De novo biosynthesis of 2-hydroxyterephthalic acid, the monomer for high-performance hydroxyl modified PBO fiber, by enzymatic Kolbe-Schmitt reaction with CO2 fixation

BACKGROUND

High-performance poly(p-phenylenebenzobisoxazole) (PBO) fiber, with excellent mechanical properties (stiffness, strength, and toughness), high thermal stability combined and light weight, are widely employed in automotive and aerospace composites, body armor and sports goods. Hydroxyl modified PBO (HPBO) fiber shows better photostability and interfacial shear strength. 2-Hydroxyterephthalic acid (2-HTA), the monomer for the HPBO fiber, is usually synthesized by chemical method, which has poor space selectivity and high energy consumption. The enzymatic Kolbe-Schmitt reaction, which carboxylates phenolic substrates to generate hydroxybenzoic acids with bicarbonate/CO2, was applied in de novo biosynthesis of 2-HTA with CO2 fixation.

RESULTS

The biosynthesis of 2-HTA was achieved by the innovative application of hydroxybenzoic acid (de)carboxylases to carboxylation of 3-hydroxybenzoic acid (3-HBA) at the para-position of the benzene carboxyl group, known as enzymatic Kolbe-Schmitt reaction. 2,3-Dihydroxybenzoic acid decarboxylase from Aspergillus oryzae (2,3-DHBD_Ao) were expressed in recombinant E. coli and showed highest activity. The yield of 2-HTA was 108.97 ± 2.21 μg/L/mg protein in the whole-cell catalysis. In addition, two amino acid substitutions, F27G and T62A, proved to be of great help in improving 2,3-DHBD activity. The double site mutation F27G/T62A increased the production of 2-HTA in the whole-cell catalysis by 24.7-fold, reaching 2.69 ± 0.029 mg/L/mg protein. Moreover, de novo biosynthetic pathway of 2-HTA was constructed by co-expression of 2,3-DHBD_Ao and 3-hydroxybenzoate synthase Hyg5 in S. cerevisiae S288C with Ura3, Aro7 and Trp3 knockout. The engineered strain synthesized 45.40 ± 0.28 μg/L 2-HTA at 36 h in the CO2 environment.

CONCLUSIONS

De novo synthesis of 2-HTA has been achieved, using glucose as a raw material to generate shikimic acid, chorismic acid, and 3-HBA, and finally 2-HTA. We demonstrate the strong potential of hydroxybenzoate (de)carboxylase to produce terephthalic acid and its derivatives with CO2 fixation.

Enzymatic Conversion of CO2: From Natural to Artificial Utilization

Enzymatic carbon dioxide fixation is one of the most important metabolic reactions as it allows the capture of inorganic carbon from the atmosphere and its conversion into organic biomass. However, due to the often unfavorable thermodynamics and the difficulties associated with the utilization of CO2, a gaseous substrate that is found in comparatively low concentrations in the atmosphere, such reactions remain challenging for biotechnological applications. Nature has tackled these problems by evolution of dedicated CO2-fixing enzymes, i.e., carboxylases, and embedding them in complex metabolic pathways. Biotechnology employs such carboxylating and decarboxylating enzymes for the carboxylation of aromatic and aliphatic substrates either by embedding them into more complex reaction cascades or by shifting the reaction equilibrium via reaction engineering. This review aims to provide an overview of natural CO2-fixing enzymes and their mechanistic similarities. We also discuss biocatalytic applications of carboxylases and decarboxylases for the synthesis of valuable products and provide a separate summary of strategies to improve the efficiency of such processes. We briefly summarize natural CO2 fixation pathways, provide a roadmap for the design and implementation of artificial carbon fixation pathways, and highlight examples of biocatalytic cascades involving carboxylases. Additionally, we suggest that biochemical utilization of reduced CO2 derivates, such as formate or methanol, represents a suitable alternative to direct use of CO2 and provide several examples. Our discussion closes with a techno-economic perspective on enzymatic CO2 fixation and its potential to reduce CO2 emissions.

