3.1 Construction of the xylose utilization pathway
The blueprint of genetic manipulation in this study was illustrated in Fig. 1. First, an expression cassette was constructed using OsRpiB gene and pYES2 plasmid for converting xylose to xylulose (Fig. S1). The cassette was transformed into T. oedocephalis and a transformant TOR was screened using xylose as substrate. TOR was first observed to assimilate xylose at the concentration of 200 g/L after 96 h fermentation (Fig. S2). The main fermentation product was identified as xylitol, at the concentration of 48.4 g/L, and no erythritol was observed. Then the XK and XDH genes were cloned from T. oedocephalis based on genome sequencing data to further develop an erythritol producing strain with xylose. An expression cassette containing the genes of three enzymes was constructed and transformed into T. oedocephalis to construct the engineered stain TORKD (Fig. S2). It should be noted that the purpose of introducing XDH was to facilitate the selection of TORKD using the substrate of xylitol instead of xylose, because the strain TOR could also assimilate xylose. Erythritol production of 10.6 g/L was achieved in the initial fermentation of TORKD with 200 g/L xylitol, and the concentration of xylitol was increased to 65.4 g/L, with a total xylose conversion rate of 83.3% at 96 h. Notably, neither TOR nor TORKD produced glycerol in fermentation with xylose, which was the main product when glucose was used as the substrate, and the product of xylose utilization performed superior purity (Fig. S3).
Biomass-derived carbon source is a promising alternative for biosynthesis industry with economic efficiency. Currently, the utilization of xylose in yeasts is mainly achieved through the non-oxidative pathway (relative to the oxidative pathway involving xylose reductase and xylitol dehydrogenase), in which isomerase plays an important role (Wasylenko and Stephanopoulos 2015). For strains which are engineered to utilize xylose, xylose isomerase (XI) is generally employed, for example in the host of Saccharomyces cerevisiae (Kim et al. 2013; Jo et al. 2017). However, XI, which is also named glucose isomerase has a wide substrate scope, which might lead to unfavorable effects in metabolic engineering. OsRpiB has the same function and efficiency as XI to convert xylose to D-xylulose, while shows no activity towards glucose (Tang et al. 2022). At 55℃ and 10 mM xylose concentration, the in vitro conversion rate of xylose reached 22% after 3 h. OsRpiB with unique substrate scope towards rare sugars has been leveraged in enzymatic cascade reactions to convert biomass hydrolysate to valuable products (Tang et al. 2023). In this study, the involvement of OsRpiB successfully endowed wild type T. oedocephalis with the ability to assimilate xylose. As the main product of strain TOR is xylitol, it is speculated that there is high activity of xylulose reductase in T. oedocephalis. Considering this activity is towards neither xylose nor xylitol, it might not been reported intensively. Then XK was further introduced aiming to increase the accumulation of D-xylulose phosphate. Previous instances had proved the importance of XK in connecting xylose assimilation with PPP, and it is speculated that raised carbon flux to PPP led to the substantial production of erythritol in this case (Zhou et al. 2012; Lee et al. 2014).
3.2 Fermentation medium optimization
The effect of xylose concentration from 100 g/L to 400 g/L on the erythritol production was investigated in flask using TORKD. The fermentation with 200 g/L xylose obtained the highest erythritol concentration of 13.4 g/L at 72 h and then decreased rapidly (Fig. 2A and Fig. S4A), and both 100 g/L and 200 g/L tests exhausted xylose substrate within the fermentation after the fermentation of 168 h (Fig. 2C). The concentration peak value of erythritol showed no increase with the xylose concentration rising from 200 g/L to 400 g/L. Besides erythritol, the concentration of the main byproduct xylitol also reached the highest, 157.1 g/L at 200 g/L xylose, and then decreased in the trails of 300 g/L and 400 g/L xylose (Fig. 2A). The cell growth and metabolism might consume produced erythritol gradually through the erythritol pathway and PPP after 72 h, which can be verified by the DCW progress in Fig. 2B, while the production of xylitol continued in this period indicating the one-way generation catalyzed by active xylulose reductase. With the increase of xylose concentration, the DCW also increased from 16 g/L to 43 g/L, and the trend slowed when the xylose concentration was higher than 200 g/L. It can be deduced that the xylose inhibition effect on both cell growth and polyol production is significant under more than 200 g/L xylose, and most carbon source flows to xylitol production, stain growth and metabolism. The fermentation data of TORKD was summarized in Table 1, and the optimal erythritol production and yield were 0.19 g/L/h and 0.22 g/g, respectively. The fermentation pH generally decreased continuously to 4.0, which commensurate with the erythritol fermentation using glucose (Fig. S4C) (Li et al. 2018b).
Table 1
Data of erythritol production at 72 h under different xylose concentrations.
Xylose (g/L) | Erythritol (g/L) | Residual sugar (g/L) | Production (g/L/h) | Yield (g/g) |
100 | 5.83 ± 0.25 | 43.28 ± 0.68 | 0.08 | 0.10 |
200 | 13.36 ± 0.60 | 139.91 ± 4.08 | 0.19 | 0.22 |
300 | 8.15 ± 0.30 | 205.54 ± 0.75 | 0.11 | 0.09 |
400 | 7.36 ± 0.45 | 297.99 ± 3.97 | 0.10 | 0.07 |
Different nitrogen sources including yeast extract, corn steep liquor, beef extract, soybean cake powder, and ammonium sulfate were tested aiming to make better utilization of the carbon source xylose. Yeast extract was found to produce the highest erythritol concentration of 14.1 g/L among five kinds of nitrogen sources, and the production of erythritol and xylitol (157.2 g/L) made no difference to previous experiments (Fig. 3A&B). Among nitrogen sources, the cultivation with corn steep liquor obtained the highest DCW, and xylose consumption rate was the highest when soybean cake powder was employed (Fig. 3C&D). The lowest productivity in ammonium sulfate medium might be due to the lowest pH of 2.3 during fermentation (data not shown), hence the pH control was be important when inorganic nitrogen source was used.
