Requirement of a Functional Flavin Mononucleotide Prenyltransferase for the Activity of a Bacterial Decarboxylase in a Heterologous Muconic Acid Pathway in Saccharomyces cerevisiae

Genome-wide host-pathway interactions affecting cis-cis-muconic acid production in yeast

The success of forward metabolic engineering depends on a thorough understanding of the behaviour of a heterologous metabolic pathway within its host. We have recently described CRI-SPA, a high-throughput gene editing method enabling the delivery of a metabolic pathway to all strains of the Saccharomyces cerevisiae knock-out library. CRI-SPA systematically quantifies the effect of each modified gene present in the library on product synthesis, providing a complete map of host:pathway interactions. In its first version, CRI-SPA relied on the colour of the product betaxanthins to quantify strains synthesis ability. However, only a few compounds produce a visible or fluorescent phenotype limiting the scope of our approach. Here, we adapt CRI-SPA to onboard a biosensor reporting the interactions between host genes and the synthesis of the colourless product cis-cis-muconic acid (CCM). We phenotype >9,000 genotypes, including both gene knock-out and overexpression, by quantifying the fluorescence of yeast colonies growing in high-density agar arrays. We identify novel metabolic targets belonging to a broad range of cellular functions and confirm their positive impact on CCM biosynthesis. In particular, our data suggests a new interplay between CCM biosynthesis and cytosolic redox through their common interaction with the oxidative pentose phosphate pathway. Our genome-wide exploration of host:pathway interaction opens novel strategies for improved production of CCM in yeast cell factories.

Screening non-conventional yeasts for acid tolerance and engineering Pichia occidentalis for production of muconic acid

Saccharomyces cerevisiae is a workhorse of industrial biotechnology owing to the organism's prominence in alcohol fermentation and the suite of sophisticated genetic tools available to manipulate its metabolism. However, S. cerevisiae is not suited to overproduce many bulk bioproducts, as toxicity constrains production at high titers. Here, we employ a high-throughput assay to screen 108 publicly accessible yeast strains for tolerance to 20 g L-1 adipic acid (AA), a nylon precursor. We identify 15 tolerant yeasts and select Pichia occidentalis for production of cis,cis-muconic acid (CCM), the precursor to AA. By developing a genome editing toolkit for P. occidentalis, we demonstrate fed-batch production of CCM with a maximum titer (38.8 g L-1), yield (0.134 g g-1 glucose) and productivity (0.511 g L-1 h-1) that surpasses all metrics achieved using S. cerevisiae. This work brings us closer to the industrial bioproduction of AA and underscores the importance of host selection in bioprocessing.

High-Level Production of Catechol from Glucose by Engineered Escherichia coli

Catechol (CA) is an aromatic compound with important applications in the fine chemical and pharmaceutical fields. As an alternative strategy to petroleum-based chemical synthesis, the production of catechol by using microbial cell factories has attracted great interest. However, the toxicity of catechol to microbial cells significantly limits the efficient production of bio-based catechol via one-step fermentation. Therefore, in this study, a two-step strategy for the efficient synthesis of CA was designed. Protocatechuic acid (PCA) was first efficiently produced by the engineered Escherichia coli strain AAA01 via fermentation, and then PCA in the fermentative broth was converted into CA by the whole-cell biocatalyst AAA12 with PCA decarboxylase. By optimizing the expression of flavin isoprenyl transferases and protocatechuic acid decarboxylases, the titer of CA increased from 3.4 g/L to 15.8 g/L in 12 h through whole-cell biocatalysis, with a 365% improvement; after further optimizing the reaction conditions for whole-cell biocatalysis, the titer of CA achieved 17.7 g/L within 3 h, which is the highest titer reported so far. This work provides an effective strategy for the green biomanufacturing of toxic compounds by Escherichia coli cell factories.

