The microbial degradation of cellulose and xylans requires several types of
enzymes such as endoglucanases (EC 126.96.36.199), cellobiohydrolases (EC 188.8.131.52)
(exoglucanases), or xylanases (EC 184.108.40.206) [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 B  or as the glycosyl hydrolases
family 6 [4,E1]. The enzymes which are currently known to belong to this
family are listed below.
Agaricus bisporus exoglucanase 3 (cel3).
Cellulomonas fimi endoglucanase A (cenA).
Cellulomonas fimi exoglucanase A (cbhA).
Microspora bispora endoglucanase A (celA).
Streptomyces halstedii endoglucanases A (celA1).
Streptomyces strain KSM-9 endoglucanase 1 (casA).
Thermomonospora fusca endoglucanase E-2 (celB).
Trichoderma reesei exoglucanase II (CBH2).
One of the conserved regions in these enzymes contains a conserved aspartic
acid residue which is potentially involved  in the catalytic mechanism;
the aspartate is followed by a cysteine which is involved in a disulfide bond
. A second conserved region contains an aspartate which seems  to be the
proton donor in the catalytic mechanism. We have used both regions as
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