Evidence for rapid uptake ofd-galacturonic acid in the yeastSaccharomyces cerevisiaeby a channel-type transport system

Use of ambr®250 to assess mucic acid production in fed-batch cultures of a marine Trichoderma sp. D-221704

Mucic acid, a diacid with potential use in the food, cosmetic, chemical and pharmaceutical industries, can be produced by microbial conversion of D-galacturonic acid, which is abundant in pectin. Using the ambr®250 bioreactor system, we found that a recently generated transformant (D-221704, formerly referred to as T2) of a marine Trichoderma species produced up to 53 g L-1 mucic acid in glucose-limited fed-batch culture with D-galacturonic acid in the feed at pH 4, with a yield of 0.99 g mucic acid per g D-galacturonic acid consumed. Yeast extract was not essential for high production, but increased the initial production rate. Reducing the amount of glucose as the co-substrate reduced the amount of mucic acid produced to 31 g L-1. Mucic acid could also be produced at pH values less than 4.0 (3.5 and 3.0), but the amount produced was less than at pH 4.0. Furthermore, the yield of mucic acid on D-galacturonic acid at the end of the cultivations (0.5 to 0.7 g g-1) at these low pH levels suggested that recovery may be more difficult at lower pH on account of the high level of crystal formation. Another strain engineered to produce mucic acid, Trichoderma reesei D-161646, produced only 31 g L-1 mucic acid under the conditions used with D-221704.

Identification of the Aldo-Keto Reductase Responsible for d-Galacturonic Acid Conversion to l-Galactonate in Saccharomyces cerevisiae

d-galacturonic acid (d-GalUA) is the main constituent of pectin, a complex polysaccharide abundant in several agro-industrial by-products such as sugar beet pulp or citrus peel. During several attempts to valorise d-GalUA by engineering the popular cell factory Saccharomyces cerevisiae, it became obvious that d-GalUA is, to a certain degree, converted to l-galactonate (l-GalA) by an endogenous enzymatic activity. The goal of the current work was to clarify the identity of the responsible enzyme(s). A protein homology search identified three NADPH-dependent unspecific aldo-keto reductases in baker’s yeast (encoded by GCY1, YPR1 and GRE3) that show sequence similarities to known d-GalUA reductases from filamentous fungi. Characterization of the respective deletion mutants and an in vitro enzyme assay with a Gcy1 overproducing strain verified that Gcy1 is mainly responsible for the detectable reduction of d-GalUA to l-GalA.

Towards valorization of pectin-rich agro-industrial residues: Engineering of Saccharomyces cerevisiae for co-fermentation of d-galacturonic acid and glycerol

Pectin-rich plant biomass residues represent underutilized feedstocks for industrial biotechnology. The conversion of the oxidized monomer d-galacturonic acid (d-GalUA) to highly reduced fermentation products such as alcohols is impossible due to the lack of electrons. The reduced compound glycerol has therefore been considered an optimal co-substrate, and a cell factory able to efficiently co-ferment these two carbon sources is in demand. Here, we inserted the fungal d-GalUA pathway in a strain of the yeast S. cerevisiae previously equipped with an NAD-dependent glycerol catabolic pathway. The constructed strain was able to consume d-GalUA with the highest reported maximum specific rate of 0.23 g g_CDW^-1 h^-1 in synthetic minimal medium when glycerol was added. By means of a ¹³C isotope-labelling analysis, carbon from both substrates was shown to end up in pyruvate. The study delivers the proof of concept for a co-fermentation of the two 'respiratory' carbon sources to ethanol and demonstrates a fast and complete consumption of d-GalUA in crude sugar beet pulp hydrolysate under aerobic conditions. The future challenge will be to achieve co-fermentation under industrial, quasi-anaerobic conditions.

D-Galacturonic acid reduction by S. cerevisiae for L-galactonate production from extracted sugar beet press pulp hydrolysate

Abstract

Pectin-rich residues are considered as promising feedstocks for sustainable production of platform chemicals. Enzymatic hydrolysis of extracted sugar beet press pulp (SBPP) releases the main constituent of pectin, d-galacturonic acid (d-GalA). Using engineered Saccharomyces cerevisiae, d-GalA is then reduced to l-galactonate (l-GalOA) with sorbitol as co-substrate. The current work addresses the combination of enzymatic hydrolysis of pectin in SBPP with a consecutive optimized biotransformation of the released d-GalA to l-GalOA in simple batch processes in stirred-tank bioreactors. Process conditions were first identified with synthetic media, where a product concentration of 9.9 g L^-1 L-GalOA was obtained with a product selectivity of 99% (L-GalOA D-GalA^-1) at pH 5 with 4% (w/v) sorbitol within 48 h. A very similar batch process performance with a product selectivity of 97% was achieved with potassium citrate buffered SBPP hydrolysate, demonstrating for the first time direct production of L-GalOA from hydrolyzed biomass using engineered S. cerevisiae. Combining the hydrolysis process of extracted SBPP and the biotransformation process with engineered S. cerevisiae paves the way towards repurposing pectin-rich residues as substrates for value-added chemicals.

