Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Metab Eng. 2013 Jan;15:134-43. doi: 10.1016/j.ymben.2012.11.005. Epub 2012 Nov 22.
Anaerobic bacteria assimilate cellodextrins from plant biomass by using a phosphorolytic pathway to generate glucose intermediates for growth. The yeast Saccharomyces cerevisiae can also be engineered to ferment cellobiose to ethanol using a cellodextrin transporter and a phosphorolytic pathway. However, strains with an intracellular cellobiose phosphorylase initially fermented cellobiose slowly relative to a strain employing an intracellular β-glucosidase. Fermentations by the phosphorolytic strains were greatly improved by using cellodextrin transporters with elevated rates of cellobiose transport. Furthermore under stress conditions, these phosphorolytic strains had higher biomass and ethanol yields compared to hydrolytic strains. These observations suggest that, although cellobiose phosphorolysis has energetic advantages, phosphorolytic strains are limited by the thermodynamics of cellobiose phosphorolysis (ΔG°=+3.6kJmol(-1)). A thermodynamic "push" from the reaction immediately upstream (transport) is therefore likely to be necessary to achieve high fermentation rates and energetic benefits of phosphorolysis pathways in engineered S. cerevisiae.
厌氧菌通过使用磷酸解途径同化植物生物质中的纤维二糖，为生长生成葡萄糖中间体。酵母酿酒酵母也可以通过使用纤维二糖转运蛋白和磷酸解途径来工程化发酵纤维二糖生成乙醇。然而，与使用细胞内β-葡萄糖苷酶的菌株相比，具有细胞内纤维二糖磷酸化酶的菌株最初发酵纤维二糖的速度较慢。通过使用具有提高的纤维二糖转运速率的纤维二糖转运蛋白，磷酸解菌株的发酵得到了极大改善。此外，在应激条件下，与水解菌株相比，这些磷酸解菌株具有更高的生物量和乙醇产量。这些观察结果表明，尽管纤维二糖磷酸解具有能量优势，但磷酸解菌株受到纤维二糖磷酸解热力学（ΔG°=+3.6kJmol(-1))的限制。因此，在工程酿酒酵母中，可能需要来自反应上游（转运）的热力学“推动”，以实现高发酵速率和磷酸解途径的能量优势。
