Carbohydrate-binding modules (CBMs) revisited: reduced amount of water counterbalances the need for CBMs

Cellulases without carbohydrate-binding modules in high consistency ethanol production process

BACKGROUND

Enzymes still comprise a major part of ethanol production costs from lignocellulose raw materials. Irreversible binding of enzymes to the residual substrate prevents their reuse and no efficient methods for recycling of enzymes have so far been presented. Cellulases without a carbohydrate-binding module (CBM) have been found to act efficiently at high substrate consistencies and to remain non-bound after the hydrolysis.

RESULTS

High hydrolysis yields could be obtained with thermostable enzymes of Thermoascus aurantiacus containing only two main cellulases: cellobiohydrolase I (CBH I), Cel7A and endoglucanase II (EG II), Cel5A. The yields were decreased by only about 10% when using these cellulases without CBM. A major part of enzymes lacking CBM was non-bound during the most active stage of hydrolysis and in spite of this, produced high sugar yields. Complementation of the two cellulases lacking CBM with CBH II (CtCel6A) improved the hydrolysis. Cellulases without CBM were more sensitive during exposure to high ethanol concentration than the enzymes containing CBM. Enzymes lacking CBM could be efficiently reused leading to a sugar yield of 90% of that with fresh enzymes. The applicability of cellulases without CBM was confirmed under industrial ethanol production conditions at high (25% dry matter (DM)) consistency.

CONCLUSIONS

The results clearly show that cellulases without CBM can be successfully used in the hydrolysis of lignocellulose at high consistency, and that this approach could provide new means for better recyclability of enzymes. This paper provides new insight into the efficient action of CBM-lacking cellulases. The relationship of binding and action of cellulases without CBM at high DM consistency should, however, be studied in more detail.

Transcriptional analysis of selected cellulose-acting enzymes encoding genes of the white-rot fungus Dichomitus squalens on spruce wood and microcrystalline cellulose

The recent discovery of oxidative cellulose degradation enhancing enzymes has considerably changed the traditional concept of hydrolytic cellulose degradation. The relative expression levels of ten cellulose-acting enzyme encoding genes of the white-rot fungus Dichomitus squalens were studied on solid-state spruce wood and in microcrystalline Avicel cellulose cultures. From the cellobiohydrolase encoding genes, cel7c was detected at the highest level and showed constitutive expression whereas variable transcript levels were detected for cel7a, cel7b and cel6 in the course of four-week spruce cultivation. The cellulolytic enzyme activities detected in the liquid cultures were consistent with the transcript levels. Interestingly, the selected lytic polysaccharide monooxygenase (LPMO) encoding genes were expressed in both cultures, but showed different transcription patterns on wood compared to those in submerged microcrystalline cellulose cultures. On spruce wood, higher transcript levels were detected for the lpmos carrying cellulose binding module (CBM) than for the lpmos without CBMs. In both cultures, the expression levels of the lpmo genes were generally higher than the levels of cellobiose dehydrogenase (CDH) encoding genes. Based on the results of this work, the oxidative cellulose cleaving enzymes of D. squalens have essential role in cellulose degrading machinery of the fungus.

Cellulose hydrolysis and binding with Trichoderma reesei Cel5A and Cel7A and their core domains in ionic liquid solutions

Ionic liquids (ILs) dissolve lignocellulosic biomass and have a high potential as pretreatment prior to total enzymatic hydrolysis. ILs are, however, known to inactivate cellulases. In this article, enzymatic hydrolysis of microcrystalline cellulose (MCC) and enzyme binding onto the cellulosic substrate were studied in the presence of cellulose-dissolving ILs. Two different ILs, 1,3-dimethylimidazolium dimethylphosphate ([DMIM]DMP) and 1-ethyl-3-methylimidazolium acetate ([EMIM]AcO), and two monocomponent cellulases, Trichoderma reesei cellobiohydrolase Cel7A and endoglucanase Cel5A, were used in the study. The role and IL sensitivity of the carbohydrate-binding module (CBM) were studied by performing hydrolysis and binding experiments with both the intact cellulases, and their respective core domains (CDs). Based on hydrolysis yields and substrate binding experiments for the intact enzymes and their CDs in the presence of ILs, the function of the CBM appeared to be very IL sensitive. Binding data suggested that the CBM was more important for the substrate binding of endoglucanase Cel5A than for the binding of cellobiohydrolase Cel7A. The CD of Cel7A was able to bind well to cellulose even without a CBM, whereas Cel5A CD had very low binding affinity. Hydrolysis also occurred with Cel5A CD even if this protein had very low binding affinity in all the studied matrices. Binding and hydrolysis were less affected by the studied ILs for Cel7A than for Cel5A. To our knowledge, this is the first systematic study of IL effects on cellulase substrate binding.

The role of carbohydrate binding module (CBM) at high substrate consistency: comparison of Trichoderma reesei and Thermoascus aurantiacus Cel7A (CBHI) and Cel5A (EGII)

The role of CBM in two fungal model cellulase systems, consisting of Cel7A and Cel5A, from Trichoderma reesei and Thermoascus aurantiacus, were compared in the hydrolysis of various substrates. For comparison, family-1 CBM's were introduced to the T. aurantiacus and removed from the T. reesei enzymes. Especially at high dry matter consistencies of lignocellulosic substrates, pretreated wheat straw and spruce, the T. aurantiacus enzymes lacking CBM outperformed the enzymes carrying the CBM. In these conditions, the CBM-less enzymes from both organisms obviously recognized and bound to the substrate at higher probability than in dilute systems. Avoiding the unproductive binding to lignin caused by the CBMs obviously enhanced the hydrolytic performance. The lignin binding effect was, however, not entirely caused by the CBM, but also by the different structures and affinities of the core enzymes to lignin. Due to decreased binding, the CBM-less enzymes would allow reuse, potentially decreasing hydrolysis costs.