A Combined Computational–Experimental Study on the Substrate Binding and Reaction Mechanism of Salicylic Acid Decarboxylase

Salicylic acid decarboxylase (SDC) from the amidohydrolase superfamily (AHS) catalyzes the reversible decarboxylation of salicylic acid to form phenol. In this study, the substrate binding mode and reaction mechanism of SDC were investigated using computational and crystallographic methods. Quantum chemical calculations show that the enzyme follows the general mechanism of AHS decarboxylases. Namely, the reaction begins with proton transfer from a metal-coordinated aspartic acid residue (Asp298 in SDC) to the C1 of salicylic acid, which is followed by the C–C bond cleavage, to generate the phenol product and release CO2. Interestingly, the calculations show that SDC is a Mg-dependent enzyme rather than the previously proposed Zn-dependent, and the substrate is shown to be bidentately coordinated to the metal center in the catalysis, which is also different from the previous proposal. These predictions are corroborated by the crystal structure of SDC solved in complex with the substrate analogue 2-nitrophenol. The mechanistic insights into SDC in the present study provide important information for the rational design of the enzyme.

Mechanisms of metal-dependent non-redox decarboxylases from quantum chemical calculations

Quantum chemical calculations are today an extremely valuable tool for studying enzymatic reaction mechanisms. In this mini-review, we summarize our recent work on several metal-dependent decarboxylases, where we used the so-called cluster approach to decipher the details of the reaction mechanisms, including elucidation of the identity of the metal cofactors and the origins of substrate specificity.

Decarboxylases are of growing potential for biocatalytic applications, as they can be used in the synthesis of novel compounds of, e.g., pharmaceutical interest. They can also be employed in the reverse direction, providing a strategy to synthesize value‐added chemicals by CO₂ fixation. A number of non-redox metal-dependent decarboxylases from the amidohydrolase superfamily have been demonstrated to have promiscuous carboxylation activities and have attracted great attention in the recent years. The computational mechanistic studies provide insights that are important for the further modification and utilization of these enzymes in industrial processes. The discussed enzymes are: 5‐carboxyvanillate decarboxylase, γ‐resorcylate decarboxylase, 2,3‐dihydroxybenzoic acid decarboxylase, and iso-orotate decarboxylase.

Synthetic Enzyme-Catalyzed CO2 Fixation Reactions

In recent years, (de)carboxylases that catalyze reversible (de)carboxylation have been targeted for application as carboxylation catalysts. This has led to the development of proof-of-concept (bio)synthetic CO2 fixation routes for chemical production. However, further progress towards industrial application has been hampered by the thermodynamic constraint that accompanies fixing CO2 to organic molecules. In this Review, biocatalytic carboxylation methods are discussed with emphases on the diverse strategies devised to alleviate the inherent thermodynamic constraints and their application in synthetic CO2 -fixation cascades.

Metal Ion Promiscuity and Structure of 2,3-Dihydroxybenzoic Acid Decarboxylase of Aspergillus oryzae

Broad substrate tolerance and excellent regioselectivity, as well as independence from sensitive cofactors have established benzoic acid decarboxylases from microbial sources as efficient biocatalysts. Robustness under process conditions makes them particularly attractive for preparative-scale applications. The divalent metal-dependent enzymes are capable of catalyzing the reversible non-oxidative (de)carboxylation of a variety of electron-rich (hetero)aromatic substrates analogously to the chemical Kolbe-Schmitt reaction. Elemental mass spectrometry supported by crystal structure elucidation and quantum chemical calculations verified the presence of a catalytically relevant Mg2+ complexed in the active site of 2,3-dihydroxybenoic acid decarboxylase from Aspergillus oryzae (2,3-DHBD_Ao). This unique example with respect to the nature of the metal is in contrast to mechanistically related decarboxylases, which generally have Zn2+ or Mn2+ as the catalytically active metal.