Key points

• Efficient bioreduction of D-GalA with S. cerevisiae in stirred-tank reactors

• Batch production of L-GalOA by engineered S. cerevisiae with high selectivity

• Direct L-GalOA production from hydrolyzed sugar beet press pulp

Graphical abstract

Mimicking natural strategies to create multi-environment enzymatic reactors: From natural cell compartments to artificial polyelectrolyte reactors

Engineering microenvironments for sequential enzymatic reactions has attracted specific interest within different fields of research as an effective strategy to improve the catalytic performance of enzymes. While in industry most enzymatic reactions occur in a single compartment carrier, living cells are however able to conduct multiple reactions simultaneously within confined sub-compartments, or organelles. Engineering multi-compartments with regulated environments and transformation properties enhances enzyme activity and stability and thus increases the overall yield of final products. In this review, we discuss current and potential methods to fabricate artificial cells for sequential enzymatic reactions, which are inspired by mechanisms and metabolic pathways developed by living cells. We aim to advance the understanding of living cell complexity and its compartmentalization and present solutions to mimic these processes in vitro. Particular attention has been given to layer-by-layer assembly of polyelectrolytes for developing multi-compartments. We hope this review paves the way for the next steps toward engineering of smart artificial multi-compartments with adoptive stimuli-responsive properties, mimicking living cells to improve catalytic properties and efficiency of the enzymes and enhance their stability.

Recent advances in the biological valorization of citrus peel waste into fuels and chemicals

In the quest to reduce global food loss and waste, fruit processing wastes, particularly citrus peel waste (CPW), have emerged as a promising and sustainable option for biorefinery without competing with human foods and animal feeds. CPW is largely produced and, as recent studies suggest, has the industrial potential of biological valorization into fuels and chemicals. In this review, the promising aspects of CPW as an alternative biomass were highlighted, focusing on its low lignin content. In addition, specific technical difficulties in fermenting CPW are described, highlighting that citrus peel is high in pectin that consist of non-fermentable sugars, mainly galacturonic acid. Last, recent advances in the metabolic engineering of yeast and other microbial strains that ferment CPW-derived sugars to produce value-added products, such as ethanol and mucic acid, are summarized. For industrially viable CPW-based biorefinery, more studies are needed to improve fermentation efficiency and to diversify product profiles.

Valorisation of pectin-rich agro-industrial residues by yeasts: potential and challenges

Pectin-rich agro-industrial residues are feedstocks with potential for sustainable biorefineries. They are generated in high amounts worldwide from the industrial processing of fruits and vegetables. The challenges posed to the industrial implementation of efficient bioprocesses are however manyfold and thoroughly discussed in this review paper, mainly at the biological level. The most important yeast cell factory platform for advanced biorefineries is currently Saccharomyces cerevisiae, but this yeast species cannot naturally catabolise the main sugars present in pectin-rich agro-industrial residues hydrolysates, in particular D-galacturonic acid and L-arabinose. However, there are non-Saccharomyces species (non-conventional yeasts) considered advantageous alternatives whenever they can express highly interesting metabolic pathways, natively assimilate a wider range of carbon sources or exhibit higher tolerance to relevant bioprocess-related stresses. For this reason, the interest in non-conventional yeasts for biomass-based biorefineries is gaining momentum. This review paper focuses on the valorisation of pectin-rich residues by exploring the potential of yeasts that exhibit vast metabolic versatility for the efficient use of the carbon substrates present in their hydrolysates and high robustness to cope with the multiple stresses encountered. The major challenges and the progresses made related with the isolation, selection, sugar catabolism, metabolic engineering and use of non-conventional yeasts and S. cerevisiae-derived strains for the bioconversion of pectin-rich residue hydrolysates are discussed. The reported examples of value-added products synthesised by different yeasts using pectin-rich residues are reviewed. Key Points • Review of the challenges and progresses made on the bioconversion of pectin-rich residues by yeasts. • Catabolic pathways for the main carbon sources present in pectin-rich residues hydrolysates. • Multiple stresses with potential to affect bioconversion productivity. • Yeast metabolic engineering to improve pectin-rich residues bioconversion. Graphical abstract.