In the optimized medium, the introduction of OsRpiB switches the strains TOR and TORXD to produce xylitol from xylose efficiently. The 157.2 g/L xylitol concentration is higher than that of most reported instances, and the productivity of 1.16 g/L/h and the yield of 0.78 were competitive (Erian and Sauer 2022). Specially, high product purity was achieved by eliminating glycerol, which was a main product in fermentation with glucose. Despite the potential of TORXD to produce more valuable products from xylose, e.g. erythritol, however the redirection of carbon flux from xylitol generation remained a major hindrance.
3.3 Comparison of glucose and xylose fermentation
The wild type T. oedocephalis produced considerable erythritol with glucose, therefore the comparison of glucose fermentation and xylose-glucose cofermentation were conducted at the concentration of 200 g/L. Erythritol of 32.5 g/L was produced with pure glucose as the carbon source, which was the highest (Fig. 4A). A significant trend was higher glucose ratio led to more erythritol production, and higher xylose ratio benefitted xylitol production. In cofermentation, the best weight ratio of xylose and glucose was 1:2, producing to 23.6 g/L erythritol and 54.5 g/L xylitol. Notably, xylose fermentation could produce more DCW than glucose fermentation and cofermentation, demonstrating the high efficiency of introduced xylose metabolic pathway (Fig. 4B). The strain growth rate in xylose medium was even 29% higher than that in glucose medium in the first 24 h and obtained 137% DCW. During cofermentation, the glucose was rapidly consumed within 48 h, and then the consumption of xylose was obvious, hinting obvious carbon catabolite repression (CCR) (Fig. 4C&D).
Cofermentation of glucose and xylose has been investigated widely to utilize the two main lignocellulosic monosaccharide simultaneously and avoid tedious separation (Kim et al. 2022). Efforts of metabolic engineering has been taken to promote the utilization of xylose when the obstacle of CCR is severe, especially on the enhancement of xylose uptake channel (Fox and Prather 2020; Guo et al. 2022). In this study, the rapid consumption of glucose before xylose indicated significant CCR. The consumption rate of xylose maintained constant before and after the exhaustion of glucose, and suggested the xylose uptake channel of T. oedocephalis was unobstructed. The main obstacle might be minor xylose entering PPP, which was competed with the intense xylitol production pathway. Nevertheless, the economic efficiency of xylose fermentation is much higher than that of glucose fermentation, therefore the following optimization was still implemented with the carbon source of 200 g/L xylose.
3.4 Effect of high osmotic fermentation on erythritol production
In our previous research, KCl of more than 5 g/L promoted the erythritol production significantly by T. oedocephalis in glucose medium (Li et al. 2017). The effect of concentration of KCl on the erythritol production by xylose was then explored. With the increase of KCl from 0.5 to 40 g/L, the production of erythritol boosted from 5.8 g/L to 46.5 g/L (Fig. 5A), and the concentration of xylitol decreased from 157.3 g/L to 130.2 g/L (Fig. 5B). Moreover, the DCW and xylose consumption rate also increased significantly (Fig. 5C&D). In the summary of Table 2, the addition of 40 g/L KCl achieved 2.53-fold erythritol productivity and 2.25-fold yield at the time point of 72 h.
Table 2
Data of erythritol production at 72 h under different KCl concentrations.
KCl (g/L) | Erythritol (g/L) | Residual xylose (g/L) | Productivity (g/L/h) | Yield (g/g) |
0.5 | 13.56 ± 0.36 | 115.29 ± 1.95 | 0.19 | 0.16 |
5 | 19.47 ± 0.81 | 103.53 ± 1.69 | 0.20 | 0.20 |
10 | 25.24 ± 0.59 | 97.89 ± 1.37 | 0.26 | 0.24 |
20 | 38.78 ± 1.16 | 90.47 ± 0.40 | 0.40 | 0.35 |
40 | 46.51 ± 1.15 | 63.63 ± 1.60 | 0.48 | 0.36 |
Polyols can serve as osmoregulatory compounds in the various microbes (Kobayashi et al. 2015b). In this study, the addition KCl at 40 g/L activated the corresponding mechanism to produce more polyol. Notably, comparing to the decreased concentration of xylitol, only the production of erythritol was enhanced and no glycerol was found, indicating that the system upregulated gene expression in PPP and/or erythritol pathway in T. oedocephalis. Yang et al. reported the high osmotic pressure redirected the carbon flux to produce more erythritol and less mannitol in the fermentation of Yarrowia lipolytica CICC 1675 with the carbon source of glycerol (Yang et al. 2014). Therefore, the mechanism might downregulate the xylitol generating step, i.e. the xylulose reductase gene expression, which could be studied in future.
3.5 Production of erythritol in a 5-L fermentor
Based on the previous optimization, the best fermentation medium was set as 200 g/L xylose, 10 g/L yeast extract and 40 g/L KCl, and a scale-up was implemented in a 5-L fermentor. From the results shown in Fig. 6, the production of erythritol was 14% less than that in flask at the time point of 72 h, and the strain growth was 26% slower than that in flask at the time point of 120 h, which was supported by the lower xylose consumption rate. Mass transfer in larger scale led to variations in pH, dissolved oxygen and other factors between the fermentation tank and shaking flask (Hollinshead et al. 2014). The concentration and the productivity of erythritol reached the peak value of 40.1 g/L and 0.56 g/L/h at 72 h, and the concentration of by-product xylitol increased continuously to 131.5 g/L at 168 h.