Structural and biochemical characterization of the prenylated flavin mononucleotide-dependent indole-3-carboxylic acid decarboxylase

The ubiquitous UbiD family of reversible decarboxylases is implicated in a wide range of microbial processes and depends on the prenylated flavin mononucleotide cofactor for catalysis. However, only a handful of UbiD family members have been characterized in detail, and comparison between these has suggested considerable variability in enzyme dynamics and mechanism linked to substrate specificity. In this study, we provide structural and biochemical insights into the indole-3-carboxylic acid decarboxylase, representing an UbiD enzyme activity distinct from those previously studied. Structural insights from crystal structure determination combined with small-angle X-ray scattering measurements reveal that the enzyme likely undergoes an open-closed transition as a consequence of domain motion, an event that is likely coupled to catalysis. We also demonstrate that the indole-3-carboxylic acid decarboxylase can be coupled with carboxylic acid reductase to produce indole-3-carboxyaldehyde from indole + CO₂ under ambient conditions. These insights provide further evidence for a common mode of action in the widespread UbiD enzyme family.

An integrated yeast-based process for cis,cis-muconic acid production

Cis,cis‐muconic acid (CCM) is a promising polymer building block. CCM can be made by whole‐cell bioconversion of lignin hydrolysates or de novo biosynthesis from sugar feedstocks using engineered microorganisms. At present, however, there is no established process for large‐scale CCM production. In this study, we developed an integrated process for manufacturing CCM from glucose by yeast fermentation. We systematically engineered the CCM‐producing Saccharomyces cerevisiae strain by rewiring the shikimate pathway flux and enhancing phosphoenolpyruvate supply. The engineered strain ST10209 accumulated less biomass but produced 1.4 g/L CCM (70 mg CCM per g glucose) in microplate assay, 71% more than the previously engineered strain ST8943. The strain ST10209 produced 22.5 g/L CCM in a 2 L fermenter with a productivity of 0.19 g/L/h, compared to 0.14 g/L/h achieved by ST8943 in our previous report under the same fermentation conditions. The fermentation process was demonstrated at pilot scale in 10 and 50 L steel tanks. In 10 L fermenter, ST10209 produced 20.8 g/L CCM with a CCM yield of 0.1 g/g glucose and a productivity of 0.21 g/L/h, representing the highest to‐date CCM yield and productivity. We developed a CCM recovery and purification process by treating the fermentation broth with activated carbon at low pH and low temperature, achieving an overall CCM recovery yield of 66.3% and 95.4% purity. In summary, we report an integrated CCM production process employing engineered S. cerevisiae yeast.

UbiD domain dynamics underpins aromatic decarboxylation

The widespread UbiD enzyme family utilises the prFMN cofactor to achieve reversible decarboxylation of acrylic and (hetero)aromatic compounds. The reaction with acrylic compounds based on reversible 1,3-dipolar cycloaddition between substrate and prFMN occurs within the confines of the active site. In contrast, during aromatic acid decarboxylation, substantial rearrangement of the substrate aromatic moiety associated with covalent catalysis presents a molecular dynamic challenge. Here we determine the crystal structures of the multi-subunit vanillic acid decarboxylase VdcCD. We demonstrate that the small VdcD subunit acts as an allosteric activator of the UbiD-like VdcC. Comparison of distinct VdcCD structures reveals domain motion of the prFMN-binding domain directly affects active site architecture. Docking of substrate and prFMN-adduct species reveals active site reorganisation coupled to domain motion supports rearrangement of the substrate aromatic moiety. Together with kinetic solvent viscosity effects, this establishes prFMN covalent catalysis of aromatic (de)carboxylation is afforded by UbiD dynamics.

In-situ muconic acid extraction reveals sugar consumption bottleneck in a xylose-utilizing Saccharomyces cerevisiae strain

Background

The current shift from a fossil-resource based economy to a more sustainable, bio-based economy requires development of alternative production routes based on utilization of biomass for the many chemicals that are currently produced from petroleum. Muconic acid is an attractive platform chemical for the bio-based economy because it can be converted in chemicals with wide industrial applicability, such as adipic and terephthalic acid, and because its two double bonds offer great versatility for chemical modification.

