Glycoside Hydrolase Family 173

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• Authors: Clelton Aparecido dos Santos and Gabriela Felix Persinoti
• Responsible Curator: Mario Murakami

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Glycoside Hydrolase Family GH173
Clan	GH-A
Mechanism	retaining (inferred)
Active site residues	known (inferred)
CAZy DB link
http://www.cazy.org/GH173.html

Substrate specificities

The GH173 family comprises members with β-galactosidase activity [1]. The founding member of the GH173 family named herein as CapGH173 (PBMDCECB_44807), was identified in a novel metagenome-assembled genome (MAG) recovered from the Bacteroidales bacterium MAG42, a novel genus among the Uncultivated Bacteria and Archaea family (UBA932). CapGH173 was found in a Polysaccharide Utilization Loci (PUL) that includes enzymes from families GH2 and GH78; however, a multidomain protein organization was also predicted among GH173 members, having an additional GH36 domain. Biochemical characterization of CapGH173 showed it is active on p-nitrophenyl-β-D-galactopyranoside (pNP-β-D-Gal) and kinetic parameters were determined from substrate saturation curves. Phylogenetic analysis showed that CapGH173 is remotely related to GH-A CAZy families, with GH30 and GH5 being the closest ones [1].

Kinetics and Mechanism

Family GH173 members are retaining enzymes (inferred [1]).

Catalytic Residues

The residues E305 and E207 correspond to the nucleophile and acid/base, respectively (inferred by structural superposition) [1].

Three-dimensional structures

Protein modeling and threading performed using AlphaFold and PDBsum, respectively, revealed that CapGH173 consists of an (α/β)₈-barrel structure, which is an archetypal scaffold of the clan GH-A. According to structural predictions, CapGH173 exhibits a two-domain architecture including an appended β-sandwich domain, which is a similar structural organization found in the GH30 family. Except for the residues defining the clan GH-A, sequence alignment with GH5 and GH30 members revealed a very low sequence conservation below the criterium for significant similarity detection (using an e-value < 0.05), demonstrating that although the domains in the tertiary structure can be similar, the sequences between these families are remarkably diverse [1].  

Family Firsts

First stereochemistry determination

CpGH173 is shown to have an inferred retaining mechanism.

First catalytic nucleophile identification

E305 (inferred).

First general acid/base residue identification

E207 (inferred).

First 3-D structure

Currently no experimental structure is available for GH173 members.

References

1. Cabral L, Persinoti GF, Paixão DAA, Martins MP, Morais MAB, Chinaglia M, Domingues MN, Sforca ML, Pirolla RAS, Generoso WC, Santos CA, Maciel LF, Terrapon N, Lombard V, Henrissat B, and Murakami MT. (2022). Gut microbiome of the largest living rodent harbors unprecedented enzymatic systems to degrade plant polysaccharides. Nat Commun. 2022;13(1):629. DOI:10.1038/s41467-022-28310-y |