Institute for Molecular Bioscience, Goethe University, Frankfurt, Germany.
FEMS Yeast Res. 2014 May;14(3):389-98. doi: 10.1111/1567-1364.12137. Epub 2014 Feb 13.
In the metabolic network of the cell, many intermediary products are shared between different pathways. d-Glyceraldehyde-3-phosphate, a glycolytic intermediate, is a substrate of GAPDH but is also utilized by transaldolase and transketolase in the scrambling reactions of the nonoxidative pentose phosphate pathway. Recent efforts to engineer baker's yeast strains capable of utilizing pentose sugars present in plant biomass rely on increasing the carbon flux through this pathway. However, the competition between transaldolase and GAPDH for d-glyceraldehyde-3-phosphate produced in the first transketolase reaction compromises the carbon balance of the pathway, thereby limiting the product yield. Guided by the hypothesis that reduction in GAPDH activity would increase the availability of d-glyceraldehyde-3-phosphate for transaldolase and thereby improve ethanol production during fermentation of pentoses, we performed a comprehensive characterization of the three GAPDH isoenzymes in baker's yeast, Tdh1, Tdh2, and Tdh3 and analyzed the effect of their deletion on xylose utilization by engineered strains. Our data suggest that overexpression of transaldolase is a more promising strategy than reduction in GAPDH activity to increase the flux through the nonoxidative pentose phosphate pathway.
在细胞的代谢网络中，许多中间产物在不同途径之间共享。d-甘油醛-3-磷酸是糖酵解的中间产物，是 GAPDH 的底物，但也被转醛醇酶和转酮醇酶在非氧化戊糖磷酸途径的重排反应中利用。最近，人们致力于工程化能够利用植物生物质中戊糖的面包酵母菌株，这依赖于增加该途径的碳通量。然而，转醛醇酶和 GAPDH 之间对第一个转酮醇酶反应产生的 d-甘油醛-3-磷酸的竞争，破坏了途径的碳平衡，从而限制了产物的产量。基于这样的假设，即降低 GAPDH 活性会增加 d-甘油醛-3-磷酸对转醛醇酶的可用性，从而在发酵戊糖时提高乙醇的产量，我们对面包酵母中的三种 GAPDH 同工酶（Tdh1、Tdh2 和 Tdh3）进行了全面表征，并分析了它们的缺失对工程菌株利用木糖的影响。我们的数据表明，过表达转醛醇酶是一种比降低 GAPDH 活性更有前途的策略，可以增加非氧化戊糖磷酸途径的通量。