Results

We have constructed a yeast cell factory converting glucose and xylose into muconic acid without formation of ethanol. We consecutively eliminated feedback inhibition in the shikimate pathway, inserted the heterologous pathway for muconic acid biosynthesis from 3-dehydroshikimate (DHS) by co-expression of DHS dehydratase from P. anserina, protocatechuic acid (PCA) decarboxylase (PCAD) from K. pneumoniae and oxygen-consuming catechol 1,2-dioxygenase (CDO) from C. albicans, eliminated ethanol production by deletion of the three PDC genes and minimized PCA production by enhancing PCAD overexpression and production of its co-factor. The yeast pitching rate was increased to lower high biomass formation caused by the compulsory aerobic conditions. Maximal titers of 4 g/L, 4.5 g/L and 3.8 g/L muconic acid were reached with glucose, xylose, and a mixture, respectively. The use of an elevated initial sugar level, resulting in muconic acid titers above 2.5 g/L, caused stuck fermentations with incomplete utilization of the sugar. Application of polypropylene glycol 4000 (PPG) as solvent for in situ product removal during the fermentation shows that this is not due to toxicity by the muconic acid produced.

Conclusions

This work has developed an industrial yeast strain able to produce muconic acid from glucose and also with great efficiency from xylose, without any ethanol production, minimal production of PCA and reaching the highest titers in batch fermentation reported up to now. Utilization of higher sugar levels remained conspicuously incomplete. Since this was not due to product inhibition by muconic acid or to loss of viability, an unknown, possibly metabolic bottleneck apparently arises during muconic acid fermentation with high sugar levels and blocks further sugar utilization.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01594-3.

Artificial ER-Derived Vesicles as Synthetic Organelles for in Vivo Compartmentalization of Biochemical Pathways

Compartmentalization in membrane-surrounded organelles has the potential to overcome obstacles associated with the engineering of metabolic pathways, such as unwanted side reactions, accumulation of toxic intermediates, drain of intermediates out of the cell, and long diffusion distances. Strategies utilizing natural organelles suffer from the presence of endogenous pathways. In our approach, we make use of endoplasmic reticulum-derived vesicles loaded with enzymes of a metabolic pathway ("metabolic vesicles"). They are generated by fusion of synthetic peptides containing the N-terminal proline-rich and self-assembling region of the maize storage protein gamma-Zein ("Zera") to the pathway enzymes. We have applied a strategy to integrate three enzymes of a cis,cis-muconic acid production pathway into those vesicles in yeast. Using fluorescence microscopy and cell fractionation techniques, we have proven the formation of metabolic vesicles and the incorporation of enzymes. Activities of the enzymes and functionality of the compartmentalized pathway were demonstrated in fermentation experiments.

Biochemistry of prenylated-FMN enzymes

The reversible (de)carboxylation of unsaturated carboxylic acids is carried out by the UbiX-UbiD system, ubiquitously present in microbes. The biochemical basis of this challenging reaction has recently been uncovered by the discovery of the UbiD cofactor, prenylated FMN (prFMN). This heavily modified flavin is synthesized by the flavin prenyltransferase UbiX, which catalyzes the non-metal dependent prenyl transfer from dimethylallyl(pyro)phosphate (DMAP(P)) to the flavin N5 and C6 positions, creating a fourth non-aromatic ring. Following prenylation, prFMN undergoes oxidative maturation to form the iminium species required for UbiD activity. prFMN^iminium acts as a prostethic group and is bound via metal ion mediated interactions between UbiD and the prFMN^iminium phosphate moiety. The modified isoalloxazine ring is place adjacent to the E(D)-R-E UbiD signature sequent motif. The fungal ferulic acid decarboxylase Fdc from Aspergillus niger has emerged as a UbiD-model system, and has yielded atomic level insight into the prFMN^iminium mediated (de)carboxylation. A wealth of data now supports a mechanism reliant on reversible 1,3 dipolar cycloaddition between substrate and cofactor for this enzyme. This poses the intriguing question whether a similar mechanism is used by all UbiD enzymes, especially those that act as carboxylases on inherently more difficult substrates such as phenylphosphate or benzene/naphthalene. Indeed, considerable variability in terms of oligomerization, domain motion and active site structure is now reported for the UbiD family.

