Deletion of the HXK2 gene in Saccharomyces cerevisiae enables mixed sugar fermentation of glucose and galactose in oxygen-limited conditions

Enhancement of Galactose Uptake from Kappaphycus alvarezii Using Saccharomyces cerevisiae through Deletion of Negative Regulators of GAL Genes

This study was aimed at enhancing galactose consumption from the red seaweed Kappaphycus alvarezii. The optimal pretreatment condition of thermal acid hydrolysis was treated with 350 mM HNO₃ for 60 min at 121 °C. The enzymatic saccharification with a 1:1 mixture of Celluclast 1.5 L and Viscozyme L showed the maximum yield of glucose; 42-g/L monosaccharide concentration was obtained with the highest yield of pretreatment and enzymatic saccharification (E_PS) and the lowest inhibitory compound concentration. The deletion of the GAL80, MIG1, CYC8, or TUP1 gene was performed to improve the galactose consumption rate. The strains with the deletion of the MIG1 gene (mig1Δ) showed higher galactose consumption rate and ethanol yield than other strains. High transcription levels of regulatory genes revealed that the mig1Δ relieved glucose repression. These results show that the mig1Δ enhances galactose consumption rate from K. alvarezii.

Enhancement of Galactose Uptake from Kappaphycus alvarezii Hydrolysate Using Saccharomyces cerevisiae Through Overexpression of Leloir Pathway Genes

A total 42.68 g/L monosaccharide with 0.10 g/L HMF was obtained from 10% (w/v) Kappaphycus alvarezii with thermal acid hydrolysis using 350 mM HNO₃ at 121 °C for 60 min and enzymatic saccharification with a 1:1 mixture of Viscozyme L and Celluclast 1.5 L for 72 h. To enhance the galactose utilization rate, fermentation was performed with overexpression of GAL1 (galactokinase), GAL7 (galactose-1-phosphate uridyltransferase), GAL10 (UDP-glucose-4-epimerase), and PGM2 (phosphoglucomutase 2) in Saccharomyces cerevisiae CEN.PK2 using CCW12 as a strong promoter. Among the strains, the overexpression of PGM2 showed twofold high galactose utilization rate (UR_gal) and produced ethanol 1.4-fold more than that of the control. Transcriptional analysis revealed the increase of PGM2 transcription level leading to enhance glucose-6-phosphate and fructose-6-phosphate and plays a key role in ensuring a higher glycolytic flux in the PGM2 strain. This finding shows particular importance in biofuel production from seaweed because galactose is one of the major monosaccharides in seaweeds such as K. alvarezii.

Effects of the carbon source and the interaction between carbon sources on the physiology of the industrial Saccharomyces cerevisiae CAT-1

During industrial fermentation, wild isolates are able to persist and even predominate in the bioreactors. Saccharomyces cerevisiae CAT-1 was one of these isolates and now is one of the yeasts mostly used in industrial ethanol processes in Brazil due to its efficient fermentation capacity. Despite it, the strain's physiology has been marginally studied so far. Since strains of the same species may have different responses to a specific cultivation condition, this work aimed to evaluate the physiology of S. cerevisiae CAT-1 in batch cultures using different carbon sources (glucose, fructose, sucrose, maltose, and galactose) as a sole carbon source and in binary mixtures, at 30 and 37 °C. The results showed that the fructose, sucrose, and maltose were the sugars that presented the highest ethanol yields on the substrate (0.40 g_ethanol g_substrate^-1) at both temperatures. Galactose was the sugar that the yeast had the lowest affinity given the lowest maximum specific growth rate (0.28 h^-1). Despite the influence of a variety of mechanisms for sugar transport, the cells consume first substrates with fewer metabolic steps to catabolism and are susceptible to adaptive evolution depending on the availability of substrate.

Enhancement of bioethanol production from Gracilaria verrucosa by Saccharomyces cerevisiae through the overexpression of SNR84 and PGM2

A total monosaccharide concentration of 47.0 g/L from 12% (w/v) Gracilaria verrucosa was obtained by hyper thermal acid hydrolysis with 0.2 M HCl at 140°C for 15 min and enzymatic saccharification with CTec2. To improve galactose utilization, we overexpressed two genes, SNR84 and PGM2, in a Saccharomyces cerevisiae CEN-PK2 using CRISPR/Cas-9. The overexpression of both SNR84 and PGM2 improved galactose utilization and ethanol production compared to the overexpression of each gene alone. The overexpression of both SNR84 and PGM2 and of PGM2 and SNR84 singly in S. cerevisiae CEN-PK2 Cas9 produced 20.0, 18.5, and 16.5 g/L ethanol with ethanol yield (Y_EtOH) values of 0.43, 0.39, and 0.35, respectively. However, S. cerevisiae CEN-PK2 adapted to high concentration of galactose consumed galactose completely and produced 22.0 g/L ethanol at a Y_EtOH value of 0.47. The overexpression of both SNR84 and PGM2 increased the transcriptional levels of GAL and regulatory genes; however, the transcriptional levels of these genes were lower than those in S. cerevisiae adapted to high galactose concentrations.

