/base residue identification

Catalytic residue proposals have been made solely on the basis of 3-D structure [1][2].

First 3-D structure

The Humicola insolens EGV (now Cel45) by the Davies group [1].

References

1. Davies GJ, Dodson GG, Hubbard RE, Tolley SP, Dauter Z, Wilson KS, Hjort C, Mikkelsen JM, Rasmussen G, and Schülein M. (1993). Structure and function of endoglucanase V. Nature. 1993;365(6444):362-4. DOI:10.1038/365362a0 |
2. Davies GJ, Tolley SP, Henrissat B, Hjort C, and Schülein M. (1995). Structures of oligosaccharide-bound forms of the endoglucanase V from Humicola insolens at 1.9 A resolution. Biochemistry. 1995;34(49):16210-20. DOI:10.1021/bi00049a037 |
3. Schou C, Rasmussen G, Kaltoft MB, Henrissat B, and Schülein M. (1993). Stereochemistry, specificity and kinetics of the hydrolysis of reduced cellodextrins by nine cellulases. Eur J Biochem. 1993;217(3):947-53. DOI:10.1111/j.1432-1033.1993.tb18325.x |

5. Sinnott, M.L. (1990) Catalytic mechanisms of enzymic glycosyl transfer. Chem. Rev. 90, 1171-1202. DOI: 10.1021/cr00105a006