Biocatalytic in Vitro and in Vivo FMN Prenylation and (De)carboxylase Activation

Reversible UbiD-like (de)carboxylases represent a large family of mostly uncharacterized enzymes, which require the recently discovered prenylated FMN (prFMN) cofactor for activity. Functional characterization of novel UbiDs is hampered by a lack of robust protocols for prFMN generation and UbiD activation. Here, we report two systems for in vitro and in vivo FMN prenylation and UbiD activation under aerobic conditions. The in vitro one-pot prFMN cascade includes five enzymes: FMN prenyltransferase (UbiX), prenol kinase, polyphosphate kinase, formate dehydrogenase, and FMN reductase, which use prenol, polyphosphate, formate, ATP, NAD⁺, and FMN as substrates and cofactors. Under aerobic conditions, this cascade produced prFMN from FMN with over 98% conversion and activated purified ferulic acid decarboxylase Fdc1 from Aspergillus niger and protocatechuic acid decarboxylase ENC0058 from Enterobacter cloaceae. The in vivo system for FMN prenylation and UbiD activation is based on the coexpression of Fdc1 and UbiX in Escherichia coli cells under aerobic conditions in the presence of prenol. The in vitro and in vivo FMN prenylation cascades will facilitate functional characterization of novel UbiDs and their applications.

Heterologous Metabolic Pathways: Strategies for Optimal Expression in Eukaryotic Hosts

Heterologous pathways are linked series of biochemical reactions occurring in a host organism after the introduction of foreign genes. Incorporation of metabolic pathways into host organisms is a major strategy used to increase the production of valuable secondary metabolites. Unfortunately, simple introduction of the pathway genes into the heterologous host in most cases does not result in successful heterologous expression. Extensive modification of heterologous genes and the corresponding enzymes on many different levels is required to achieve high target metabolite production rates. This review summarizes the essential techniques used to create heterologous biochemical pathways, with a focus on the key challenges arising in the process and the major strategies for overcoming them.

Recent progress on bio-based production of dicarboxylic acids in yeast

Dicarboxylic acids are widely used in fine chemical and food industries as well as the monomer for polymerisation of high molecular material. Given the problems of environmental contamination and sustainable development faced by traditional production of dicarboxylic acids based on petrol, new approaches such as bio-based production of dicarboxylic acids drew more attentions. The yeast, Saccharomyces cerevisiae, was regarded as an ideal organism for bio-based production of dicarboxylic acids with high tolerance to acidic and hyperosmotic environments, robust growth using a broad range of substrates, great convenience for genetic manipulation, stable inheritance via sub-cultivation, and food compatibility. In this review, the production of major dicarboxylates via S. cerevisiae was concluded and the challenges and opportunities facing were discussed.Key Points• Summary of current production of major dicarboxylic acids by Saccharomyces cerevisiae.• Discussion of influence factors on four-carbon dicarboxylic acids production by Saccharomyces cerevisiae.• Outlook of potential production of five- and six-carbon dicarboxylic acids by Saccharomyces cerevisiae.

Improvement of cis,cis-Muconic Acid Production in Saccharomyces cerevisiae through Biosensor-Aided Genome Engineering

