On the whole, the sequenced-based groupings do a good job clustering enzymes with like structures and activities (Table 1). However, the presence of multiple activ­ities within single GH families suggests that divergent evolution to new activities is not uncommon.[4] At the same time, structural comparisons show that a number of different GH families have the same fold ([58]; see Table 1, Fig. 3).

This is particularly true for (b/a)8-barrels (which can be decorated with addi­tional domains, Fig. 3), (a/a)6-barrels, and b-jelly rolls. It seems likely that the ubiquity of these folds reflect common, albeit more distant, evolutionary origins than are captured in the sequence families. To include structural similarity, a broader “clan” classification has been devised to group GH-families with the same fold ( [29] , also see [58]). Cellulases in different GH families with similar structures can be grouped into clans A, B, C, K, and M (Table 1). In addition, some GH families that have substantial numbers of characterized cellulases lack structural similarity to other GH families, and thus do not belong to a specific clan.