Department of Systems Biology and Bioinformatics, University of Rostock, Ulmenstr. 69, 18051 Rostock, Germany.
Appl Microbiol Biotechnol. 2013 Jul;97(14):6451-66. doi: 10.1007/s00253-013-4860-7. Epub 2013 May 3.
In response to changing extracellular pH levels, phosphate-limited continuous cultures of Clostridium acetobutylicum reversibly switches its metabolism from the dominant formation of acids to the prevalent production of solvents. Previous experimental and theoretical studies have revealed that this pH-induced metabolic switch involves a rearrangement of the intracellular transcriptomic, proteomic and metabolomic composition of the clostridial cells. However, the influence of the population dynamics on the observations reported has so far been neglected. Here, we present a method for linking the pH shift, clostridial growth and the acetone-butanol-ethanol fermentation metabolic network systematically into a model which combines the dynamics of the external pH and optical density with a metabolic model. Furthermore, the recently found antagonistic expression pattern of the aldehyde/alcohol dehydrogenases AdhE1/2 and pH-dependent enzyme activities have been included into this combined model. Our model predictions reveal that the pH-induced metabolic shift under these experimental conditions is governed by a phenotypic switch of predominantly acidogenic subpopulation towards a predominantly solventogenic subpopulation. This model-driven explanation of the pH-induced shift from acidogenesis to solventogenesis by population dynamics casts an entirely new light on the clostridial response to changing pH levels. Moreover, the results presented here underline that pH-dependent growth and pH-dependent specific enzymatic activity play a crucial role in this adaptation. In particular, the behaviour of AdhE1 and AdhE2 seems to be the key factor for the product formation of the two phenotypes, their pH-dependent growth, and thus, the pH-induced metabolic switch in C. acetobutylicum.
为了响应不断变化的细胞外 pH 值水平，受磷酸盐限制的连续培养的丙酮丁醇梭菌（Clostridium acetobutylicum）会可逆地将其代谢从主要形成酸转变为主要产生溶剂。先前的实验和理论研究表明，这种 pH 诱导的代谢转换涉及到梭菌细胞内转录组、蛋白质组和代谢组组成的重新排列。然而，到目前为止，种群动态对观察结果的影响尚未被忽视。在这里，我们提出了一种将 pH 变化、梭菌生长和丙酮丁醇乙醇发酵代谢网络系统地联系起来的方法，该方法将外部 pH 和光密度的动态与代谢模型结合起来。此外，最近发现的醛/醇脱氢酶 AdhE1/2 的拮抗表达模式和 pH 依赖性酶活性也被纳入到这个组合模型中。我们的模型预测表明，在这些实验条件下，由主要产酸亚群向主要产溶剂亚群的表型转换控制着 pH 诱导的代谢转变。这种基于模型的对由种群动态引起的从产酸到产溶剂的 pH 诱导转变的解释，为梭菌对不断变化的 pH 值的反应提供了全新的视角。此外，这里呈现的结果强调了 pH 依赖性生长和 pH 依赖性特定酶活性在这种适应中的关键作用。特别是，AdhE1 和 AdhE2 的行为似乎是两种表型的产物形成、它们的 pH 依赖性生长以及因此在 C. acetobutylicum 中 pH 诱导的代谢转换的关键因素。
