Figure 7

Current view on fungal enzymatic degradation of cellulose. Abbreviations: EG, endoglucanase; CBH, cellobiohydrolase; CDH, cellobiose-dehydrogenase; CBM, carbohydrate-binding module. Note that many cellulolytic enzyme systems have multiple EG and/or CBH that may act on various parts of the substrate, e.g. different crystal faces or parts differing in terms of crystallinity and accessibility. The picture shows a C1 and a C4 oxidizing GH61 which would generate optimal (i.e. non-oxidized) ends for the CBH2 and the CBH1, respectively (oxidized sugars are colored red). Note that the combined action of C1 and C4 oxidizing enzymes may produce native cello-oligosaccharides from the middle of the cellulose chain. The possible consequence of GH61 action is illustrated in the lower left part of the picture, where new attacking points for CBHs are indicated by arrows. CDH may provide GH61s with electrons, but it must be noted that not all organisms have genes encoding for both of these enzyme families in their genome (e.g. Postia placenta has four genes encoding GH61s, but none encoding CDH [81]). Also other non-enzymatic reductants (electron donors) have been demonstrated to induce oxidative activity (e.g. reduced glutathione, ascorbic acid and gallic acid). For more information on the various glucanases and the mechanisms for their synergy, the reader is referred to Kostylev and Wilson, 2012 [43].