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Glycoside Hydrolase Family 45
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|Glycoside Hydrolase Family GH45|
|Active site residues||known (but see discussion)|
|CAZy DB link|
Glycoside hydrolases of GH45 are endoglucanases (EC 18.104.22.168); mainly the hydrolysis of soluble β -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 α-D anomer of the oligosaccaride 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.
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" ; 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 .
Family GH45 enzymes are structurally related to plant  and bacterial  expansins. Indeed they even display some of the catalytic centre motifs such as the catalytic acid. The putative catalytic base is absent in expansins.
- First sterochemistry determination
- As part of an analysis of many families reported in .
- 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 .
- 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. 365, 362-4. DOI:10.1038/365362a0 |
- 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. 34, 16210-20.
- Ludvigsen S and Poulsen FM. (1992) Three-dimensional structure in solution of barwin, a protein from barley seed. Biochemistry. 31, 8783-9.
- Yennawar NH, Li LC, Dudzinski DM, Tabuchi A, and Cosgrove DJ. (2006) 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. 103, 14664-71. DOI:10.1073/pnas.0605979103 |
- Kerff F, Amoroso A, Herman R, Sauvage E, Petrella S, Filée P, Charlier P, Joris B, Tabuchi A, Nikolaidis N, and Cosgrove DJ. (2008) Crystal structure and activity of Bacillus subtilis YoaJ (EXLX1), a bacterial expansin that promotes root colonization. Proc Natl Acad Sci U S A. 105, 16876-81. DOI:10.1073/pnas.0809382105 |
- 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. 217, 947-53.