Nanoscale Structure of the Cell Wall Protecting Cellulose from Enzyme Attack

The cell wall structure protects cellulose from enzymatic attack and its successive fermentation. The nature of this protection consists in the very complex macroscopic and microscopic structure of cell wall that limits transport. Explaining this kind of protection is critical in future research to improve cell polymer availability for enzymatic attack. This research shows that the complete description of the cell wall topography at a nanoscale level allows a mechanistic understanding of cellulose protection. For this purpose, we used gas adsorption methods (CO2 at 273 K and N2 at 77 K) to detect mesoporosity (pore size of 1.5−30 nm diameter; MeS) and microporosity (pore size of 0.3−1.5 nm diameter; MiS) of the cell wall of five energy crops, i.e., giant cane, rivet wheat straw, miscanthus, proso millet, and sorghum. The presence of both hemicelluloses in the spaces between cellulose fibrils and the unhydrolyzable and highly cross-linked lignocarbohydrate complex (LCC) determines a microporous (80% pores having diameters below 0.8 nm) structure of the cell wall that prevents the cellulase enzymes from coming into direct contact with the cellulose, as their sizes exceed the cell wall pore size. On the other hand, the removal of the hemicelluloses and of the LCC complex determines a reduction of the MiS and an increase of the available surface for enzymatic attack, i.e., pores >5 nm diameter. This was confirmed by the good negative (r = −0.87, P < 0.001, n = 11) and positive (r = 0.78, P < 0.005, n = 11) correlations found for microporosity and mesoporosity (pores of diameters >5 nm), respectively, vs the glucose production, by cellulase enzyme attack in specific enzymatic hydrolysis tests performed on biomass samples.