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Glycoside Hydrolase Family 8

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Glycoside Hydrolase Family 8
Clan GH-M
Mechanism inverting
Active site residues known
CAZy DB link

Substrate specificities

Glycoside hydrolases of family 8 cleave β-1,4 linkages of β-1,4 glucans, xylans (or xylooligosaccharides), chitosans, and lichenans (1,3-1,4-β-D-glucan). All of GH8 members have been found from bacteria, and there are no members from Eukaryotic or Archaeal origin. The majority of the enzymes are endo-acting enzymes, but one member has an exo-activity that releases β-D-xylose residues from the reducing end of xylooligosaccharides. The substrate specificities found in GH8 are: chitosanase (EC, cellulase (EC, licheninase (EC, endo-1,4-β-xylanase (EC and reducing-end-xylose releasing exo-oligoxylanase (EC GH8 was one of the first glycoside hydrolase families classified by hydrophobic cluster analysis, and was previously known as "Cellulase Family D" [1, 2].

Kinetics and Mechanism

Enzymes of glycoside hydrolase family 8 are inverting enzymes, as first shown by Fierobe et al. who monitored the reaction of endoglucanase C from Clostridium cellulolyticum (CelCCC) using proton NMR spectroscopy [3]. Hydrolysis by CelA was computationally simulated with QM/MM metadynamics [4].

Glycosynthase engineering

Reducing-end-xylose releasing exo-oligoxylanase from Bacillus halodurans C-125 is the first inverting GH that was converted to glycosynthase by mutating the general base residue [5].

Catalytic Residues

The general acid (proton donor to the leaving group) was first identified in CelA from C. thermocellum as Glu95 [6]. The general base (proton acceptor from the nucleophilic water) of GH8a subfamily was first identified in CelA from C. thermocellum as Asp278 [6]. The general base of GH8b subfamily was first identified in chitosanase from Bacillus sp. K17 as Glu309 based on its crystal structure and by making E309Q mutant [7].

Three-dimensional structures

Several three-dimensional structures of GH8 members from bacterial origin have been solved. The first solved 3-D structure was endoglucanase CelA from Clostridium thermocellum (PDB ID 1cem) in 1996 [6]. As members of Clan GH-M they have a (α/α)6 fold similar to Glycoside Hydrolase Family 48. The general acid residue is located at the N-terminal end of α4 helix. Position of the general base differ among #Subfamilies. Atomic (0.94 Å) resolution structure of CelA in complex with substrate (PDB ID 1kwf) has been determined [8].


GH8 enzymes are divided into at least three subfamilies, depending on the position of the general base [7]. GH8a has the general base (Asp) at the N-terminal end of α8 helix. GH8a contains cellulases, xylanases and other enzymes. In GH8b enzymes, the Asp residue is replaced by Asn, and the general base is a Glu residue located in a long loop inserted between α7 and α8 helices. GH8b contains chitosanases, licheninases, cellulases and other enzymes. The position of the general base in GH8c is unknown.

Family Firsts

First sequence identification
Cellulase (celA) from Clostridium thermocellum [9]
First sterochemistry determination
Endoglucanase C from Clostridium cellulolyticum (CelCCC) [3]
First general acid residue identification
Cellulase (CelA) from Clostridium thermocellum [6]
First general base residue identification of GH8a
Cellulase (CelA) from Clostridium thermocellum [6]
First general base residue identification of GH8b
Chitosanase from Bacillus sp. K17 by crystal structure and a mutant [7].
First 3-D structure
Endoglucanase CelA from Clostridium thermocellum by X-ray crystallography (PDB ID 1cem) [6].


  1. Henrissat B, Claeyssens M, Tomme P, Lemesle L, and Mornon JP. (1989). Cellulase families revealed by hydrophobic cluster analysis. Gene. 1989;81(1):83-95. DOI:10.1016/0378-1119(89)90339-9 | PubMed ID:2806912 [Henrissat1989]
  2. Gilkes NR, Henrissat B, Kilburn DG, Miller RC Jr, and Warren RA. (1991). Domains in microbial beta-1, 4-glycanases: sequence conservation, function, and enzyme families. Microbiol Rev. 1991;55(2):303-15. DOI:10.1128/mr.55.2.303-315.1991 | PubMed ID:1886523 [Gilkes1991]
  3. Fierobe HP, Bagnara-Tardif C, Gaudin C, Guerlesquin F, Sauve P, Belaich A, and Belaich JP. (1993). Purification and characterization of endoglucanase C from Clostridium cellulolyticum. Catalytic comparison with endoglucanase A. Eur J Biochem. 1993;217(2):557-65. DOI:10.1111/j.1432-1033.1993.tb18277.x | PubMed ID:8223599 [Fierobe1993]
  4. Petersen L, Ardèvol A, Rovira C, and Reilly PJ. (2009). Mechanism of cellulose hydrolysis by inverting GH8 endoglucanases: a QM/MM metadynamics study. J Phys Chem B. 2009;113(20):7331-9. DOI:10.1021/jp811470d | PubMed ID:19402614 [Petersen2009]
  5. Honda Y and Kitaoka M. (2006). The first glycosynthase derived from an inverting glycoside hydrolase. J Biol Chem. 2006;281(3):1426-31. DOI:10.1074/jbc.M511202200 | PubMed ID:16301312 [Honda2006]
  6. Alzari PM, Souchon H, and Dominguez R. (1996). The crystal structure of endoglucanase CelA, a family 8 glycosyl hydrolase from Clostridium thermocellum. Structure. 1996;4(3):265-75. DOI:10.1016/s0969-2126(96)00031-7 | PubMed ID:8805535 [Alzari1996]
  7. Adachi W, Sakihama Y, Shimizu S, Sunami T, Fukazawa T, Suzuki M, Yatsunami R, Nakamura S, and Takénaka A. (2004). Crystal structure of family GH-8 chitosanase with subclass II specificity from Bacillus sp. K17. J Mol Biol. 2004;343(3):785-95. DOI:10.1016/j.jmb.2004.08.028 | PubMed ID:15465062 [Adachi2004]
  8. Guérin DM, Lascombe MB, Costabel M, Souchon H, Lamzin V, Béguin P, and Alzari PM. (2002). Atomic (0.94 A) resolution structure of an inverting glycosidase in complex with substrate. J Mol Biol. 2002;316(5):1061-9. DOI:10.1006/jmbi.2001.5404 | PubMed ID:11884144 [Guerin2002]
  9. Béguin P, Cornet P, and Aubert JP. (1985). Sequence of a cellulase gene of the thermophilic bacterium Clostridium thermocellum. J Bacteriol. 1985;162(1):102-5. DOI:10.1128/jb.162.1.102-105.1985 | PubMed ID:3980433 [Beguin1985]

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