Exploring the Promoting Mechanisms of Bovine Serum Albumin, Lignosulfonate, Polyethylene Glycol for Lignocellulose Saccharification from Perspective of Molecular Interactions with Cellulase

Background: Bovine serum albumin (BSA), polyethylene glycol (PEG) and lignosulfonate (LS) have been extensively employed as synergistic agents in lignocellulose saccharification. Nevertheless, the promoting mechanisms have not been fully understood and there are a number of controversial opinions existed. All attention has been paid to the interactions between respective additive and substrate. However, rarely attention has been paid to the interactions between additives and cellulase. The interaction between respective additive and cellulase is actually more important since cellulase interacts with the additives before it contacts with substrate.Results: This investigation showed that BSA and LS could bind to cellulase to form complexes, whereas PEG did not. However, PEG had a high affinity to lignin or lignin derivatives. The complexes of Cell-BSA consisted of one BSA and four cellulase molecules; while the complexes of Cell-LS were composed of one cellulase and four LS molecules in the testing conditions. Regarding to the enzymatic hydrolytic efficiency for Avicel and lignocellulose substrates, the results showed that BSA and PEG promoted the enzymatic hydrolysis for both substrates, while LS had a promoting effect for lignocellulose only and inhibited some extent for Avicel. Conclusions: This study showed that synergistic agents of LS, BSA, and PEG have different interaction modes with cellulase. BSA and LS can form complexes with cellulase and the formed complexes prevent them from nonproductive binding by residue lignin; what’s more, the cellulase-BSA complexes improve the hydrolytic capability of pristine enzyme whereas cellulase-LS complexes reduce. The promoting mechanism of PEG has to be attributed to two aspects: one is to prevent cellulase from nonproductive binding to residue lignin by forming a thin layer that actually serves as steric hindrance on residue lignin; the other is the structure change of substrate induced by PEG addition. This investigation will help us to understand the sophisticated interactions among the components in the complicated enzymatic system, especially the interactions between enzymes and synergistic agents. It will be helpful in the design and utilization of synergistic additives in the lignocellulose biorefinery process as well.