The Exo-beta-D-glucosaminidases (exochitosanases)

Official name (IUBMB): exo-1,4-beta-D-glucosaminidase; EC
The EC symbol for these enzymes has been created in 2008. Link to IUBMB page

The first report of exo-1,4-ß-D-glucosaminidase activity (GlcNase) came from the work of Nanjo et al., (1990) (10). These authors purified an extracellular, secreted GlcNase from the actinomycete Nocardia orientalis. The enzyme acted specifically on chitosan and chitosan oligosaccharides and did not hydrolyze a- or ß-glucosides, galactosides, N-acetylglucosaminides including substrates such as colloidal chitin, cellulose, carboxymethylcellulose or cello-oligosaccharides.

Following this first report, enzymes with similar properties were isolated and purified from several organisms. Most were characterized in fungal organisms: Trichoderma reesei (Nogawa et al., 1998)(70); Aspergillus fumigatus (Jung et al., 2005)(71); Penicillium funiculosum (Matsumura et al., 1999)(72); Aspergillus oryzae (Zhang et al., 2000)(73).

More recently, studies on the Nocardia enzyme (present name: Amycolatopsis orientalis) were extended in order to understand the catalytic mechanism and the molecular properties of this GlcNase (Côté et al., 2006)(74); (Fukamizo et al., 2006)(75), reviewed by Fukamizo and Brzezinski, 2007 (76).
We cloned the GlcNase-encoding gene (csxA) and established a recombinant system for the easy production of GlcNase. We also purified a GlcNase from another actinomycete, Streptomyces avermitilis with similar properties (74).

Also in 2006, Ike et al., (77) cloned and characterized the GlcNase-encoding gene (gls93) from Trichoderma reesei.

In 2009, the crystallization of the glucosaminidase from Amycolatopsis orientalis has been achieved. Essential amino acids participating in substrate binding and catalysis has been reported. The suggested mechanism of hydrolysis also explained why this enzyme catalyzes so poorly the hydrolysis of the paranitrophenyl derivative of glucosamine (Van Bueren et al, 2009) (79). The common properties of the fungal and actinobacterial GlcNases studied so far are the following:
Extracellular enzymes
Active on high molecular weight chitosan and on chitosan oligosaccharides with similar specific activities toward this full range of substrates; can be designated as an “exochitosanase”
No detectable activity toward non-GlcN oligosaccharides
Belong to family 2 of glycoside hydrolases
Analysis of fully sequenced genomes reveal the presence of homologs only in other fungi and actinobacteria.

A different type of GlcNase has been purified from a hyperthermophilic archaeon, Thermococcus kodakaraensis (Tanaka et al., 2003)(78). These authors also cloned the gene encoding this protein. The differences between the archaeal gene and the fungal/actinobacterial enzymes are substantial.

Properties of the archaeal GlcNase:
Intracellular enzyme
Dimeric enzyme (two identical subunits)
Active on chitosan oligosaccharides; activity decreases with increase of increase of oligosaccharide length; very slow hydrolysis of high molecular weight chitosan (12 – 140-times slower than against chitosan oligosaccharides)
Weak activity toward cellobiose and laminaribiose
Belongs to family 35 of glycoside hydrosases; weak homology to family 42
Analysis of fully sequenced genomes shows the presence of homologs only in other archaeons.

An important common property of all the GlcNases studied so far is that they do not possess any detectable beta-N-acetyl-glucosaminidase (GlcNAcase) activity.

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This page was created by Ryszard Brzezinski and Andrzej Neugebauer.
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Last updated: January 2009