Muconic acid is a potential platform chemical for the production of nylon, polyurethanes, and terephthalic acid. It is also an attractive functional copolymer in plastics due to its two double bonds. At this time, no economically viable process for the production of muconic acid exists. To harness novel genetic targets for improved production of cis,cis-muconic acid (CCM) in the yeast Saccharomyces cerevisiae, we employed a CCM-biosensor coupled to GFP expression with a broad dynamic response to screen UV-mutagenesis libraries of CCM-producing yeast. Via fluorescence activated cell sorting we identified a clone Mut131 with a 49.7% higher CCM titer and 164% higher titer of biosynthetic intermediate-protocatechuic acid (PCA). Genome resequencing of the Mut131 and reverse engineering identified seven causal missense mutations of the native genes (PWP2, EST2, ATG1, DIT1, CDC15, CTS2, and MNE1) and a duplication of two CCM biosynthetic genes, encoding dehydroshikimate dehydratase and catechol 1,2-dioxygenase, which were not recognized as flux controlling before. The Mut131 strain was further rationally engineered by overexpression of the genes encoding for PCA decarboxylase and AROM protein without shikimate dehydrogenase domain (Aro1p^ΔE), and by restoring URA3 prototrophy. The resulting engineered strain produced 20.8 g/L CCM in controlled fed-batch fermentation, with a yield of 66.2 mg/g glucose and a productivity of 139 mg/L/h, representing the highest reported performance metrics in a yeast for de novo CCM production to date and the highest production of an aromatic compound in yeast. The study illustrates the benefit of biosensor-based selection and brings closer the prospect of biobased muconic acid.

The microenvironment surrounding FAD mediates its conversion to 8-formyl-FAD in Aspergillus oryzae RIB40 formate oxidase

Aspergillus oryzae RIB40 formate oxidase has Arg87 and Arg554 near the formyl group and O(4) atom of 8-formyl-flavin adenine dinucleotide (FAD), respectively, with Asp396 neighbouring Arg554. Herein, we probed the roles of these three residues in modification of FAD to 8-formyl-FAD. Replacement of Arg87 or Arg554 with Lys or Ala decreased and abolished the modification, respectively. Replacement of Asp396 with Ala or Asn lowered the modification rate. The observation of unusual effects of maintaining pH 7.0 on the modification in R87K, R554K and D396 variants indicates initial and subsequent processes with different pH dependencies. Comparison of the initial process at pH 4.5 and 7.0 suggests that the microenvironment around Arg87 and the protonation state of Asp396 affect the initial process in the native enzyme. Comparison of the crystal structures of native and R554 variants showed that the replacements had minimal effect on catalytic site structure. The positively charged Arg87 might contribute to the formation of an anionic quinone-methide tautomer intermediate, while the positively charged Arg554, in collaboration with the negatively charged Asp396, might stabilize this intermediate and form a hydrogen bonding network with the N(5)/O(4) region, thereby facilitating efficient FAD modification.

Bacterial bifunctional chorismate mutase-prephenate dehydratase PheA increases flux into the yeast phenylalanine pathway and improves mandelic acid production

Mandelic acid is an important aromatic fine chemical and is currently mainly produced via chemical synthesis. Recently, mandelic acid production was achieved by microbial fermentations using engineered Escherichia coli and Saccharomyces cerevisiae expressing heterologous hydroxymandelate synthases (hmaS). The best-performing strains carried a deletion of the gene encoding the first enzyme of the tyrosine biosynthetic pathway and therefore were auxotrophic for tyrosine. This was necessary to avoid formation of the competing intermediate hydroxyphenylpyruvate, the preferred substrate for HmaS, which would have resulted in the predominant production of hydroxymandelic acid. However, feeding tyrosine to the medium would increase fermentation costs. In order to engineer a tyrosine prototrophic mandelic acid-producing S. cerevisiae strain, we tested three strategies: (1) rational engineering of the HmaS active site for reduced binding of hydroxyphenylpyruvate, (2) compartmentalization of the mandelic acid biosynthesis pathway by relocating HmaS together with the two upstream enzymes chorismate mutase Aro7 and prephenate dehydratase Pha2 into mitochondria or peroxisomes, and (3) utilizing a feedback-resistant version of the bifunctional E. coli enzyme PheA (PheA^fbr) in an aro7 deletion strain. PheA has both chorismate mutase and prephenate dehydratase activity. Whereas the enzyme engineering approaches were only successful in respect to reducing the preference of HmaS for hydroxyphenylpyruvate but not in increasing mandelic acid titers, we could show that strategies (2) and (3) significantly reduced hydroxymandelic acid production in favor of increased mandelic acid production, without causing tyrosine auxotrophy. Using the bifunctional enzyme PheA^fbr turned out to be the most promising strategy, and mandelic acid production could be increased 12-fold, yielding titers up to 120 mg/L. Moreover, our results indicate that utilizing PheA^fbr also shows promise for other industrial applications with S. cerevisiae that depend on a strong flux into the phenylalanine biosynthetic pathway.

