Enzymatic hydrolysis of cellulosic material is an essential step in the bioethanol production process. However, complete cellulose hydrolysis by cellulase is difficult due to the irreversible adsorption of cellulase onto cellulose. Thus, part of the cellulose remains in crystalline form after hydrolysis. In this study, after 96-h hydrolysis of Avicel crystalline cellulose, 47.1 % of the cellulase was adsorbed on the cellulose surface with 10.8 % crystalline cellulose remaining. In simultaneous saccharification and fermentation of 100 g/L Avicel with 1.0 filter paper unit/mL cellulase, a wild-type yeast strain produced 44.7 g/L ethanol after 96 h. The yield of ethanol was 79.7 % of the theoretical yield. On the other hand, a recombinant yeast strain displaying various cellulases, such as β-glucosidase, cellobiohydrolase, and endoglucanase, produced 48.9 g/L ethanol, which corresponds to 87.3 % of the theoretical yield. Higher ethanol production appears to be attributable to higher efficiency of cellulase displayed on the cell surface. These results suggest that cellulases displayed on the yeast cell surface improve hydrolysis of Avicel crystalline cellulose. Indeed, after the 96-h simultaneous saccharification and fermentation using the cellulase-displaying yeast, the amount of residual cellulose was 1.5 g/L, one quarter of the cellulose remaining using the wild-type strain, a result of the alleviation of irreversible adsorption of cellulases on the crystalline cellulose.

译文

纤维素材料的酶水解是生物乙醇生产过程中必不可少的步骤。然而,由于纤维素酶不可逆地吸附在纤维素上,纤维素酶很难完全水解纤维素。因此,部分纤维素在水解后保持结晶形式。在这项研究中,在Avicel结晶纤维素水解96小时后,纤维素酶的47.1% 吸附在纤维素表面,剩余10.8% 结晶纤维素。在用1.0滤纸单位/mL纤维素酶同时糖化和发酵100g/L Avicel的过程中,野生型酵母菌株在96小时后产生44.7g/L乙醇。乙醇的产率是理论产率的79.7%。另一方面,显示各种纤维素酶 (例如 β-葡萄糖苷酶、纤维二糖水解酶和内切葡聚糖酶) 的重组酵母菌株产生48.9g/L乙醇,这相当于理论产量的87.3%。较高的乙醇产量似乎归因于细胞表面显示的纤维素酶效率较高。这些结果表明,酵母细胞表面显示的纤维素酶可改善Avicel结晶纤维素的水解。实际上,在使用显示纤维素酶的酵母进行96小时的同时糖化和发酵之后,残留纤维素的量为1.5g/L,为使用野生型菌株剩余纤维素的四分之一,这是纤维素酶在结晶纤维素上不可逆吸附的减轻的结果。

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