Understanding and application of a biological Kolbe-Schmitt reaction: aromatic C-H activation coupled to CO2 fixation.

Carboxylase enzymes are able to link CO2 to other molecules, thereby using it as a readily accessible and cheap one-carbon building block. Furthermore, carboxylase activity contributes to the reduction of global CO2 levels. Indeed, most life on the planet directly or indirectly depends on the photosynthesis driven action of a key carboxylase able to generate biomass from CO2. Thus, carboxylases could serve to meet the challenge of reducing atmospheric CO2 levels and creating a more sustainable economy. However, engineering and application of these enzymes has often met with slow progress due to the complexity and/or strict requirements of these proteins for activity. In contrast, decarboxylase enzymes (that normally achieve the opposite reaction, i.e. cleaving a CO2 from a molecule) in the reverse direction has met with some success. Achieving carboxylation traditionally requires either large amounts of CO2 (pushing the reaction in the right direction) or additional enzymes that are highly efficient in rapidly converting the carboxylate product (pulling the reaction in the right direction). A third option is to couple the decarboxylase reaction directly to a favourable reaction such that the decarboxylase effectively runs in reverse under ambient CO2 and yields the carboxylated product at the expense of the reagents from the coupled reaction. Nature has achieved this feat in the form of the phenol carboxylase enzyme system, which is involved in bacterial phenol degradation. This enzyme catalyses the biological equivalent of the industrial Kolbe-Schmitt reaction, which uses high pressure CO2 and temperatures exceeding 100 degrees Celsius to carboxylate phenol. In contrast, this intriguing enzyme system operates and ambient conditions, and uses ATP (the natural "energy currency") to drive phenol + CO2 yielding the product para-hydroxybenzoic acid. We seek to determine how the phenol carboxylase achieves the coupling of both reactions (i.e. carboxylation and ATP consumption) to determine the natural engineering principles and apply these to development of new pathways that incorporate ATP-dependent carboxylase enzymes such as the phenol carboxylase. We will make use of protein crystallography to determine the structure of the various components of this enzyme, and combine that with detailed computational and solution studies to present a complete picture of the molecular choreography that underpins activity. We will use the insights generated as a blueprint for the laboratory guided evolution of this system to expand the substrate repertoire to non-phenolic aromatic alcohols. Evolved carboxylase enzymes will be used to generate new and renewable pathways for the production of key chemical commodities such as FDCA (furan dicarboxylic acid) or terephthalate from biomass.

Detailed understanding and application of carboxylases will undoubtedly contribute to solutions aimed at a developing a circular (bio)economy and reduction of atmospheric CO2 levels. The phenol carboxylase has been reported since the late 90s to catalyse the biological equivalent of the Kolbe-Schmitt reaction at ambient conditions and consists of a phenol phosphate synthase and a phenol phosphate carboxylase (PPC) working in tandem. The latter couples dephosphorylation to carboxylation in a hitherto ill-understood process. Our group has recently reported on the discovery of prenylated flavin (prFMN) as a cofactor supporting reversible decarboxylase function in the ubiquitous UbiD enzyme family. The PPC contains two separate UbiD-like modules in addition to a putative phosphatase. We have recently reconstituted the active PPC system in vitro from heterologous produced components as well as determined crystal structures of the individual non-interacting modules. This has allowed us to propose a hypothesis for the natural engineering principles that underpin the coupling between dephosphorylation and carboxylation in this complex system. In addition, we propose simpler natural PPC variants exist that exhibit less efficient coupling between both reactions. We will use an interdisciplinary approach integrating structural biology, computational studies and in vitro enzyme kinetics combined with in vivo studies on the entire pathway expressed in a model organism to confirm our hypothesis. This will generate the detailed understanding necessary to guide engineering and laboratory evolution of the PPC system towards alternative aromatic substrates. We seek to evolve new enzyme systems that are able to carboxylate non-phenolic aromatic alcohols by combining PPC variants with appropriate alcohol kinases, aiming the create new sustainable pathways towards the production of commodity chemicals such as FDCA and terephthalate through aromatic C-H activation via CO2 fixation.