Non-Oxidative Enzymatic (De)Carboxylation of (Hetero)Aromatics and Acrylic Acid Derivatives

The utilization of carbon dioxide as a C1-building block for the production of valuable chemicals has recently attracted much interest. Whereas chemical CO2 fixation is dominated by C-O and C-N bond forming reactions, the development of novel concepts for the carboxylation of C-nucleophiles, which leads to the formation of carboxylic acids, is highly desired. Beside transition metal catalysis, biocatalysis has emerged as an attractive method for the highly regioselective (de)carboxylation of electron-rich (hetero)aromatics, which has been recently further expanded to include conjugated α,β-unsaturated (acrylic) acid derivatives. Depending on the type of substrate, different classes of enzymes have been explored for (i) the ortho-carboxylation of phenols catalyzed by metal-dependent ortho-benzoic acid decarboxylases and (ii) the side-chain carboxylation of para-hydroxystyrenes mediated by metal-independent phenolic acid decarboxylases. Just recently, the portfolio of bio-carboxylation reactions was complemented by (iii) the para-carboxylation of phenols and the decarboxylation of electron-rich heterocyclic and acrylic acid derivatives mediated by prenylated FMN-dependent decarboxylases, which is the main focus of this review. Bio(de)carboxylation processes proceed under physiological reaction conditions employing bicarbonate or (pressurized) CO2 when running in the energetically uphill carboxylation direction. Aiming to facilitate the application of these enzymes in preparative-scale biotransformations, their catalytic mechanism and substrate scope are analyzed in this review.

Carboxylation of Hydroxyaromatic Compounds with HCO3− by Enzyme Catalysis: Recent Advances Open the Perspective for Valorization of Lignin-Derived Aromatics

This review focuses on recent advances in the field of enzymatic carboxylation reactions of hydroxyaromatic compounds using HCO3− (as a CO2 source) to produce hydroxybenzoic and other phenolic acids in mild conditions with high selectivity and moderate to excellent yield. Nature offers an extensive portfolio of enzymes catalysing reversible decarboxylation of hydroxyaromatic acids, whose equilibrium can be pushed towards the side of the carboxylated products. Extensive structural and mutagenesis studies have allowed recent advances in the understanding of the reaction mechanism of decarboxylase enzymes, ultimately enabling an improved yield and expansion of the scope of the reaction. The topic is of particular relevance today as the scope of the carboxylation reactions can be extended to include lignin-related compounds in view of developing lignin biorefinery technology.

Strategies towards the synthesis of anti-tuberculosis drugs

In this report, we reviewed the strategies towards the synthesis of anti-tuberculosis drugs. They include semisynthetic approaches, resolution based strategies, microbial transformations, solid phase synthesis, and asymmetric synthesis.

Biochemical characterization and substrate profiling of a reversible 2,3-dihydroxybenzoic acid decarboxylase for biocatalytic Kolbe-Schmitt reaction

Reversible benzoic acid decarboxylases are versatile biocatalysts by taking advantage of both decarboxylation and carboxylation reactions, especially for the biocatalytic Kolbe-Schmitt reaction. In the course of developing a benzoic acid decarboxylase tool-box, a putative benzoic acid decarboxylase gene from Fusarium oxysporum was heterologously over-expressed in Escherichia coli, the recombinant protein was purified and characterized. The purified enzyme exhibited relatively high catalytic efficiencies for the decarboxylation of 2, 3-dihydroxybenzoic acid and carboxylation of catechol (k_cat/K_m = 2.03 × 10² and 1.88 mM^-1 min^-1, respectively), and thus characterized as 2, 3-dihydroxybenzoic acid decarboxylase (2, 3-DHBD_Fo). The enzyme also catalyzed the decarboxylation of various substituted salicylic acids with different groups at varied positions except 5-position and the carboxylation of phenol and the substituted phenols. In a preparative reaction, catechol was carboxylated into 2, 3-dihydroxybenoic acid with 95% conversion by adding dodecyldimethylbenzylammonium chloride into the reaction system, and the product was isolated in 72% yield. These results demonstrate that 2, 3-DHBD_Fo is a valuable addition to the benzoic acid decarboxylase tool-box with potential practical applications.

