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Glycoside Hydrolase Family 67
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| Glycoside Hydrolase Family GHnn | |
| Clan | GH-x |
| Mechanism | retaining/inverting |
| Active site residues | known/not known |
| CAZy DB link | |
| http://www.cazy.org/fam/GHnn.html | |
Substrate specificities
GH67 contains enzymes that display alpha-glucuronidase activity. The enzymes target the glucuronic acid appended to the C2-OH of the xylose at the non-reducing end of xylooligosaccharides. The enzymes display a preference for 4-O-methyl-D-glucuronic acid side chains. The length of the oligosacchride does not influence catalytic rate indicating that the enzyme only interacts with the uronic acid and the linked xylose. These enzymes do not remove glucuronic acid from internal regions of xylan (cite)#1#2(/cite). The enzymes are generally intracellular or membrane associated (cite)#3#4(/cite)suggesting that they play a terminal role in uncapping decorated xyloooligosacchrides, making these molecules available to beta-xylosidases produced by the host.
Kinetics and Mechanism
Alpha-glucuronidases hydrolyse their target glycoside bond through a single displacement acid-base assisted mechanism, and thus the released glucuronic acid released is in a beta conformation (cite)#5(/cite).
Catalytic Residues
ypical of single displacement glycoside hydrolases, GH67 enzymes contain a catalytic acid that protonates the scissile glycosidic oxygen promoting leaving group departure. This residue, Glu292 in the Cellvibrio japonicus GH67 (cite)#5(/cite ) and Glu285 in the Geobacillus stearothermophilus GH67 enzymes (cite)#6(/cite) is a conserved glutamate within GH67. There are a pair of carboxylic acids that make hydrogen bonds with the catalytic water (attacks the anomeric carbon of the scissile glycosidic bond), and are predicted to activate the solvent molecule, thus acting as the catalytic base. Which of these highly conserved residues, Glu393/Asp365 and Glu392/Asp364 in the C. japonicus and G. stearothermophilus enzymes, respectively, act as the catalytic base is unclear. Mutational studies suggested that Asp365 in the C. japonicus enzyme may be the catalytic base (cite)#5(/cite ), although similar mutagenesis studies on the Geobacillus glucuronidase indicate that mutation of either possible catalytic bases results in almost complete inactivation of the enzyme (cite)#7(/cite ).
Three-dimensional structures
Family Firsts
- First sterochemistry determination
- Cite some reference here, with a short explanation [1].
- First catalytic nucleophile identification
- First general acid/base residue identification
- First 3-D structure
References
- Comfort DA, Bobrov KS, Ivanen DR, Shabalin KA, Harris JM, Kulminskaya AA, Brumer H, and Kelly RM. (2007). Biochemical analysis of Thermotoga maritima GH36 alpha-galactosidase (TmGalA) confirms the mechanistic commonality of clan GH-D glycoside hydrolases. Biochemistry. 2007;46(11):3319-30. DOI:10.1021/bi061521n |