Evaluation of the Substrate Scope of Benzoic Acid (De)carboxylases According to Chemical and Biochemical Parameters

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.

Structural Basis of Salicylic Acid Decarboxylase Reveals a Unique Substrate Recognition Mode and Access Channel

Salicylic acid (SA) decarboxylase from Trichosporon moniliiforme (TmSdc), which reversibly catalyses the decarboxylation of SA to yield phenol, is of significant interest because of its potential for the production of benzoic acid derivatives under environmentally friendly reaction conditions. TmSdc showed a preference for C2 hydroxybenzoate derivatives, with k_cat/K_m of SA being 3.2 × 10³ M^-1 s^-1. Here, we presented the first crystal structures of TmSdc, including a complex with SA. The three conserved residues of Glu8, His169, and Asp298 are the catalytic residues within the TIM-barrel domain of TmSdc. Trp239 forms a unique hydrophobic recognition site by interacting with the phenyl ring of SA, while Arg235 is responsible for recognizing the hydroxyl group at the C2 of SA via hydrogen bond interactions. Using a semi-rational combinatorial active-site saturation test, we obtained the TmSdc mutant MT3 (Y64T/P191G/F195V/E302D), which exhibited a 26.4-fold increase in k_cat/K_m with SA, reaching 8.4 × 10⁴ M^-1 s^-1. Steered molecular dynamics simulations suggested that the structural changes in MT3 relieved the steric hindrance within the substrate access channel and enlarged the substrate-binding pocket, leading to the increased activity by improving substrate access. Our data elucidate the unique substrate recognition mode and the substrate entrance tunnel of SA decarboxylase.

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.

Loop engineering of aryl sulfotransferase B for improving catalytic performance in regioselective sulfation

Loop engineering of aryl sulfotransferase B improves catalytic performance in regioselective sulfation.

Exploring the substrate scope of ferulic acid decarboxylase (FDC1) from Saccharomyces cerevisiae

Ferulic acid decarboxylase from Saccharomyces cerevisiae (ScFDC1) was described to possess a novel, prenylated flavin mononucleotide cofactor (prFMN) providing the first enzymatic 1,3-dipolar cycloaddition mechanism. The high tolerance of the enzyme towards several non-natural substrates, combined with its high quality, atomic resolution structure nominates FDC1 an ideal candidate as flexible biocatalyst for decarboxylation reactions leading to synthetically valuable styrenes. Herein the substrate scope of ScFDC1 is explored on substituted cinnamic acids bearing different functional groups (–OCH₃, –CF₃ or –Br) at all positions of the phenyl ring (o−, m−, p−)_, as well as on several biaryl and heteroaryl cinnamic acid analogues or derivatives with extended alkyl chain. It was found that E. coli whole cells expressing recombinant ScFDC1 could transform a large variety of substrates with high conversion, including several bulky aryl and heteroaryl cinnamic acid analogues, that characterize ScFDC1 as versatile and highly efficient biocatalyst. Computational studies revealed energetically favoured inactive binding positions and limited active site accessibility for bulky and non-linear substrates, such as 2-phenylthiazol-4-yl-, phenothiazine-2-yl- and 5-(4-bromophenyl)furan-2-yl) acrylic acids. In accordance with the computational predictions, site-directed mutagenesis of residue I330 provided variants with catalytic activity towards phenothiazine-2-yl acrylic acid and provides a basis for altering the substrate specificity of ScFDC1 by structure based rational design.

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.

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.

Mechanism and Structure of γ-Resorcylate Decarboxylase

γ-Resorcylate decarboxylase (γ-RSD) has evolved to catalyze the reversible decarboxylation of 2,6-dihydroxybenzoate to resorcinol in a nonoxidative fashion. This enzyme is of significant interest because of its potential for the production of γ-resorcylate and other benzoic acid derivatives under environmentally sustainable conditions. Kinetic constants for the decarboxylation of 2,6-dihydroxybenzoate catalyzed by γ-RSD from Polaromonas sp. JS666 are reported, and the enzyme is shown to be active with 2,3-dihydroxybenzoate, 2,4,6-trihydroxybenzoate, and 2,6-dihydroxy-4-methylbenzoate. The three-dimensional structure of γ-RSD with the inhibitor 2-nitroresorcinol (2-NR) bound in the active site is reported. 2-NR is directly ligated to a Mn²⁺ bound in the active site, and the nitro substituent of the inhibitor is tilted significantly from the plane of the phenyl ring. The inhibitor exhibits a binding mode different from that of the substrate bound in the previously determined structure of γ-RSD from Rhizobium sp. MTP-10005. On the basis of the crystal structure of the enzyme from Polaromonas sp. JS666, complementary density functional calculations were performed to investigate the reaction mechanism. In the proposed reaction mechanism, γ-RSD binds 2,6-dihydroxybenzoate by direct coordination of the active site manganese ion to the carboxylate anion of the substrate and one of the adjacent phenolic oxygens. The enzyme subsequently catalyzes the transfer of a proton to C1 of γ-resorcylate prior to the actual decarboxylation step. The reaction mechanism proposed previously, based on the structure of γ-RSD from Rhizobium sp. MTP-10005, is shown to be associated with high energies and thus less likely to be correct.

Amine-Mediated Enzymatic Carboxylation of Phenols Using CO2 as Substrate Increases Equilibrium Conversions and Reaction Rates

A variety of strategies is applied to alleviate thermodynamic and kinetic limitations in biocatalytic carboxylation of metabolites in vivo. A key feature to consider in enzymatic carboxylations is the nature of the cosubstrate: CO₂ or its hydrated form, bicarbonate. The substrate binding and activation mechanism determine what the actual carboxylation agent is. Dihydroxybenzoic acid (de)carboxylases catalyze the reversible regio-selective ortho-(de)carboxylation of phenolics. These enzymes have attracted considerable attention in the last 10 years due to their potential in substituting harsh conditions typical of chemical carboxylations (100-200 °C, 5-100 bar) with, ideally, greener ones (20-40 °C, 1 bar). They are reported to use bicarbonate as substrate, needed in large excess to overcome thermodynamic and kinetic limitations. Therefore, CO₂ can be used as substrate by these enzymes only if it is converted into bicarbonate in situ. In this contribution, we report the simultaneous amine-mediated conversion of CO₂ into bicarbonate and the ortho-carboxylation of different phenolic molecules catalyzed by 2,3-dihydroxybenzoic acid (de)carboxylase from Aspergillus oryzae. Our results show that under the newly developed conditions a significant thermodynamic (up to twofold increase in conversion) and kinetic improvement (up to approx. fivefold increase in rate) of the biocatalytic carboxylation of catechol is achieved.