Glycoside Hydrolase Family 8

• Author: Motomitsu Kitaoka
• Responsible Curator: Shinya Fushinobu

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Glycoside Hydrolase Family 8
Clan	GH-M
Mechanism	inverting
Active site residues	known
CAZy DB link
http://www.cazy.org/fam/GH8.html

Substrate specificities

GH8 enzymes cleave β-1,4 linkages of β-1,4 glucans, xylans (xylooligosaccharides), chitosans, and lichenans (1,3-1,4-β-D-glucan). 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 3.2.1.132), cellulase (EC 3.2.1.4), licheninase (EC 3.2.1.73), endo-1,4-β-xylanase (EC 3.2.1.8) and reducing-end-xylose releasing exo-oligoxylanase (EC 3.2.1.156).

Kinetics and Mechanism

GH8 enzymes are inverting enzymes, as first shown by ... (ref) on ... from ....

Catalytic Residues

The catalytic acid (proton donor) was first suggested/identified in ... as Glu... The catalytic base (proton acceptor) of GH8a subfamily was first suggested/identified in ... as Asp... The catalytic 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 [1].

Subfamilies

GH8 enzymes are divided into at least three subfamilies, depending on the position of the catalytic base [1]. GH8a has the catalytic base (Asp) at the N-terminal position of α8 helix. GH8a contains cellulases, xylanases and other enzymes. In GH8b enzymes, the Asp residue is replaced by Asn, and the catalytic 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 catalytic base in GH8c is unknown.

Three-dimensional structures

Several three-dimensional structures have been solved of GH8 members of bacterial origin. The first solved 3-D structure was endoglucanase CelA from Clostridium thermocellum (PDB 1cem) in 1996 [2]. As members of Clan GH-M they have a (α/α)₆ fold similar to Glycoside_Hydrolase_Family_48 GH48. Atomic (0.94 Å) resolution structure of CelA in complex with substrate (PDB 1kwf) has been determined [3].

Family Firsts

First sterochemistry determination

Cite some reference here, with a short explanation [4].

First catalytic acid residue identification

First catalytic base residue identification of GH8a

First catalytic base residue identification of GH8b

First 3-D structure

endoglucanase CelA from Clostridium thermocellum by X-ray crystallography (PDB 1cem) [2].

Glycosynthase

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

References

1. 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 |
2. 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 |
3. 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 |
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 |