Glycoside Hydrolase Family 53

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• Author: ^^^Leila Lo Leggio^^^
• Responsible Curator: ^^^Leila Lo Leggio^^^

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Glycoside Hydrolase Family GH53
Clan	GH-A
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/fam/GH53.html

Substrate specificities

The only known specificity for this family is beta-1,4-galactanase (EC 3.2.1.89) and the only reported function is the microbial degradation of galactans and arabinogalactans in the pectic component of plant cell walls. A number of patents on industrial applications of GH53 have been filed.

Kinetics and Mechanism

GH53 beta-1,4-galactanases follow a classical retaining mechanism as first demonstrated by following the stereochemical course of rection for the endo-beta-1,4-galactanase of the bacterium Cellvibrio japonicus (at that time referred to as Pseudomonas fluorescens subspecies cellulosa) [1]. Most characterized members have been reported to have an endo-mode of action, although processivity has been suggested in one case [2].

Catalytic Residues

The catalytic residues were first identified for the endo-beta-1,4-galactanase of the bacterium Cellvibrio japonicus [1] (at that time referred to as Pseudomonas fluorescens subspecies cellulosa). As expected for a member for clan GH-A, the two catalytic residues were, by a combination of mutagenesis and kinetic analysis, identified to be two glutamates, one acting as an acid-base (E161) and the other as a nucleophile (E270).

Three-dimensional structures

As for all members of Clan GH-A [3,4], structurally characterized GH53 enzymes [5-7] display a (beta/alpha)8 barrel structure for the catalytic domain, usually with fairly compact loop structure and a sequence under 400 residues in length. The catalytic residues are typically positioned at the C-terminal ends of beta strands 4 and 7 in the barrel. Somewhat unusually, none of the four structurally characterized GH53 catalytic domains was accompanied by other catalytic domains or accessory modules, but modularity can be inferred by sequence in other members of the family. A disulphide bridging two loops (beta/alpha loops 7 and 8) in 3 known fungal structures [5-6], is replaced functionally by a calcium ion in one bacterial structure [7]. For one bacterial member of the family ligand complexes with products have been obtained crystallographically, occupying subsites -4 to -2 and +1 to +2 [7-8]. Based on these crystal structures, binding of a galactononaose fragment has also been computationally modelled [8].

Family Firsts

First sterochemistry determination

Cite some reference here, with a short (1-2 sentence) explanation [1].

First catalytic nucleophile identification

Cite some reference here, with a short (1-2 sentence) explanation [1].

First general acid/base residue identification

Cite some reference here, with a short (1-2 sentence) explanation [1].

First 3-D structure

Cite some reference here, with a short (1-2 sentence) explanation [2].

References

1. Braithwaite KL, Barna T, Spurway TD, Charnock SJ, Black GW, Hughes N, Lakey JH, Virden R, Hazlewood GP, Henrissat B, and Gilbert HJ. (1997). Evidence that galactanase A from Pseudomonas fluorescens subspecies cellulosa is a retaining family 53 glycosyl hydrolase in which E161 and E270 are the catalytic residues. Biochemistry. 1997;36(49):15489-500. DOI:10.1021/bi9712394 |
3. Hinz SW, Pastink MI, van den Broek LA, Vincken JP, and Voragen AG. (2005). Bifidobacterium longum endogalactanase liberates galactotriose from type I galactans. Appl Environ Microbiol. 2005;71(9):5501-10. DOI:10.1128/AEM.71.9.5501-5510.2005 |
4. Ryttersgaard C, Le Nours J, Lo Leggio L, Jørgensen CT, Christensen LL, Bjørnvad M, and Larsen S. (2004). The structure of endo-beta-1,4-galactanase from Bacillus licheniformis in complex with two oligosaccharide products. J Mol Biol. 2004;341(1):107-17. DOI:10.1016/j.jmb.2004.05.017 |