Multi-omic characterization of laboratory-evolved Saccharomyces cerevisiae HJ7-14 with high ability of algae-based ethanol production

In this study, an evolved Saccharomyces cerevisiae HJ7-14 with high ability of algae-based ethanol production was characterized by multi-omic approaches. Genome sequencing of the HJ7-14 revealed a point mutation in the GAL83 gene (G703A) involved in the catabolite repression as well as the galactose metabolism. Cultural and transcriptional analyses of a S. cerevisiae mutant with chromosomal GAL83(G703A) indicated that the catabolite repression onto the galactose metabolism was considerably relieved in all cell growth stages. Untargeted metabolomic approach revealed that metabolic phenotypes between the control D452-2 and HJ7-14 strains were clearly discriminated in time-dependent manner. Especially in early growth stage at 6 h, the HJ7-14 showed dramatic and coordinated alteration in central carbon and amino acid metabolisms. Through metabolomic re-organization, fold changes in fatty acid metabolism and metabolites related to stress response system were also found upon glucose depletion and active galactose utilization. Multi-omic characterization using genome sequencing, transcription, and metabolome profiling clearly unveiled that the GAL83 gene mutation partially relieved glucose-dependent catabolite repression and allowed the evolved HJ7-14 to efficiently convert algal sugars to ethanol. Our finding could be applicable for engineering of S. cerevisiae able to covert red algal biomass to other biofuels and biochemicals.

Value-added biotransformation of cellulosic sugars by engineered Saccharomyces cerevisiae

The substantial research efforts into lignocellulosic biofuels have generated an abundance of valuable knowledge and technologies for metabolic engineering. In particular, these investments have led to a vast growth in proficiency of engineering the yeast Saccharomyces cerevisiae for consuming lignocellulosic sugars, enabling the simultaneous assimilation of multiple carbon sources, and producing a large variety of value-added products by introduction of heterologous metabolic pathways. While microbial conversion of cellulosic sugars into large-volume low-value biofuels is not currently economically feasible, there may still be opportunities to produce other value-added chemicals as regulation of cellulosic sugar metabolism is quite different from glucose metabolism. This review summarizes these recent advances with an emphasis on employing engineered yeast for the bioconversion of lignocellulosic sugars into a variety of non-ethanol value-added products.

Evolutionary engineering of Saccharomyces cerevisiae for efficient conversion of red algal biosugars to bioethanol

The aim of this work was to apply the evolutionary engineering to construct a mutant Saccharomyces cerevisiae HJ7-14 resistant on 2-deoxy-D-glucose and with an enhanced ability of bioethanol production from galactose, a mono-sugar in red algae. In batch and repeated-batch fermentations, HJ7-14 metabolized 5-fold more galactose and produced ethanol 2.1-fold faster than the parental D452-2 strain. Transcriptional analysis of genes involved in the galactose metabolism revealed that moderate relief from the glucose-mediated repression of the transcription of the GAL genes might enable HJ7-14 to metabolize galactose rapidly. HJ7-14 produced 7.4 g/L ethanol from hydrolysates of the red alga Gelidium amansii within 12 h, which was 1.5-times faster than that observed with D452-2. We demonstrate conclusively that evolutionary engineering is a promising tool to manipulate the complex galactose metabolism in S. cerevisiae to produce bioethanol from red alga.

Deletion of hxk1 gene results in derepression of xylose utilization in Scheffersomyces stipitis

A major problem in fermenting xylose in lignocellulosic substrates is the presence of glucose and mannose which inhibit xylose utilization. Previous studies showed that catabolite repression in some yeasts is associated with hexokinases and that deletion of one of these gene(s) could result in derepressed mutant strain(s). In this study, the hxk1 encoding hexokinase 1 in Scheffersomyces stipitis was disrupted. The ∆hxk1 SS6 strain retained the ability to utilize the main hexoses and pentoses commonly found in lignocellulosic hydrolysates as efficiently as the wild-type (WT) strain. SS6 also fermented the dominant sugars to ethanol; however, on xylose, the ∆hxk1 strain produced more xylitol and less ethanol than the WT. On mixed sugars, as expected the WT utilized glucose ahead of xylose and xylose utilization did not commence until all the glucose was consumed. In contrast, the ∆hxk1 mutant showed derepression in that it started to utilize xylose even when considerable glucose (about 1.72 %, w/v) remained in the medium. Similarly, mannose did not repress xylose utilization by the ∆hxk1 mutant and xylose and mannose were simultaneously utilized. The results are of interest in efforts to engineer yeast strains capable of efficiently utilizing glucose and xylose simultaneously for lignocellulosic biomass conversion.