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The GH173 family comprises so far members with β-galactosidase activity [1]. The founding member of the GH173 family named herein as CapGH173 (PBMDCECB_44807), was identified in a metagenome-assembled genome (Bacteroidales bacterium MAG42) recovered from capybara gut microbiota, which potentially represents a novel genus from the UBA932 family. The enzyme CapGH173 was found in a Polysaccharide Utilization Loci (PUL) that includes enzymes from the families GH2 and GH78, yet the target carbohydrate remains unclear. GH173 members often exhibit a modular architecture including a GH36 domain. Substrate kinetics characterization of CapGH173 was demonstrated on synthetic substrate p-nitrophenyl-β-D-galactopyranoside (pNP-β-D-Gal). Phylogenetic analysis showed that this family belongs to the GH-A clan, being remotely related to the families GH30 and GH5 [1].
Kinetics and Mechanism
GH173 members are inferred to be retaining enzymes [1].
Catalytic Residues
The residues E305 and E207 (sequence numbering based on the founding member, CapGH173) was inferred to correspond to the nucleophile and acid/base, respectively, based on the structural analysis of AlphaFold models [1].
Three-dimensional structures
Protein modeling and threading performed using AlphaFold and PDBsum, respectively, suggest 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 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
CapGH173 is inferred to operate by a 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.
