Metabolic pathway analysis of the xylose-metabolizing yeast protoplast fusant ZLYRHZ7

Enhanced dextran production by Weissella confusa in co-culture with Candida shehatae and its quorum sensing regulation mechanism

Lactic acid bacteria (LAB) are well-known for its expertise in synthesizing exopolysaccharides (EPS), which are linked to significant health benefits, such as its prebiotic effects and ability to modulate the immune system. However, the synthesis of EPS is hindered by low yields. The objective of this study was to investigate the impact of co-cultivation on EPS output by Weissella confusa XG-3 when paired with Candida shehatae. The structure and characteristics of the purified EPS were analyzed. Gel permeation chromatography (GPC) revealed a molecular weight of 3.45 × 106 Da. High-performance liquid chromatography (HPLC), nuclear magnetic resonance (NMR) spectroscopy and fourier transform infrared (FT-IR) spectroscopy showed that the EPS was a linear homopolysaccharide, mainly composed of glucose units, with a proposed structure of dextran featuring α-(1, 6) glycosidic linkages and occasional α-(1,3) branching. Fermentation processes were conducted in both mono-culture and co-culture settings, with tracking of pivotal gene expression. The result showed that the expression of luxS, ackA and wzb was significantly up-regulated in the co-culture system, which was positively correlated with dextransucrase activity and dextran production. C. shehatae promoted the growth of W. confusa XG-3 by consuming organic acids in the culture system through direct contact, initiating quorum sensing (QS), inducing dsr expression, increasing dextransucrase activity, and ultimately promoting dextran synthesis.

Pentose metabolism and conversion to biofuels and high-value chemicals in yeasts

Pentose sugars are widespread in nature and two of them, D-xylose and L-arabinose belong to the most abundant sugars being the second and third by abundance sugars in dry plant biomass (lignocellulose) and in general on planet. Therefore, it is not surprising that metabolism and bioconversion of these pentoses attract much attention. Several different pathways of D-xylose and L-arabinose catabolism in bacteria and yeasts are known. There are even more common and really ubiquitous though not so abundant pentoses, D-ribose and 2-deoxy-D-ribose, the constituents of all living cells. Thus, ribose metabolism is example of endogenous metabolism whereas metabolism of other pentoses, including xylose and L-arabinose, represents examples of the metabolism of foreign exogenous compounds which normally are not constituents of yeast cells. As a rule, pentose degradation by the wild-type strains of microorganisms does not lead to accumulation of high amounts of valuable substances; however, productive strains have been obtained by random selection and metabolic engineering. There are numerous reviews on xylose and (less) L-arabinose metabolism and conversion to high value substances; however, they mostly are devoted to bacteria or the yeast Saccharomyces cerevisiae. This review is devoted to reviewing pentose metabolism and bioconversion mostly in non-conventional yeasts, which naturally metabolize xylose. Pentose metabolism in the recombinant strains of S. cerevisiae is also considered for comparison. The available data on ribose, xylose, L-arabinose transport, metabolism, regulation of these processes, interaction with glucose catabolism and construction of the productive strains of high-value chemicals or pentose (ribose) itself are described. In addition, genome studies of the natural xylose metabolizing yeasts and available tools for their molecular research are reviewed. Metabolism of other pentoses (2-deoxyribose, D-arabinose, lyxose) is briefly reviewed.

Heterologous expression and biochemical characterization of a thermostable xylulose kinase from Bacillus coagulans IPE22

Xylulose kinase is an important enzyme involved in xylose metabolism, which is considered as essential biocatalyst for sustainable lignocellulosic-derived pentose utilization. Bacillus coagulans IPE22 is an ideal bacterium for refinery due to its strong ability to ferment xylose at high temperature. However, the B. coagulans xylose utilization mechanism remains unclear and the related promising enzymes need to be developed. In the present study, the gene coding for xylulose kinase from B. coagulans IPE22 (Bc-XK) was expressed in Escherichia coli BL21 (DE3). Bc-XK has a 1536 bp open reading frame, encoding a protein of 511 amino acids (56.15 kDa). Multiple sequence alignments were performed and a phylogenetic tree was built to evaluate differences among Bc-XK and other bacteria homologs. Bc-XK showed a broad adaptability to high temperature and the enzyme displayed its best performance at pH 8.0 and 60 °C. Bc-XK was activated by Mg²⁺ , Mn²⁺ , and Co²⁺ . Meanwhile, the enzyme could keep activity at 60 °C for at least 180 min. K_M values of Bc-XK for xylulose and ATP were 1.29 mM and 0.76 mM, respectively. The high temperature stability of Bc-XK implied that it was an attractive candidate for industrial application.