Glycoside Hydrolase Family 55

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

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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 of family 55 exclusively consists of β-1,3-glucanases, including both exo- and endo-enzymes, at this moment. All of biochemically characterized members of this family have fungal origin, but not from yeast. Several homologous genes are found from Bacterial genomes, but none of their gene products are characterized.

The enzymes belonging to this family are generally called "laminarinase" because the enzyme hydrolyzes laminarin (β-1,3-glucan having single β-1,6-glucoside side chains: β-1,3/1,6-glucan) from brown algae. But 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). Exo-β-1,3-glucanases in this family cleave the terminal β-1,3-glycosidic linkage at non-reducing end of β-1,3-glucans or β-1,3/1,6-glucans. Many of them produces gentiobiose (β-D-glucopyranosyl-1,6-D-glucose) in addition to glucose during degradation of β-1,3/1,6-glucan[1][2].

Bgn13.1 from Trichoderma harzianum [3] and LamAI fromTrichoderma viride [4] were described as endo-type enzymes (EC3.2.1.39) based on the experimental results.

Kinetics and Mechanism

Family 55 enzymes are inverting enzyme, 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], but the genes for these enzymes have not cloned yet.

Catalytic Residues

From the crystal structure of Lam55A from Phanerochaete chrysospoirum complexed with gluconolactone, Glu633 appears to be the general acid. A possible nucleophilic water was found near the C-1 atom of gluconolactone, but no acidic residue that can be the general base was found around the water.

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

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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