Glycoside hydrolase

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Glycoside hydrolases (also called glycosidases) catalyze the hydrolysis of the glycosidic linkage to generate two smaller sugars. They are extremely common enzymes with roles in nature including degradation of biomass such as cellulose and hemicellulose, in anti-bacterial defense strategies (eg lysozyme, in pathogenesis mechanisms (eg viral neuraminidases) and in normal cellular function (eg trimming mannosidases involved in N-linked glycoprotein biosynthesis). Together with glycosyltransferases, glycosidases form the major catalytic machinery for the synthesis and breakage of glycosidic bonds.

1 Classification
- 1.1 Sequence-based classification
2 Mechanism
- 2.1 Inverting glycoside hydrolases
- 2.2 Retaining glycoside hydrolases
3 Nomenclature and examples
4 Uses
5 See also
6 References
7 External links

[edit] Classification

Glycoside hydrolases are classified into EC 3.2.1 as enzymes catalyzing the hydrolysis of O- or S-glycosides. Glycoside hydrolases can also be classified according to the stereochemical outcome of the hydrolysis reaction: thus they can be classified as either retaining or inverting enzymes.^[1] Glycoside hydrolases can also be classified as exo or endo acting, dependent upon whether they act at the (usually non-reducing) end or in the middle, respectively, of an oligo/polysaccharide chain. Glycoside hydrolases may also be classified by sequence based methods.

[edit] Sequence-based classification

Sequence-based classifications are among the most powerful predictive method for suggesting function for newly sequenced enzymes for which function has not been biochemically demonstrated. The sequence based classification of Henrissat and colleagues, available through the online database carbohydrate active enzymes (CAZy) provides a series of regularly updated sequence based classification that allow reliable prediction of mechanism (retaining/inverting), active site residues and possible substrates. Based on three dimensional structural similarities, the sequence-based families have been classified into 'clans' of related structure. Recent progress in glycosidase sequence analysis and 3D structure comparison has allowed the proposal of an extended hierarchical classification of the glycoside hydrolases.^[2]

[edit] Mechanism

[edit] Inverting glycoside hydrolases

Inverting enzymes utilize two enzymic residues, typically carboxylate residues, that act as acid and base respectively, as shown below for a beta-glucosidase.

[edit] Retaining glycoside hydrolases

Retaining glycosidases operate through a two-step mechanism, with each step resulting in inversion, for a net retention of stereochemistry. Again, two residues are involved, which are usually enzyme-borne carboxylates. One acts as a nucleophile and the other as an acid/base. In the first step the nucleophile attacks the anomeric centre, resulting in the formation of a glycosyl enzyme intermediate, with acidic assistance provided by the acidic carboxylate. In the second step the now deprotoned acidic carboxylate acts as a base and assists a nucleophilic water to hydrolyze the glycosyl enzyme intermediate, giving the hydrolyzed product. The mechanism is illustrated below for hen egg white lysozyme.^[3]

An alternative mechanism for hydrolysis with retention of stereochemistry can occur that proceeds through a nucleophilic residue that is bound to the substrate, rather than being attached to the enzyme. Such mechanisms are common for certain N-acetylhexosaminidases, which have an acetamido group capable of neighboring group participation to form an intermediate oxazoline or oxazolinium ion. Again, the mechanism proceeds in two steps through individual inversions to lead to a net retention of configuration.

[edit] Nomenclature and examples

Glycoside hydrolases are typically named after the substrate that they act upon. Thus glucosidases catalyze the hydrolysis of glucosidases and xylanases catalyze the cleavage of the xylose based homopolymer xylan. Other examples include lactase, amylase, chitinase, sucrase, maltase, neuraminidase, invertase, hyaluronidase and lysozyme.

[edit] Uses

Glycoside hydrolases have a variety of uses including degradation of plant materials (eg cellulases for degrading cellulose to glucose, which can be used for ethanol production), in the food industry (invertase for use in making soft-centered chocolates), and in the paper industry (xylanases for removing hemicelluloses from paper pulp).

In organic chemistry, glycoside hydrolases can be used as synthetic catalysts to form glycosidic bonds through either reverse hydrolysis (kinetic approach) where the equilibrium position is reversed; or by transglycosylation (kinetic approach) whereby retaining glycoside hydrolases can catalyze the transfer of a glycosyl moiety from an activated glycoside to an acceptor alcohol to afford a new glycoside.

Mutant glycoside hydrolases termed glycosynthases have been developed that can achieve the synthesis of glycosides in high yield from activated glycosyl donors such as glycosyl fluorides.