Microbial hexuronate catabolism in biotechnology

The most abundant hexuronate in plant biomass is D-galacturonate. D-Galacturonate is the main constituent of pectin. Pectin-rich biomass is abundantly available as sugar beet pulp or citrus processing waste and is currently mainly used as cattle feed. Other naturally occurring hexuronates are D-glucuronate, L-guluronate, D-mannuronate and L-iduronate. D-Glucuronate is a constituent of the plant cell wall polysaccharide glucuronoxylan and of the algal polysaccharide ulvan. Ulvan also contains L-iduronate. L-Guluronate and D-mannuronate are the monomers of alginate. These raw materials have the potential to be used as raw material in biotechnology-based production of fuels or chemicals. In this communication, we will review the microbial pathways related to these hexuronates and their potential use in biotechnology.

Spotlight on fungal pectin utilization-from phytopathogenicity to molecular recognition and industrial applications

Pectin is a complex polysaccharide with D-galacturonic acid as its main component that predominantly accumulates in the middle lamella of the plant cell wall. Integrity and depolymerization of pectic structures have long been identified as relevant factors in fungal phytosymbiosis and phytopathogenicity in the context of tissue penetration and carbon source supply. While the pectic content of a plant cell wall can vary significantly, pectin was reported to account for up to 20-25% of the total dry weight in soft and non-woody tissues with non- or mildly lignified secondary cell walls, such as found in citrus peel, sugar beet pulp, and apple pomace. Due to their potential applications in various industrial sectors, pectic sugars from these and similar agricultural waste streams have been recognized as valuable targets for a diverse set of biotechnological fermentations.Recent advances in uncovering the molecular regulation mechanisms for pectinase expression in saprophytic fungi have led to a better understanding of fungal pectin sensing and utilization that could help to improve industrial, pectin-based fermentations. Related research in phytopathogenic fungi has furthermore added to our knowledge regarding the relevance of pectinases in plant cell wall penetration during onset of disease and is therefore highly relevant for agricultural sciences and the agricultural industry. This review therefore aims at summarizing (i) the role of pectinases in phytopathogenicity, (ii) the global regulation patterns for pectinase expression in saprophytic filamentous fungi as a highly specialized class of pectin degraders, and (iii) the current industrial applications in pectic sugar fermentations and transformations.

Engineering Saccharomyces cerevisiae for co-utilization of D-galacturonic acid and D-glucose from citrus peel waste

Pectin-rich biomasses, such as citrus peel and sugar beet pulp, hold promise as inexpensive feedstocks for microbial fermentations as enzymatic hydrolysis of their component polysaccharides can be accomplished inexpensively to yield high concentrations of fermentable sugars and D-galacturonic acid (D-galUA). In this study, we tackle a number of challenges associated with engineering a microbial strain to convert pectin-rich hydrolysates into commodity and specialty chemicals. First, we engineer D-galUA utilization into yeast, Saccharomyces cerevisiae. Second, we identify that the mechanism of D-galUA uptake into yeast is mediated by hexose transporters and that consumption of D-galUA is inhibited by D-glucose. Third, we enable co-utilization of D-galUA and D-glucose by identifying and expressing a heterologous transporter, GatA, from Aspergillus niger. Last, we demonstrate the use of this transporter for production of the platform chemical, meso-galactaric acid, directly from industrial Navel orange peel waste.

Methyl-selective isotope labeling using α-ketoisovalerate for the yeast Pichia pastoris recombinant protein expression system

Methyl-detected NMR spectroscopy is a useful tool for investigating the structures and interactions of large macromolecules such as membrane proteins. The procedures for preparation of methyl-specific isotopically-labeled proteins were established for the Escherichia coli (E. coli) expression system, but typically it is not feasible to express eukaryotic proteins using E. coli. The Pichia pastoris (P. pastoris) expression system is the most common yeast expression system, and is known to be superior to the E. coli system for the expression of mammalian proteins, including secretory and membrane proteins. However, this system has not yet been optimized for methyl-specific isotope labeling, especially for Val/Leu-methyl specific isotope incorporation. To overcome this difficulty, we explored various culture conditions for the yeast cells to efficiently uptake Val/Leu precursors. Among the searched conditions, we found that the cultivation pH has a critical effect on Val/Leu precursor uptake. At an acidic cultivation pH, the uptake of the Val/Leu precursor was increased, and methyl groups of Val and Leu in the synthesized recombinant protein yielded intense ¹H-¹³C correlation signals. Based on these results, we present optimized protocols for the Val/Leu-methyl-selective ¹³C incorporation by the P. pastoris expression system.