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.

Regioselective Enzymatic Carboxylation of Bioactive (Poly)phenols

In order to extend the applicability of the regioselective enzymatic carboxylation of phenols, the substrate scope of o-benzoic acid (de)carboxylases has been investigated towards complex molecules with an emphasis on flavouring agents and polyphenols possessing antioxidant properties. o-Hydroxycarboxylic acid products were obtained with perfect regioselectivity, in moderate to excellent yields. The applicability of this method was proven by the regioselective bio-carboxylation of resveratrol on a preparative scale with 95% yield.

Biocatalysis for the application of CO2 as a chemical feedstock

Biocatalysts, capable of efficiently transforming CO2 into other more reduced forms of carbon, offer sustainable alternatives to current oxidative technologies that rely on diminishing natural fossil-fuel deposits. Enzymes that catalyse CO2 fixation steps in carbon assimilation pathways are promising catalysts for the sustainable transformation of this safe and renewable feedstock into central metabolites. These may be further converted into a wide range of fuels and commodity chemicals, through the multitude of known enzymatic reactions. The required reducing equivalents for the net carbon reductions may be drawn from solar energy, electricity or chemical oxidation, and delivered in vitro or through cellular mechanisms, while enzyme catalysis lowers the activation barriers of the CO2 transformations to make them more energy efficient. The development of technologies that treat CO2-transforming enzymes and other cellular components as modules that may be assembled into synthetic reaction circuits will facilitate the use of CO2 as a renewable chemical feedstock, greatly enabling a sustainable carbon bio-economy.

Pushing the equilibrium of regio-complementary carboxylation of phenols and hydroxystyrene derivatives

The enzymatic carboxylation of electron-rich aromatics, which represents a promising 'green' equivalent to the chemical Kolbe-Schmitt reaction, is thermodynamically disfavored and is therefore impeded by incomplete conversions. Optimization of the reaction conditions, such as pH, temperature, substrate concentration and the use of organic co-solvents and/or ionic liquids allowed to push the conversion in favor of carboxylation by a factor of up to 50%. Careful selection of the type of bicarbonate salt used as CO2 source was crucial to ensure optimal activities. Among two types of carboxylases tested with their natural substrates, benzoic acid decarboxylase from Rhizobium sp. proved to be significantly more stable than phenolic acid decarboxylase from Mycobacterium colombiense; it tolerated reaction temperatures of up to 50 °C and substrate concentrations of up to 100mM and allowed efficient biocatalyst recycling.

Evolutionary history and biotechnological future of carboxylases

Carbon dioxide (CO2) is a potent greenhouse gas whose presence in the atmosphere is a critical factor for global warming. At the same time atmospheric CO2 is also a cheap and readily available carbon source that can in principle be used to synthesize value-added products. However, as uncatalyzed chemical CO2-fixation reactions usually require quite harsh conditions to functionalize the CO2 molecule, not many processes have been developed that make use of CO2. In contrast to synthetical chemistry, Nature provides a multitude of different carboxylating enzymes whose carboxylating principle(s) might be exploited in biotechnology. This review focuses on the biochemical features of carboxylases, highlights possible evolutionary scenarios for the emergence of their reactivity, and discusses current, as well as potential future applications of carboxylases in organic synthesis, biotechnology and synthetic biology.

Regioselective enzymatic carboxylation of phenols and hydroxystyrene derivatives

The enzymatic carboxylation of phenol and styrene derivatives using (de)carboxylases in carbonate buffer proceeded in a highly regioselective fashion: Benzoic acid (de)carboxylases selectively formed o-hydroxybenzoic acid derivatives, phenolic acid (de)carboxylases selectively acted at the β-carbon atom of styrenes forming (E)-cinnamic acids.