Biochemistry and molecular biology of exocellular fungal β‐(1,3)‐ and β‐(1,6)‐glucanases

Many fungi produce exocellular β‐glucan‐degrading enzymes, the β‐glucanases including the noncellulolytic β‐(1,3)‐ and β‐(1,6)‐glucanases, degrading β‐(1,3)‐ and β‐(1,6)‐glucans. An ability to purify several exocellular β‐glucanases attacking the same linkage type from a single fungus is common, although unlike the β‐1,3‐glucanases, production of multiple β‐1,6‐glucanases is quite rare in fungi. Reasons for this multiplicity remain unclear and the multiple forms may not be genetically different but arise by posttranslational glycosylation or proteolytic degradation of the single enzyme. How their synthesis is regulated, and whether each form is regulated differentially also needs clarifying. Their industrial potential will only be realized when the genes encoding them are cloned and expressed in large quantities. This review considers what is known in molecular terms about their multiplicity of occurrence, regulation of synthesis and phylogenetic diversity. It discusses how this information assists in understanding their functions in the fungi producing them. It deals largely with exocellular β‐glucanases which here refers to those recoverable after the cells are removed, since those associated with fungal cell walls have been reviewed recently by Adams (2004). It also updates the earlier review by Pitson et al. (1993).