The introduction of the fungal D-galacturonate pathway enables the consumption of D-galacturonic acid by Saccharomyces cerevisiae

BACKGROUND

Pectin-rich wastes, such as citrus pulp and sugar beet pulp, are produced in considerable amounts by the juice and sugar industry and could be used as raw materials for biorefineries. One possible process in such biorefineries is the hydrolysis of these wastes and the subsequent production of ethanol. However, the ethanol-producing organism of choice, Saccharomyces cerevisiae, is not able to catabolize D-galacturonic acid, which represents a considerable amount of the sugars in the hydrolysate, namely, 18 % (w/w) from citrus pulp and 16 % (w/w) sugar beet pulp.

RESULTS

In the current work, we describe the construction of a strain of S. cerevisiae in which the five genes of the fungal reductive pathway for D-galacturonic acid catabolism were integrated into the yeast chromosomes: gaaA, gaaC and gaaD from Aspergillus niger and lgd1 from Trichoderma reesei, and the recently described D-galacturonic acid transporter protein, gat1, from Neurospora crassa. This strain metabolized D-galacturonic acid in a medium containing D-fructose as co-substrate.

CONCLUSION

This work is the first demonstration of the expression of a functional heterologous pathway for D-galacturonic acid catabolism in Saccharomyces cerevisiae. It is a preliminary step for engineering a yeast strain for the fermentation of pectin-rich substrates to ethanol.

Fermentative production of l-galactonate by using recombinant Saccharomyces cerevisiae containing the endogenous galacturonate reductase gene from Cryptococcus diffluens

The GAR1 gene, encoding d-galacturonate reductase in Cryptococcus diffluens, was isolated, and the GAR1-expression plasmid was constructed by insertion of GAR1 downstream of the yeast constitutive promoter in the yeast-integrating vector. Recombinant Saccharomyces cerevisiae expressing C. diffluensd-galacturonate reductase from a genome integrated copy of the gene was cultured for use the conversion of d-galacturonic acid to l-galactonic acid. The optimum conditions for l-galactonic acid production were determined in terms of the initial concentration of d-galacturonic acid, fermentation pH, and mixed sugars. The following conditions yielded high efficiency in the conversion of d-galacturonic acid to l-galactonic acid in large-scale cultures: 0.1% initial d-galacturonic acid concentration, pH 3.5, and glucose as additional sugar. The aerobic condition was necessary for the conversion of d-galacturonic acid. Subculture of that recombinant was not showing to decrease of the d-galacturonic acid conversion rate even though it was repeated in ten generations. Culturing in scale-up, the conversion rate of d-galacturonic acid to l-galactonic acid was increased.

Involvement of Agrobacterium tumefaciens Galacturonate Tripartite ATP-Independent Periplasmic (TRAP) Transporter GaaPQM in Virulence Gene Expression

Monosaccharides capable of serving as nutrients for the soil bacterium Agrobacterium tumefaciens are also inducers of the vir regulon present in the tumor-inducing (Ti) plasmid of this plant pathogen. One such monosaccharide is galacturonate, the predominant monomer of pectin found in plant cell walls. This ligand is recognized by the periplasmic sugar binding protein ChvE, which interacts with the VirA histidine kinase that controls vir gene expression. Although ChvE is also a member of the ChvE-MmsAB ABC transporter involved in the utilization of many neutral sugars, it is not involved in galacturonate utilization. In this study, a putative tripartite ATP-independent periplasmic (TRAP) transporter, GaaPQM, is shown to be essential for the utilization of galacturonic acid; we show that residue R169 in the predicted sugar binding site of the GaaP is required for activity. The gene upstream of gaaPQM (gaaR) encodes a member of the GntR family of regulators. GaaR is shown to repress the expression of gaaPQM, and the repression is relieved in the presence of the substrate for GaaPQM. Moreover, GaaR is shown to bind putative promoter regions in the sequences required for galacturonic acid utilization. Finally, A. tumefaciens strains carrying a deletion of gaaPQM are more sensitive to galacturonate as an inducer of vir gene expression, while the overexpression of gaaPQM results in strains being less sensitive to this vir inducer. This supports a model in which transporter activity is crucial in ensuring that vir gene expression occurs only at sites of high ligand concentration, such as those at a plant wound site.

