Glycoside Hydrolase Family 55

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

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

Substrate specificities

Glycoside Hydrolase 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 the corresponding gene products have been 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 (EC 3.2.1.58), 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 Hypocrea lixii (formerly known as Trichoderma harzianum) [3] and LamAI from Trichoderma viride [4] were characterised as endo-acting enzymes (EC 3.2.1.39).

Kinetics and Mechanism

Family 55 enzymes are inverting enzymes, as shown by ¹NMR analysis on ExgS from Aspergillus phoenicis (formerly known as Aspergillus saitoi) [5]. Release of α-glucose was subsequently confirmed by polarimetric analysis on family 55 enzymes from Acremonium persicinum [1]. These results are 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 exo-β-1,3-glucanase Lam55A from Phanerochaete chrysospoirum K-3 complexed with gluconolactone (PDB ID 3eqo) 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 Sporotrichum dimorphosporum (formerly known as 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 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 H-NMR analysis [5]. See kinetics and mechanism.

First gene cloning

BGN13.1 from T. harzianum (Uniprot P53626) [3], EXG1 from Cochliobolus carbonum (partial gene coning and gene knockout) (Uniprot P49426) [10].

First general acid residue identification

First general base residue identification

First 3-D structure

Lam55A from P. chrysosporium by X-ray crystallography [9]. A duplicated motif had been found in the primary sequence of EXG1 from C. carbonum [11].

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