Cellobiose Consumption Uncouples Extracellular Glucose Sensing and Glucose Metabolism in Saccharomyces cerevisiae

Glycolysis is central to energy metabolism in most organisms, and is highly regulated to enable optimal growth. In the yeast Saccharomyces cerevisiae, feedback mechanisms that control flux through glycolysis span transcriptional control to metabolite levels in the cell. Using a cellobiose consumption pathway, we decoupled glucose sensing from carbon utilization, revealing new modular layers of control that induce ATP consumption to drive rapid carbon fermentation. Alterations of the beta subunit of phosphofructokinase (PFK2), H+-plasma membrane ATPase (PMA1), and glucose sensors (SNF3, RGT2) revealed the importance of coupling extracellular glucose sensing to manage ATP levels in the cell. Controlling the upper bound of cellular ATP levels may be a general mechanism used to regulate energy levels in cells, via a regulatory network that can be uncoupled from ATP concentrations under perceived starvation conditions

Effects of overexpression of endogenous enzymes in the alternative xylose utilization pathway.

<p>Strains overexpressing the three endogenous components–<i>FBA1</i>, <i>ADH1</i>, and <i>GRE2</i>—were compared. <i>xks1Δ</i> XI-RnKHK were used as the background strain for all the overexpression comparisons. Concentrations of <b>(A)</b> xylose and <b>(B)</b> ethylene glycol (EG) were shown. OE, overexpression. Error bars indicated standard errors, N = 2.</p

Product titers and relative abundance of metabolites in fermentations with the alternative pathway.

<p>The traditional xylose isomerase pathway (denoted as “PPP”) and the alternative xylose utilization pathway (<i>xks1Δ</i> XI-RnKHK-<i>FBA1</i>-CD denoted as “Bypass”) were compared. <b>(A)</b> Ethanol and ethylene glycol (EG) production. <b>(B)</b> Relative abundance of intracellular ᴅ-xylulose-1-phosphate (X1P), ᴅ-xylulose-5-phosphate (X5P), ᴅ-ribose-5-phosphate (R5P) and ᴅ-sedoheptulose-7-phosphate (S7P), after 4 days of fermentation. Values are normalized to 1 for levels of each compound observed in the Bypass strain. <b>(C)</b> Abundance of metabolites in the Bypass strain provided with xylose (X), a mixture of xylose and cellobiose (XC) and cellobiose only (C) are shown. Samples taken after 4 days of fermentation, and values are normalized to 1 for the levels of each compound observed in the cellobiose-only fermentation. In all three panels, error bars indicated standard errors, N = 5.</p

Synergistic effects of cellobiose and xylose co-utilization.

<p><b>(A)</b> Intracellular concentrations of ATP, NAD⁺ and NADH are shown for fermentations with xylose, cellobiose and its mixture (denoted, X, C and XC respectively) provided to the <i>xks1Δ</i> XI-RnKHK-<i>FBA1</i>-CD strain. <b>(B)</b> <i>xks1Δ</i> XI-RnKHK-CD (denoted in blue) and <i>xks1Δ</i> XI-RnKHK-<i>FBA1</i>-CD (denoted in green) strains were provided with xylose in the presence and absence of cellobiose. Concentrations of ethylene glycol (EG) are shown. X, xylose; C, cellobiose; XC, mixture of xylose and cellobiose; OE, overexpression. Error bars indicated standard errors, N = 5.</p

Deciphering bacterial xylose metabolism and metabolic engineering of industrial microorganisms for use as efficient microbial cell factories

The goal of sustainable production of biochemicals and biofuels has driven the engineering of microbial cell as factories that convert low-value substrates to high-value products. Xylose is the second most abundant sugar substrate in lignocellulosic hydrolysates. We analyzed the mechanisms of xylose metabolism using genome sequencing data of 492 industrially relevant bacterial species in the mini-review. The analysis revealed the xylose isomerase and Weimberg pathways as the major routes across diverse routes of bacterial xylose metabolism. In addition, we discuss recent developments in metabolic engineering of xylose metabolism in industrial microorganisms. Genome-scale analyses have revealed xylose pathway-specific flux landscapes. Overall, a comprehensive understanding of bacterial xylose metabolism could be useful for the feasible development of microbial cell factories