Glycoside Hydrolase Family 55

• Authors: Takuya Ishida and ^^^Kiyohiko Igarashi^^^
• Responsible Curator: ^^^User:ShinyaFushinobu|Shinya Fushinobu^^^

---

Glycoside Hydrolase Family 55
Clan	none
Mechanism	inverting
Active site residues	not known
CAZy DB link
http://www.cazy.org/fam/GH55.html

Substrate specificities

Glycoside Hydrolases family 55 consists exclusively of β-1,3-glucanases, including both exo- and endo-enzymes. All biochemically characterized members of this family are of fungal origin, although there are no yeast homologues. Several homologous genes have been identified in bacterial genomes, but none of their gene products are characterized.

The enzymes belonging to this family are generally called "laminarinases," because they hydrolyze laminarin from brown algae (β-1,3-glucan having single β-1,6-glucoside side chains: β-1,3/1,6-glucan). However, the physiological substrate for the enzymes might be fungal cell wall, whose major component is also β-1,3/1,6-glucan.

The majority of the members in this family are exo-glucan-1,3-β-glucosidases (EC3.2.1.58), which cleave the terminal β-1,3-glycosidic linkage at the non-reducing end of β-1,3-glucans or β-1,3/1,6-glucans. Many produce gentiobiose (β-D-glucopyranosyl-1,6-D-glucose) in addition to glucose during the degradation of β-1,3/1,6-glucan[1, 2].

Bgn13.1 from Trichoderma harzianum [3] and LamAI fromTrichoderma viride [4] were characterised as endo-acting enzymes (EC3.2.1.39).

Kinetics and Mechanism

Family 55 enzymes are inverting enzymes, as shown by NMR analysis on ExgS from Aspergillus saitoi [5]. This result is consistent with many classical reports on gentiobiose-producing exo-β-1,3-glucanases from fungi[6, 7], although the genes for these enzymes have not yet been described.

Catalytic Residues

The crystal structure of Lam55A from Phanerochaete chrysospoirum complexed with gluconolactone, suggests that Glu633 is the general acid. A candidate nucleophilic water was found near the C-1 atom of gluconolactone, but no acidic residue corresponding to the general base was identified in the vicinity of the water molecule.

In classical studies of a exo-β-1,3-glucanase from Basidiomycete QM-806, Jeffcoat and Kirkwood reported that chemical modification of histidine in the catalytic site of the enzyme caused irreversible loss of activity, suggesting a crucial role of the histidine residues [8].

Three-dimensional structures

The first solved 3-D structure was exo-β-1,3-glucanase Lam55A from P. chrysosporium [9]. In this structure, two tandem β-helix domains are positioned side-by-side to form a rib cage-like structure. The active site is located between the two β-helix domains.

Family Firsts

First sterochemistry determination

Probably ExgS from A. saitoi by ¹H-NMR analysis [5]. See kinetics and mechanism.

First gene cloning

BGN13.1 from T. harzianum. [6].

First general acid residue identification

First general base residue identification

First 3-D structure

Lam55A from Phanerochaete chrysosporium K-3 by X-ray crystallography (PDB 3eqo) [9].

References

Error fetching PMID 8948426:
Error fetching PMID 12594027:
Error fetching PMID 7592488:
Error fetching PMID 12843664:
Error fetching PMID 5416668:
Error fetching PMID 3100526:

5. Kasahara S, Nakajima T, Miyamoto C, Wada K, Furuichi Y, and Ichishima E. Characterization and mode of action of exo-1,3-β-D-glucanase from Aspergillus saitoi. J Ferment Bioeng 74 (4), 238-240 (1992).DOI:10.1016/0922-338X(92)90118-E

7. Nagasaki N, Saito K, and Yarnamoto S. Purification and characterization of an exo-β-l,3-glucanase from a fungi imperfecti. Agric Biol Cbem 41, 493-502 (1977).JOI:JST.Journalarchive/bbb1961/41.493

9. Ishida T, Fushinobu S, Kawai R, Kitaoka M, Igarashi K, and Samejima M. (2009). Crystal structure of glycoside hydrolase family 55 {beta}-1,3-glucanase from the basidiomycete Phanerochaete chrysosporium. J Biol Chem. 2009;284(15):10100-9. DOI:10.1074/jbc.M808122200 |