Glycoside Hydrolase Family 188

This page is currently under construction. This means that the Responsible Curator has deemed that the page's content is not quite up to CAZypedia's standards for full public consumption. All information should be considered to be under revision and may be subject to major changes.

• Author:
• Responsible Curator: Spencer Williams

---

Glycoside Hydrolase Family GH188
Clan	GH-x
Mechanism	NAD-dependent hydrolysis
Active site residues	known
CAZy DB link
http://www.cazy.org/GH188.html

Substrate specificities

The glycoside hydrolases of this family are found in bacteria, algae, plants and a small number of archaea [1]. The family contains enzymes with sulfoquinovosidase activity (EC 3.2.1.199), namely the ability to cleave glycosides of 6-deoxy-6-sulfoquinovose. Sulfoquinovosidases are also found in family GH31 [2]. Enzymes of this family have the ability to cleave both α- and β-glycosides, and are dependent on an NAD⁺ cofactor.

Kinetics and Mechanism

GH188 enzymes utilize an NAD-dependent hydrolysis mechanism that proceeds through oxidation-elimination-addition-reduction steps. Exchange of the substrate C2 proton with solvent deuterium during the enzyme-catalyzed reaction was demonstrated by mass spectrometry [1]. The following chemical mechanism is proposed: (1) C3 hydride abstraction via the reduction of NAD⁺ cofactor to NADH and simultaneous oxidation of the C3 hydroxyl group; (2) α to the ketone functionality, the C2 proton is deprotonated by a general catalytic base residue; (3) cleavage of the C1-O1 bond occurs in an α,β-elimination, producing an α,β-unsaturated ketone intermediate; (4) 1,4-Michael-like addition of a water molecule at C1; and (5) reduction of the C3 carbonyl functionality by the enzyme-bound NADH generates the product.

Catalytic Residues

Catalytic residues can be inferred on the basis of X-ray crystallographic data for complexes of GH188 enzymes with NAD⁺ and sulfoquinovose [1]. Tyr136 in Arthrobacter sp. strain U41 SqgA is conserved in all family GH188 members and is located close to C2-OH, suggesting a possible role as a general base. His321 is located close to the C1-OH and may act as general acid (along with Tyr136) for the glycosidic oxygen facilitating glycosidic bond scission. The sulfonate group is recognized by a triad of amino acids: one oxygen H-bonds to Arg166 (2.6 Å), a second to Lys172 (2.9 Å), and a third to the backbone amide of Leu170 (2.8 Å).

Three-dimensional structures

Crystallographic data is available for at least two GH188 enzymes, including as complexes with NADH, and NADH plus sulfoquinovose. The 3D X-ray crystal structures include those of Flavobacterium sp. strain K172 SqgA (PDB 8QC8, 8QC2) and Arthrobacter sp. strain U41 SqgA (PDB 8QC3, 8QC6. 8QC5) [1]. Each enzyme possesses an N-terminal dinucleotide-binding Rossman fold. The GH188 enzymes show structural similarities to inositol-2-dehydrogenase, glucose-fructose/IDH/MocA-like oxidoreductase, and NAD⁺-dependent N-acetylgalactosaminidase of family GH109.

Family Firsts

First stereochemistry determination

not applicable

First catalytic residue identification

Arthrobacter sp. strain U41 SqgA using 3D X-ray crystal structure [1]

First 3-D structural determination

Flavobacterium sp. strain K172 SqgA and Arthrobacter sp. strain U41 SqgA [1]

First GH188 enzyme shown to hydrolyze both α- and β-substrates

Flavobacterium sp. strain K172 SqgA [1]

1. Kaur2024 pmid=38100472

1. Speciale2016 pmid=26878550

</biblio>