Follstad B D, Stephanopoulos G
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge 02139, USA.
Eur J Biochem. 1998 Mar 15;252(3):360-71. doi: 10.1046/j.1432-1327.1998.2520360.x.
The pentose phosphate pathway plays several key roles in metabolism including supply of biosynthetic carbon skeletons and reducing power. Previous research has focused on determining the fluxes through the reactions of this pathway using carbon-labeled substrates and models that make certain assumptions about the reversibility of the transketolase and transaldolase reactions in the nonoxidative pathway. These assumptions, however, have resulted in inconsistencies between the predicted carbon label distributions using these models and those determined experimentally. A general metabolic reaction network model developed in this paper and applied to the pentose phosphate pathway not only incorporates reaction reversibility but also accounts for the effect of individually varying extents of reaction reversibility on labeled carbon fractional enrichment values for intermediate metabolites. In addition, an algorithm is presented that can be used to calculate the three individual transaldolase and transketolase extents of reversibility. The results of this method show that varying extents of reaction reversibility have an observable effect on the metabolite carbon label distributions which can in turn affect flux calculation for other parts of the metabolic network such as the tricarboxylic acid cycle. In addition, the observability of reversibility extent and accuracy of flux calculations depend on the particular choice of metabolite carbon enrichments measured. In particular, [6-13C]hexose 6-phosphate and [4-13C]erythrose 4-phosphate carbon enrichment values resulting from [1-13C]glucose feeding contained more information as compared to those from ribose 5-phosphate. This analysis was applied to literature data of metabolite carbon labeling that resulted from supplying either 13C- or 14C-enriched substrates to several cell types growing under various conditions. The specific activities of metabolite carbon atoms taken from rat epididymal adipose tissue, goosefish islet cells, Corynebacterium glutamicum, and Escherichia coli supplied with either [2-14C]glucose or [1-13C]glucose demonstrate how reversibility is present in the pentose phosphate pathway and the extents of reversibility can be estimated from labeled carbon data sets.
磷酸戊糖途径在新陈代谢中发挥着几个关键作用，包括提供生物合成碳骨架和还原力。先前的研究集中于使用碳标记底物和对非氧化途径中转酮醇酶和转醛醇酶反应的可逆性做出某些假设的模型来确定通过该途径反应的通量。然而，这些假设导致使用这些模型预测的碳标记分布与实验确定的分布之间存在不一致。本文开发并应用于磷酸戊糖途径的通用代谢反应网络模型不仅纳入了反应可逆性，还考虑了反应可逆性程度的个体差异对中间代谢物标记碳分数富集值的影响。此外，还提出了一种算法，可用于计算转醛醇酶和转酮醇酶的三个个体可逆性程度。该方法的结果表明，反应可逆性程度的变化对代谢物碳标记分布有明显影响，进而可能影响代谢网络其他部分（如三羧酸循环）的通量计算。此外，可逆性程度的可观测性和通量计算的准确性取决于所测量的代谢物碳富集的特定选择。特别是，与来自5-磷酸核糖的相比，由[1-¹³C]葡萄糖喂养产生的[6-¹³C]6-磷酸己糖和[4-¹³C]4-磷酸赤藓糖碳富集值包含更多信息。该分析应用于代谢物碳标记的文献数据，这些数据是通过向在各种条件下生长的几种细胞类型供应¹³C或¹⁴C富集底物而产生的。从大鼠附睾脂肪组织、鮟鱇胰岛细胞、谷氨酸棒杆菌和大肠杆菌中获取的代谢物碳原子的比活度，这些细胞分别供应[2-¹⁴C]葡萄糖或[1-¹³C]葡萄糖，证明了磷酸戊糖途径中可逆性的存在，并且可以从标记碳数据集估计可逆性程度。