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Increased mandelic acid production in tyrosine prototrophic S. cerevisiae.

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Bifunctional E. coli enzyme PheA increases flux into yeast phenylalanine branch.

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PheA allows mandelic acid production in prototrophic S. cerevisiae.

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Compartmentalized mandelic acid production in yeast mitochondria/peroxisomes.

An Engineered Aro1 Protein Degradation Approach for Increased cis,cis-Muconic Acid Biosynthesis in Saccharomyces cerevisiae

Muconic acid (MA) is a chemical building block and precursor to adipic and terephthalic acids used in the production of nylon and polyethylene terephthalate polymer families. Global demand for these important materials, coupled to their dependence on petrochemical resources, provides substantial motivation for the microbial synthesis of MA and its derivatives. In this context, the Saccharomyces cerevisiae yeast shikimate pathway can be sourced as a precursor for the formation of MA. Here we report a novel strategy to balance MA pathway performance with aromatic amino acid prototrophy by destabilizing Aro1 through C-terminal degron tagging. Coupling of a composite MA production pathway to degron-tagged Aro1 in an aro3Δ aro4Δ mutant background led to the accumulation of 5.6 g/liter protocatechuic acid (PCA). However, metabolites downstream of PCA were not detected, despite the inclusion of genes mediating their biosynthesis. Because CEN.PK family strains of S. cerevisiae lack the activity of Pad1, a key enzyme supporting PCA decarboxylase activity, chromosomal expression of intact PAD1 alleviated this bottleneck, resulting in nearly stoichiometric conversion (95%) of PCA to downstream products. In a fed-batch bioreactor, the resulting strain produced 1.2 g/liter MA under prototrophic conditions and 5.1 g/liter MA when supplemented with amino acids, corresponding to a yield of 58 mg/g sugar.IMPORTANCE Previous efforts to engineer a heterologous MA pathway in Saccharomyces cerevisiae have been hindered by a bottleneck at the PCA decarboxylation step and the creation of aromatic amino acid auxotrophy through deleterious manipulation of the pentafunctional Aro1 protein. In light of these studies, this work was undertaken with the central objective of preserving amino acid prototrophy, which we achieved by employing an Aro1 degradation strategy. Moreover, resolution of the key PCA decarboxylase bottleneck, as detailed herein, advances our understanding of yeast MA biosynthesis and will guide future strain engineering efforts. These strategies resulted in the highest titer reported to date for muconic acid produced in yeast. Overall, our study showcases the effectiveness of careful tuning of yeast Aro1 activity and the importance of host-pathway dynamics.

Regioselective para-Carboxylation of Catechols with a Prenylated Flavin Dependent Decarboxylase

The utilization of CO2 as a carbon source for organic synthesis meets the urgent demand for more sustainability in the production of chemicals. Herein, we report on the enzyme-catalyzed para-carboxylation of catechols, employing 3,4-dihydroxybenzoic acid decarboxylases (AroY) that belong to the UbiD enzyme family. Crystal structures and accompanying solution data confirmed that AroY utilizes the recently discovered prenylated FMN (prFMN) cofactor, and requires oxidative maturation to form the catalytically competent prFMNiminium species. This study reports on the in vitro reconstitution and activation of a prFMN-dependent enzyme that is capable of directly carboxylating aromatic catechol substrates under ambient conditions. A reaction mechanism for the reversible decarboxylation involving an intermediate with a single covalent bond between a quinoid adduct and cofactor is proposed, which is distinct from the mechanism of prFMN-associated 1,3-dipolar cycloadditions in related enzymes.