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  • br Experimental br Results and discussion br Conclusions

    2019-09-12


    Experimental
    Results and discussion
    Conclusions In this work we presented the direct electrochemical behaviour of the Mo containing formate dehydrogenase, purified from D. desulfuricans, where the Mo centre redox features were observed. The results allowed to calculate the reduction potentials associated with the couples Mo(VI/IV) (by CV) and Mo(VI/V) and Mo(V/IV) (by DPV). The catalytic currents associated with the carbon dioxide reduction were attained using direct cyclic voltammetry (without mediators) and different methodologies to add CO2 into the solution. When both CO2 and formate are present, the Mo centre redox features are not observed, but the development of characteristic sigmoidal catalytic currents towards the CO2 reduction and formate oxidation are observed. We believe that these qualitative results constitute a first step as a proof of concept that DdFDH can be used as a basis for a BES device for the CO2 reduction, using gas-diffusion type electrodes, allowing the direct injection of atmospheric carbon dioxide and its reduction in situ.
    Abbreviations
    Acknowledgments This work was supported by the Associate Laboratory for Green Chemistry-LAQV, with national funds from FCT/MCTES (UID/QUI/50006/2019). CMC acknowledges FCT/MCTES for funding her “Research Position” (signed with FCT NOVA in accordance with DL.57/2016 and Lei 57/2017). LBM thanks to FCT/MCTES for the CEEC-Individual 2017 Program Contract.
    Introduction Fungi are rich producers of secondary (or specialized) metabolites [[1], [2], [3], [4]]. Humans have taken advantage of fungal secondary metabolites as pharmaceuticals. For example, the indole alkaloid ergotamine offers relief from migraine attacks, while non-ribosomal peptide cyclosporine is used as an immunosuppressant [5,6]. Aromatic WYE-132 clinical are the precursor of many secondary metabolites of plants and fungi, such as alkaloids, pigments, and vitamins [[7], [8], [9], [10], [11]]. In the fungal phylum Basidiomycota, tyrosine is the precursor of a unique class of pigments, betalains, only found in the genera of Amanita and Hygrocybe [12,13] and the plant order Caryophyllales [14]. In the phylum Ascomycota, tyrosine-derived pigments (i.e. melanin) and tyrosine betaine are associated with stress tolerance (e.g. temperature, radiation) and pathogenicity [[15], [16], [17], [18], [19], [20]]. Chorismate, the final product of the shikimate pathway, is the precursor of all three aromatic amino acids, L-tryptophan, L-phenylalanine, and L-tyrosine [7,21]. Chorismate is converted to prephenate, which is used to synthesize phenylalanine and tyrosine via two alternative pathways. In most plants and some bacteria (e.g. α and δ-proteobacteria, spirochaetes), prephenate is first transaminated to produce arogenate, which is oxidatively decarboxylated by NADP+-dependent arogenate dehydrogenase enzyme (TyrAa/ADH, EC 1.3.1.78 and EC 1.3.1.43) to produce tyrosine [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. In contrast, many bacteria first use NAD+-dependent prephenate dehydrogenase (TyrAp/PDH, EC 1.3.1.12 and EC 1.3.1.13) to convert prephenate into 4-hydroxyphenylpyruvate, which is subsequently transaminated to tyrosine (Fig. 1). However, there are some exceptions to this general pattern and legume plants in particular have both TyrAa/ADH and TyrAp/PDH enzymes [24], whereas some bacteria, such as Pseudomonas aeruginosa, have a single TyrA enzyme that can use both arogenate and prephenate substrates and NADP+ as the cofactor [[32], [33], [34]]. Since phenylalanine and tyrosine biosynthesis compete for the prephenate or arogenate precursor, the pathway is highly regulated by feedback inhibition [7,[35], [36], [37], [38], [39]]. However, prior studies have shown that TyrA enzymes are not always inhibited by tyrosine, and some groups of bacteria and plants possess TyrA enzymes that are insensitive or less sensitive to feedback inhibition by tyrosine, including legume TyrAp/PDH enzymes that are completely insensitive to feedback inhibition by tyrosine [[23], [24], [25], [26], [27],40].