Genome-wide analysis of pectate-induced gene expression in Botrytis cinerea: identification and functional analysis of putative d-galacturonate transporters

The fungal plant pathogen Botrytis cinerea produces a spectrum of cell wall degrading enzymes for the decomposition of host cell wall polysaccharides and the consumption of the monosaccharides that are released. Especially pectin is an abundant cell wall component, and the decomposition of pectin by B. cinerea has been extensively studied. An effective concerted action of the appropriate pectin depolymerising enzymes, monosaccharide transporters and catabolic enzymes is important for complete d-galacturonic acid utilization by B. cinerea. In this study, we performed RNA sequencing to compare genome-wide transcriptional profiles between B. cinerea cultures grown in media containing pectate or glucose as sole carbon source. Transcript levels of 32 genes that are induced by pectate were further examined in cultures grown on six different monosaccharides, by means of quantitative RT-PCR, leading to the identification of 8 genes that are exclusively induced by d-galacturonic acid. Among these, the hexose transporter encoding genes Bchxt15 and Bchxt19 were functionally characterised. The subcellular location was studied of BcHXT15-GFP and BcHXT19-GFP fusion proteins expressed under control of their native promoter, in a B. cinerea wild-type strain. Both genes are expressed during growth on d-galacturonic acid and the fusion proteins are localized in plasma membranes and intracellular vesicles. Target gene knockout analysis revealed that BcHXT15 contributes to d-galacturonic acid uptake at pH 5∼5.6. The virulence of all B. cinerea hexose transporter mutants tested was unaltered on tomato and Nicotiana benthamiana leaves.

Identification and characterization of a galacturonic acid transporter from Neurospora crassa and its application for Saccharomyces cerevisiae fermentation processes

BACKGROUND

Pectin-rich agricultural wastes potentially represent favorable feedstocks for the sustainable production of alternative energy and bio-products. Their efficient utilization requires the conversion of all major constituent sugars. The current inability of the popular fermentation host Saccharomyces cerevisiae to metabolize the major pectic monosaccharide D-galacturonic acid (D-GalA) significantly hampers these efforts. While it has been reasoned that the optimization of cellular D-GalA uptake will be critical for the engineering of D-GalA utilization in yeast, no dedicated eukaryotic transport protein has been biochemically described. Here we report for the first time such a eukaryotic D-GalA transporter and characterize its functionality in S. cerevisiae.

RESULTS

We identified and characterized the D-GalA transporter GAT-1 out of a group of candidate genes obtained from co-expression analysis in N. crassa. The N. crassa Δgat-1 deletion strain is substantially affected in growth on pectic substrates, unable to take up D-GalA, and impaired in D-GalA-mediated signaling events. Moreover, expression of a gat-1 construct in yeast conferred the ability for strong high-affinity D-GalA accumulation rates, providing evidence for GAT-1 being a bona fide D-GalA transport protein. By recombinantly co-expressing D-galacturonate reductase or uronate dehydrogenase in yeast we furthermore demonstrated a transporter-dependent conversion of D-GalA towards more reduced (L-galactonate) or oxidized (meso-galactaric acid) downstream products, respectively, over a broad concentration range.

CONCLUSIONS

By utilizing the novel D-GalA transporter GAT-1 in S. cerevisiae we successfully generated a transporter-dependent uptake and catalysis system for D-GalA into two products with high potential for utilization as platform chemicals. Our data thereby provide a considerable first step towards a more complete utilization of biomass for biofuel and value-added chemicals production.

Toward pectin fermentation by Saccharomyces cerevisiae: expression of the first two steps of a bacterial pathway for D-galacturonate metabolism

Saccharomyces cerevisiae cannot metabolize D-galacturonate, an important monomer of pectin. Use of S. cerevisiae for production of ethanol or other compounds of interest from pectin-rich feedstocks therefore requires introduction of a heterologous pathway for D-galacturonate metabolism. Bacterial D-galacturonate pathways involve D-galacturonate isomerase, D-tagaturonate reductase and three additional enzymes. This study focuses on functional expression of bacterial D-galacturonate isomerases in S. cerevisiae. After demonstrating high-level functional expression of a D-tagaturonate reductase gene (uxaB from Lactococcus lactis), the resulting yeast strain was used to screen for functional expression of six codon-optimized bacterial D-galacturonate isomerase (uxaC) genes. The L. lactis uxaC gene stood out, yielding a tenfold higher enzyme activity than the other uxaC genes. Efficient expression of D-galacturonate isomerase and D-tagaturonate reductase represents an important step toward metabolic engineering of S. cerevisiae for bioethanol production from D-galacturonate. To investigate in vivo activity of the first steps of the D-galacturonate pathway, the L. lactis uxaB and uxaC genes were expressed in a gpd1Δ gpd2Δ S. cerevisiae strain. Although D-tagaturonate reductase could, in principle, provide an alternative means for re-oxidizing cytosolic NADH, addition of D-galacturonate did not restore anaerobic growth, possibly due to absence of a functional D-altronate exporter in S. cerevisiae.