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 had been limited to fungal enzymes until the extensive work by Bianchetti, Takasuka, et al., who reported the crystallography of exo-β-1,3-glucanase SacteLam55A from Streptmyces sp. SirexAA-E (Uniprot G2NFJ9), together with characterization of many other bacterial enzymes [1].

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 (β-1,3-glucan having single β-1,6-glucoside side chains, also known as laminarin, from brown algae). Many produce gentiobiose (β-D-glucopyranosyl-1,6-D-glucose) in addition to glucose during the degradation of β-1,3/1,6-glucan [2, 3]. Due to activity on laminarin, GH55 members may be referred to as "laminarinases." However, the physiological substrate for these enzymes might be fungal cell walls, whose major component is also β-1,3/1,6-glucan.

Bgn13.1 from Hypocrea lixii (formerly known as Trichoderma harzianum) [4] and LamAI from Trichoderma viride [5] 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) [6]. Release of α-glucose was subsequently confirmed by polarimetric analysis on family 55 enzymes from Acremonium persicinum [2]. These results are consistent with many classical reports on gentiobiose-producing exo-β-1,3-glucanases from fungi [7, 8], although the genes for these enzymes have not yet been described.

Catalytic Residues

A crystal structure of exo-β-1,3-glucanase Lam55A from Phanerochaete chrysospoirum K-3 (PcLam55A) complexed with gluconolactone (PDB ID 3eqo) suggested that Glu633 is the general acid. A candidate nucleophilic water was found near the C-1 atom of gluconolactone. A crystal structure of the bacterial enzyme SacteLam55A complexed with laminarihexaose (PDB ID 4pf0), together with kinetic analysis of site-directed mutants, revealed that corresponding glutamic acid (Glu502 in SacteLam55A) also functions as the general acid in bacterial enzymes.

The identification of the general base in this family is less clear, as is common with several other inverting GH families. In the crystal structures of both PcLam55A and SacteLam55A, the candidate nucleophilic water has no direct interaction with a sidechain carboxylate, but rather with a highly conserved glutamine residue that is, in turn, hydrogen-bonded to a conserved glutamic acid (Glu480 in SacteLam55A). Mutations of this glutamic acid (E480Q and E480A of SacteLam55A) significantly reduce catalytic activity. Based on these kinetic and structural observations, a proton relay system for the activation of water has been proposed [1].

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 residues in the catalytic site of the enzyme caused irreversible loss of activity, suggesting a crucial role for this residue [9].

Three-dimensional structures

The first solved 3-D structure was Lam55A from P. chrysosporium [10]. 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. A duplicated motif had been found in the primary sequence of EXG1 from Cochliobolus carbonum [11], predicting the presence of two structurally similar domains in this family.

The SacteLam55A E502A structure complexed with laminarioligosaccharides revealed the positioning of the substrate in the active site and the conformation of the proposed catalytic residues. The structure also shows a solvent-exposed Surface Binding Site [1].

Family Firsts

First sterochemistry determination

Probably ExgS from A. saitoi by H-NMR analysis [6]. See kinetics and mechanism.

First gene cloning

BGN13.1 from T. harzianum (Uniprot P53626) [4] and EXG1 from C. carbonum (partial gene coning and gene knockout) (Uniprot P49426) [12]. First bacterial gene was cloned from Arthrobacter sp. NHB-10 (Uniprot A4PHQ5) [13].

First general acid residue identification

SacteLam55A from Streptmyces sp. SirexAA-E (Uniprot G2NFJ9) by crystal structure and kinetic analysis on mutants [1].

First general base residue identification

SacteLam55A from Streptmyces sp. SirexAA-E (Uniprot G2NFJ9) by crystal structure and kinetic analysis on mutants [1].

First 3-D structure

Lam55A from P. chrysosporium by X-ray crystallography [10].

References

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1. Bianchetti CM, Takasuka TE, Deutsch S, Udell HS, Yik EJ, Bergeman LF, and Fox BG. (2015). Active site and laminarin binding in glycoside hydrolase family 55. J Biol Chem. 2015;290(19):11819-32. DOI:10.1074/jbc.M114.623579 |

6. 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

8. 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