Some cellulases exhibit a higher level of complexity whereby more than one catalytic module and/or CBM is included in the same protein. Examples of such enzymes are the very similar cellulases from Anaerocellum thermophilum (Zverlov et al. 1998) and Caldocellum saccharolyticum (Te’o et al. 1995), both of which contain a GH9 and a GH48 catalytic module. Other paired catalytic modules include those from GH44 and either GH5 or GH9. Such an arrangement presumably indicates close cooperation between two particular catalytic domains, which may lead to synergistic action on the cellulosic substrate.

Some xylanases also exhibit such a multi-modular structure. GH10 and GH11 xylanases may be linked in the same polypeptide chain either to each other, to GH5, GH16, and GH43 catalytic modules or to ferulic, coumaric, or xylan acetic acid esterases from different families of carbohydrate esterase. One particularly interesting combination of multifunctional catalytic modules that appear in the same polypeptide chain is a typical GH10 xylanase together with a CE1 feruloyl esterase, which presumably allows enhanced cleavage of the xylan-lignin linkage in the plant cell wall (Grepinet et al. 1988; Fontes et al. 1995).

It is striking to note that many of the multifunctional enzymes in nature occur in bacteria that inhabit extreme environments, for example, growing at 60°C-90°C. One exception is evident in the rumen bacterium, Ruminococcus flavefaciens, which produces some very intriguing multifunctional xylanases (Flint et al. 1993; Laurie et al. 1997). One could argue, however, that the rumen environment is indeed a very specialized environment (Flint et al. 2008). Why this bacterium “chooses” to produce such complicated enzymes remains a mystery.