Hahn-Hägerdal B, Lindén T, Senac T, Skoog K
Applied Microbiology, Lund University Chemical Center, Sweden.
Appl Biochem Biotechnol. 1991 Spring;28-29:131-44. doi: 10.1007/BF02922595.
In the fermentation of lignocellulose hydrolysates to ethanol, two major problems are encountered: the fermentation of the pentose sugar xylose, and the presence of microbial inhibitors. Xylose can be directly fermented with yeasts, such as Pachysolen tannophilus, Candida shehatae, and Pichia stipis, or by isomerization of xylose to xylulose with the enzyme glucose (xylose) isomerase (XI; EC 5.3.1.5), and subsequent fermentation with bakers' yeast, Saccharomyces cerevisiae. The direct fermentation requires low, carefully controlled oxygenation, as well as the removal of inhibitors. Also, the xylose-fermenting yeasts have a limited ethanol tolerance. The combined isomerization and fermentation with XI and S. cerevisiae gives yields and productivities comparable to those obtained in hexose fermentations without oxygenation and removal of inhibitors. However, the enzyme is not very stable in a lignocellulose hydrolysate, and S. cerevisiae has a poorly developed pentose phosphate shunt. Different strategies involving strain adaptation, and protein and genetic engineering adopted to overcome these different obstacles, are discussed.
在将木质纤维素水解产物发酵为乙醇的过程中，会遇到两个主要问题：戊糖木糖的发酵以及微生物抑制剂的存在。木糖可以用酵母直接发酵，如嗜鞣管囊酵母、嗜杀假丝酵母和树干毕赤酵母，或者先用葡萄糖（木糖）异构酶（XI；EC 5.3.1.5）将木糖异构化为木酮糖，随后用面包酵母酿酒酵母进行发酵。直接发酵需要低水平的、经过仔细控制的氧合作用，以及去除抑制剂。此外，木糖发酵酵母的乙醇耐受性有限。将异构化与XI和酿酒酵母联合发酵所得到的产量和生产率，与在无氧合作用和去除抑制剂的己糖发酵中所获得的产量和生产率相当。然而，该酶在木质纤维素水解产物中不太稳定，并且酿酒酵母的戊糖磷酸途径发育不良。本文讨论了为克服这些不同障碍而采用的涉及菌株适应性、蛋白质和基因工程的不同策略。
