Abstract
BackgroundThe effectiveness of the enzymatic hydrolysis of cellulose in plant cell wall is strongly influenced by the access of enzymes to cellulose, which is at least in part limited by the presence of lignin. Although physicochemical treatments preceding the enzymatic catalysis significantly overcome this recalcitrance, the residual lignin can still play a role in the process. Lignin is suggested to act as a barrier, hindering cellulose and limiting the access of the enzymes. It can also unspecifically bind cellulases, reducing the amount of enzymes available to act on cellulose. However, the limiting role of the lignin present in pretreated sugarcane bagasses has not been fully understood yet.ResultsA set of sugarcane bagasses pretreated by five leading pretreatment technologies was created and used to assess their accessibility and the unproductive binding capacity of the resulting lignins. Steam explosion and alkaline sulfite pretreatments resulted in more accessible substrates, with approximately 90% of the cellulose hydrolyzed using high enzyme loadings. Enzymatic hydrolysis of alkaline-treated (NaOH) and steam-exploded sugarcane bagasses were strongly affected by unproductive binding at the lowest enzyme loading tested. Analysis of the extracted lignins confirmed the superior binding capacity of these lignins. Sulfite-based pretreatments (alkaline sulfite and acid sulfite) resulted in lignins with lower binding capacities compared to the analogue pretreatments without sulfite (alkaline and acidic). Strong acid groups present in sulfite-based pretreated substrates, attributed to sulfonated lignins, corroborated the lower binding capacities of the lignin present in these substrates. A more advanced enzyme preparation (Cellic CTec3) was shown to be less affected by unproductive binding at low enzyme loading.ConclusionsPretreatments that increase the accessibility and modify the lignin are necessary in order to decrease the protein binding capacity. The search for the called weak lignin-binding enzymes is of major importance if hydrolysis with low enzyme loadings is the goal for economically viable processes.
Highlights
The effectiveness of the enzymatic hydrolysis of cellulose in plant cell wall is strongly influenced by the access of enzymes to cellulose, which is at least in part limited by the presence of lignin
In order to understand the limiting role of lignin in sugarcane bagasse, an important source of sugars after enzymatic hydrolysis [50], this work first aimed to prepare a range of pretreated sugarcane bagasse (PSCB) samples with varying lignin contents and properties by pretreating bagasse with five thermochemical leading pretreatment methods
Pretreatments that efficiently removed (Na2SO3/NaOH) or relocated (SO2/steam pretreatment) lignin resulted in substrates with greater cellulose accessibility and, were more efficiently digested by cellulases
Summary
The effectiveness of the enzymatic hydrolysis of cellulose in plant cell wall is strongly influenced by the access of enzymes to cellulose, which is at least in part limited by the presence of lignin. Because of the strong interaction of components, plant cell walls are highly recalcitrant and the utilization of their components in the biorefinery concept requires cell wall fractionation, at least partially. This step, known as pretreatment, is crucial to reduce cell wall recalcitrance and enhance cellulose hydrolysis [4]. Since most of the hemicellulose remains in the solid fraction, accessory enzymes (xylanases, esterases, and some oxidases) are necessary in the enzyme mixture [16, 17]
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