Catalytic domains

The catalytic domains of cellulases and xylanases are relatively large and represent more than 70% of the total protein. Sequence analysis of these domains from various cellulases and xylanases revealed extensive conservation of sequence and, using hydrophobic cluster analysis, the catalytic domains of cellulases and xylanases were initially grouped into 11 structurally related families (Henrissat et al., 1989). According to this classification, the members of a single family possess the same protein fold and display the same stereoselectivity (Gebler et al., 1992). This suggested that the enzymes of a given family would follow either retaining or inverting mechanisms (Table 2.3). Subsequently, the catalytic domains of some 1000 glycosyl hydrolases have been classified into 64 families (Henrissat and Bairoch, 1993, 1996; Henrissat, 1998). The cellulases and xylanases are contained in families 1, 3, 5-12, 26, 43-45, 60 and 61 (Henrissat and Davies, 1997; Davies, 1998; Henrissat, 1998). This sequence-based classification has been enormously beneficial in reflecting common structural features and evolutionary relationships, and as a tool for deducing mechanistic information.

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