Aix-Marseille Université, CNRS, LCB-UMR7283, Marseille, France.
mBio. 2021 Dec 21;12(6):e0220621. doi: 10.1128/mBio.02206-21. Epub 2021 Nov 9.
Xyloglucan utilization by was formerly shown to imply the uptake of large xylogluco-oligosaccharides, followed by cytosolic depolymerization into glucose, galactose, xylose, and cellobiose. This raises the question of how the anaerobic bacterium manages the simultaneous presence of multiple sugars. Using genetic and biochemical approaches targeting the corresponding metabolic pathways, we observed that, surprisingly, all sugars are catabolized, collectively, but glucose consumption is prioritized. Most selected enzymes display unusual features, especially the GTP-dependent hexokinase of glycolysis, which appeared reversible and crucial for xyloglucan utilization. In contrast, mutant strains lacking either galactokinase, cellobiose-phosphorylase, or xylulokinase still catabolize xyloglucan but display variably altered growth. Furthermore, the xylogluco-oligosaccharide depolymerization process appeared connected to the downstream pathways through an intricate network of competitive and noncompetitive inhibitions. Altogether, our data indicate that xyloglucan utilization by relies on an energy-saving central carbon metabolism deviating from current bacterial models, which efficiently prevents carbon overflow. The study of the decomposition of recalcitrant plant biomass is of great interest as the limiting step of terrestrial carbon cycle and to produce plant-derived valuable chemicals and energy. While extracellular cellulose degradation and catabolism have been studied in detail, few publications describe the complete metabolism of hemicelluloses and, to date, the published models are limited to the extracellular degradation and sequential entry of simple sugars. Here, we describe how the model anaerobic bacterium Ruminiclostridium cellulolyticum deals with the synchronous intracellular release of glucose, galactose, xylose, and cellobiose upon cytosolic depolymerization of imported xyloglucan oligosaccharides. The described novel metabolic strategy involves the simultaneous activity of different metabolic pathways coupled to a network of inhibitions controlling the carbon flux and is distinct from the ubiquitously observed sequential uptake and metabolism of carbohydrates known as the diauxic shift. Our results highlight the diversity of cellular responses related to a complex environment.
木葡聚糖的利用以前被认为需要摄取大量的木葡糖醛酸寡糖，然后在细胞质中解聚成葡萄糖、半乳糖、木糖和纤维二糖。这就提出了一个问题，即这种厌氧细菌如何同时处理多种糖。通过针对相应代谢途径的遗传和生化方法，我们观察到，令人惊讶的是，所有的糖都被集体代谢，但优先消耗葡萄糖。大多数选定的酶具有不寻常的特征，特别是糖酵解的 GTP 依赖性己糖激酶，它似乎是可逆的，对木葡聚糖的利用至关重要。相比之下，缺乏半乳糖激酶、纤维二糖磷酸化酶或木酮糖激酶的突变菌株仍然可以代谢木葡聚糖，但生长情况发生了不同程度的改变。此外，木葡糖醛酸寡糖的解聚过程似乎通过一个复杂的竞争和非竞争抑制网络与下游途径相连。总的来说，我们的数据表明，通过依赖于一种节能的中心碳代谢来利用木葡聚糖，这种代谢模式与当前的细菌模型不同，有效地防止了碳溢出。研究难降解植物生物质的分解对于陆地碳循环的限速步骤以及生产植物衍生的有价值的化学品和能源具有重要意义。虽然已经详细研究了细胞外纤维素的降解和分解代谢，但很少有文献描述半纤维素的完整代谢，而且迄今为止，已发表的模型仅限于细胞外降解和简单糖的顺序进入。在这里，我们描述了模式厌氧细菌 Ruminiclostridium cellulolyticum 如何处理同步细胞内释放的葡萄糖、半乳糖、木糖和纤维二糖，这些糖是在细胞质中解聚进口木葡聚糖寡糖后产生的。所描述的新型代谢策略涉及不同代谢途径的同时活性，以及控制碳通量的抑制网络，与普遍观察到的碳水化合物顺序摄取和代谢（称为双相转换）不同。我们的研究结果强调了与复杂环境相关的细胞反应的多样性。
