Cellulase-lignin interactions-the role of carbohydrate-binding module and pH in non-productive binding.

Non-productive cellulase adsorption onto lignin is a major inhibitory mechanism preventing enzymatic hydrolysis of lignocellulosic feedstocks. Therefore, understanding of enzyme-lignin interactions is essential for the development of enzyme mixtures and processes for lignocellulose hydrolysis. We have studied cellulase-lignin interactions using model enzymes, Melanocarpus albomyces Cel45A endoglucanase (MaCel45A) and its fusions with native and mutated carbohydrate-binding modules (CBMs) from Trichoderma reesei Cel7A. Binding of MaCel45A to lignin was dependent on pH in the presence and absence of the CBM; at high pH, less enzyme bound to isolated lignins. Potentiometric titration of the lignin preparations showed that negatively charged groups were present in the lignin samples and that negative charge in the samples was increased with increasing pH. The results suggest that electrostatic interactions contributed to non-productive enzyme adsorption: Reduced enzyme binding at high pH was presumably due to repulsive electrostatic interactions between the enzymes and lignin. The CBM increased binding of MaCel45A to the isolated lignins only at high pH. Hydrophobic interactions are probably involved in CBM binding to lignin, because the same aromatic amino acids that are essential in CBM-cellulose interaction were also shown to contribute to lignin-binding.

Screening of microbes for novel acidic cutinases and cloning and expression of an acidic cutinase from Aspergillus niger CBS 513.88.

Isolates from gardening waste compost and 38 culture collection microbes were grown on agar plates at pH 4.0 with the cutinase model substrate polycaprolactone as a carbon source. The strains showing polycaprolactone hydrolysis were cultivated in liquid at acidic pH and the cultivations were monitored by assaying the p-nitrophenyl butyrate esterase activities. Culture supernatants of four strains were analyzed for the hydrolysis of tritiated apple cutin at different pHs. Highest amounts of radioactive hydrolysis products were detected at pHs below 5. The hydrolysis of apple cutin by the culture supernatants at acidic pH was further confirmed by GC-MS analysis of the hydrolysis products. On the basis of screening, the acidic cutinase from Aspergillus niger CBS 513.88 was chosen for heterogeneous production in Pichia pastoris and for analysis of the effects of pH on activity and stability. The recombinant enzyme showed activity over a broad range of pHs with maximal activity between pH 5.0 and 6.5. Activity could be detected still at pH 3.5.

Inhibition of enzymatic hydrolysis by residual lignins from softwood--study of enzyme binding and inactivation on lignin-rich surface.

Lignin-derived inhibition is a major obstacle restricting the enzymatic hydrolysis of cell wall polysaccharides especially with softwood lignocellulosics. Enzyme adsorption on lignin is suggested to contribute to the inhibitory effect of lignin. The interaction of cellulases with softwood lignin was studied in the present work with commercial Trichoderma reesei cellulases (Celluclast) and lignin-rich residues isolated from steam pretreated softwood (SPS) by enzymatic and acid hydrolysis. Both lignin preparations inhibited the hydrolysis of microcrystalline cellulose (Avicel) and adsorbed the major cellulases present in the commercial cellulase mixture. The adsorption phenomenon was studied at low temperature (4°C) and at the typical hydrolysis temperature (45°C) by following activities of free and lignin-bound enzymes. Severe inactivation of the lignin-bound enzymes was observed at 45°C, however at 4°C the enzymes retained well their activity. Furthermore, SDS-PAGE analysis of the lignin-bound enzymes indicated that very strong interactions form between the residue and the enzymes at 45°C, because the enzymes were not released from the residue in the electrophoresis. These results suggest that heat-induced denaturation may take place on the surface of softwood lignin at the hydrolysis temperature.