Glycoside Hydrolase Family 117

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• Author: Etienne Rebuffet
• Responsible Curator: Mirjam Czjzek

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Glycoside Hydrolase Family GH117
Clan	None
Mechanism	Not known
Active site residues	Not known
CAZy DB link
http://www.cazy.org/GH117.html

Substrate specificities

The only activity so far characterized within this recently discovered family of glycoside hydrolases is that of α-1,3-L-(3,6-anhydro)-galactosidase [1, 2, 3, 4, 5]. Nevertheless phylogenetic analyses (Figure 1) of this family together with activity tests for another member, Zg3597 (Clade C), show that the family GH117 most probably is polyspecific [3].

Figure 1: Phylogeny of GH117 family (click to enlarge). From [3].

Kinetics and Mechanism

The stereochemical outcome of members of glycoside hydrolase family GH117 is still not determined experimentally. Nevertheless a mechanism based on the structure of an inactive mutant (BpGH117 E303Q) complexed to a neoagarobiose has been proposed [5] (Figure 2). In this unusual inverting catalytic mechanism an aspartic acid acting as the base and a histidine acting as the acid. An analogous Asp-His dyad has been similarly reported to act as the general base catalyst in the retaining mechanism of select GH3 members [6].

Figure 2: Proposed mechanism of α-1,3-L-(3,6-anhydro)-galactosidase. From [5]

Two of the three 3D structures revealed the presence of a divalent cation, directly coordinated only by water molecules, close to the active site, which could activate the catalytic water molecule and provide the energy needed for the enzymatic reaction [3, 5]. Sequence alignments suggest that the enzymes of clades B and C do not bind divalent cation, which could be related to their difference in substrate specificity [3].

Catalytic Residues

From structural analysis and sequence alignments the catalytic residues have been predicted to be Asp-90 as the base and His-302 as the acid BpGH117 numbering) [5].

Three-dimensional structures

Three crystal structures of GH117 family have been reported. Two are enzymes from marine bacteria, one from Saccharophagus degradans (PDB: 3R4Y) [4] and one from Zobellia galactanivorans (PDB: 3P2N) [3], the third one is from the human gut bacterium Bacteroidetes plebeius (PDB: 4AK5) [5]. GH117 adopts a five-bladed β-propeller fold and forms a dimer via domain-swapping of the N-terminal HTH (Helix-Turn-Helix) domain (Figure 3) [3]. Interestingly, previous sequences reported from Vibrio sp. JT0107 and Bacillus sp. MK03 contain the conserved domain-swapping signature SxAxxR in the HTH domain. Consistently, these proteins were reported to form multimers (a dimer and an octamer respectively), based on calibrated gel filtration estimations [1, 2]. In contrast, RB13146 (Clade B) lacks the domain-swapping signature, in which the crucial residues are missing. This enzyme from R. baltica thus likely occurs as a monomer and may represent an ‘ancestral’ form of the GH117 family, which would be limited to the catalytic β-propeller domain [3]. Structure of SdNABH and BpGH117 possess a ordered C terminus part which also interact with the adjacent monomer [4, 5]. Moreover in the case of BpGH117, His-392 from the C terminus of the monomer A participate in the substrate binding in the binding pocket of monomer B, and aims versa [5].

Figure 3: Structure of the dimer of AghA. From [3].

Family Firsts

First stereochemistry determination

not determined yet.

First catalytic nucleophile identification

not determined yet.

First general acid/base residue identification

not determined yet.

First 3-D structure

The first 3D structure was reported in 2011 for an α-1,3-L-(3,6-anhydro)-galactosidase (AhgA or Zg4663) from the marine bacteria Zobellia galactanivorans, PDB: 3p2n [3].

References

1. Sugano Y, Kodama H, Terada I, Yamazaki Y, and Noma M. (1994). Purification and characterization of a novel enzyme, alpha-neoagarooligosaccharide hydrolase (alpha-NAOS hydrolase), from a marine bacterium, Vibrio sp. strain JT0107. J Bacteriol. 1994;176(22):6812-8. DOI:10.1128/jb.176.22.6812-6818.1994 |
2. Suzuki H, Sawai Y, Suzuki T, and Kawai K. (2002). Purification and characterization of an extracellular alpha-neoagarooligosaccharide hydrolase from Bacillus sp. MK03. J Biosci Bioeng. 2002;93(5):456-63. DOI:10.1016/s1389-1723(02)80092-5 |
3. Rebuffet E, Groisillier A, Thompson A, Jeudy A, Barbeyron T, Czjzek M, and Michel G. (2011). Discovery and structural characterization of a novel glycosidase family of marine origin. Environ Microbiol. 2011;13(5):1253-70. DOI:10.1111/j.1462-2920.2011.02426.x |
4. Ha SC, Lee S, Lee J, Kim HT, Ko HJ, Kim KH, and Choi IG. (2011). Crystal structure of a key enzyme in the agarolytic pathway, α-neoagarobiose hydrolase from Saccharophagus degradans 2-40. Biochem Biophys Res Commun. 2011;412(2):238-44. DOI:10.1016/j.bbrc.2011.07.073 |
5. Hehemann JH, Smyth L, Yadav A, Vocadlo DJ, and Boraston AB. (2012). Analysis of keystone enzyme in Agar hydrolysis provides insight into the degradation (of a polysaccharide from) red seaweeds. J Biol Chem. 2012;287(17):13985-95. DOI:10.1074/jbc.M112.345645 |
6. Litzinger S, Fischer S, Polzer P, Diederichs K, Welte W, and Mayer C. (2010). Structural and kinetic analysis of Bacillus subtilis N-acetylglucosaminidase reveals a unique Asp-His dyad mechanism. J Biol Chem. 2010;285(46):35675-84. DOI:10.1074/jbc.M110.131037 |