The microbial degradation of cellulose and xylans requires several types of
enzymes such as endoglucanases (EC 3.2.1.4), cellobiohydrolases (EC 3.2.1.91)
(exoglucanases), or xylanases (EC 3.2.1.8) [1,2]. Fungi and bacteria produces
a spectrum of cellulolytic enzymes (cellulases) and xylanases which, on the
basis of sequence similarities, can be classified into families. One of these
families is known as the cellulase family G [3] or as the glycosyl hydrolases
family 11 (GH11) [4,E1,E2]. Family 11 is monospecific, only consisting of
xylanases. The enzymes which are currently known to belong to this family are
listed below.
Aspergillus awamori xylanase C (xynC).
Bacillus circulans, pumilus, stearothermophilus and subtilis xylanase
(xynA).
Clostridium acetobutylicum xylanase (xynB).
Clostridium stercorarium xylanase A (xynA).
Fibrobacter succinogenes xylanase C (xynC) which consist of two catalytic
domains that both belong to family 10.
Neocallimastix patriciarum xylanase A (xynA).
Ruminococcus flavefaciens bifunctional xylanase XYLA (xynA). This protein
consists of three domains: a N-terminal xylanase catalytic domain that
belongs to family 11 of glycosyl hydrolases; a central domain composed of
short repeats of Gln, Asn an Trp, and a C-terminal xylanase catalytic
domain that belongs to family 10 of glycosyl hydrolases.
Schizophyllum commune xylanase A.
Streptomyces lividans xylanases B (xlnB) and C (xlnC).
Trichoderma reesei xylanases I and II.
The GH11 domain folds into a jelly-roll shape likened to a partially closed
right hand (see <PDB:4HK8>). Several anti-parallel β-strands bend almost
90° to produce a substrate-binding groove characteristic of the GH11 domain
active sites. Two catalytic Glu residues face each other from opposite sides
of the groove. The hydrolysis reaction is believed to follow a double-displacement mechanism, with one Glu residue acting as a general acid/base
catalyst and the other as a nucleophile.
Two of the conserved regions in these enzymes are centered on glutamic acid
residues which have both been shown [5], in Bacillus pumilis xylanase, to be
necessary for catalytic activity. We have used both regions as signature
patterns. We have also developed a profile that covers the entire GH11 domain.
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