CAZy Mechanisms

Two major mechanisms

Retaining Glycoside Hydrolases


Description

Retaining glycoside hydrolases operate via a double displacement mechanism that leads to the retention of the configuration at the anomeric carbon of the sugar ring undergoing catalytis. These enzymes often display transglycosylating abilities. The catalytic machinery of these enzymes involves two catalytic carboxylates located on opposite sides of the sugar plane and that perform two separate chemical steps. In the first step (glycosylation), a carboxylic group provides general acid-catalysed leaving group departure simultaneously with a nucleophilic attack by the second carboxylate to form a glycosyl-enzyme intermediate. In the second step (deglycosylation), the first residue functions as a general base to activate the incoming nucleophile (a water molecule in the case of hydrolysis, and an alcohol in the case of transglycosylation) which hydrolyzes the glycosyl-enzyme. The distance between the two carboxylates is approximately 5.5 Å. Many retaining glycoside hydrolases hydrolysing beta-N-acetyl glucosaminic bonds utilise a double-displacement mechanism in which the nucleophile is donated not by the enzyme, but by the C-2 N-acetamido group of the substrate itself [3].


References


[1]	Sinnott ML (1990) Catalytic mechanisms of enzymic glycoside transfer. Chem. Rev. 90:1170-1202.

[2]	McCarter, J.D., Withers, S.G. (1994) Mechanisms of enzymatic glycoside hydrolysis. Curr. Opin. Struct. Biol 4:885-892 [PMID: 7712292].

[3]	Terwisscha van Scheltinga A, Armand S, Kalk KH, Isogai A, Henrissat B, Dijkstra BW (1995) Stereochemistry of chitin hydrolysis by a plant chitinase/lysozyme and x-ray structure of a complex with allosamidin. Evidence for substrate assisted catalysis. Biochemistry 34:15619-15623 [PMID: 7495789].
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Inverting Glycoside Hydrolases


Description

Inverting glycoside hydrolases lead to an inversion of the anomeric configuration via a single nucleophilic displacement. Hydrolysis of a beta-glycosidic bond thus creates a product with the alpha-configuration or vice-versa. The catalytic machinery of these enzymes involves two catalytic carboxylates in order to provide (i) general acid-catalysed leaving group departure and (ii) general base-assistance to nucleophilic attack by a water molecule from the opposite side of the sugar ring. The distance between the two carboxylates is less constrained as for the retaining enzymes and is in the range 6.5-9.5 Å.


References


[1]	Sinnott ML (1990) Catalytic mechanisms of enzymic glycoside transfer. Chem. Rev. 90:1170-1202.

[2]	McCarter, J.D., Withers, S.G. (1994) Mechanisms of enzymatic glycoside hydrolysis. Curr. Opin. Struct. Biol 4:885-892 [PMID: 7712292].
	or close