The studied aldehyde oxidoreductase enzyme (AOR) incorporates in its active centre a tungsten ion, the heaviest element with biological function. Tungsten-containing enzymes occur only in bacteria and archaea, are very oxygen-sensitive and catalyze reactions with very low redox potential.

The studied AOR was isolated from Aromatoleum aromaticum (AORAa) a mesophilic and facultative anaerobic bacterium. AORAa characteristics differ significantly from the best-studied examples of AOR from the hyperthermophilic archaea. It is less oxygen-sensitive and has a more complicated structure. The AORAa catalyses the oxidation of a wide variety of aldehydes to the respective acids with either viologen dyes or NAD+ as electron acceptors.[1]

 

 

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Fig 1. AOR catalyzes the O2-independent oxidation of aldehydes to corresponding carboxylic acids or the reverse reaction in a two-electron redox reaction.

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Recently we have discovered that the enzyme can also catalyze a reverse reaction i.e., reduction of carboxylic acids to aldehydes and using molecular hydrogen as a sole electron donor [5]. This allows us to apply the enzyme in the synthesis of aldehydes or pure recovery of NADH. 

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In cooperation with the SYNMIKRO centre from Marburg, we have also managed to solve the structure of the enzyme by means of the cryo-EM method (Fig.2). The enzyme exhibits an unusual wire-like structure, with ferredoxin-like subunit AorA oligomerizing into the electron-conducting helix decorated with tungsten-containing AorB subunit which can catalyse the oxidation of aldehydes or hydrogen. The reduction of NAD to NADH takes place in the AorC subunit at the flavin cofactor [7]. 
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Fig. 2. The reconstruction of the wire-like structure of AOR from Aromatoleum aromaticum based on cryo-EM studies. Yellow: AorB catalytic subunit containing the tungsten cofactor (green - WCo) as a catalytic centre and one Fe4S4 cluster, violet -the electron transfer subunit (AorA) containing four Fe4S4 clusters in each subunit, green - subunit containing a FAD cofactor.

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Currently, our research aims at the further characterization of the catalytic properties of this unusual enzyme.  We are developing it as a novel NADH recycling system as well as a valuable element of the enzymatic cascades. We also aim at the elucidation of the reaction mechanism by means of kinetic studies and molecular modelling. 

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Up to date, the project was supported by the NCN grant PRELUDIUM No 2017/27/N/ST4/02676: ‘Tungsten-containing aldehyde oxidoreductase from Aromatoleum aromaticum - a study of catalytic reaction mechanism’ and Interdisciplinary Environmental Doctoral Studies "Physical, Chemical and Biophysical Foundations of Modern Technologies and Materials Engineering" (FCB). The project is conducted in close cooperation with the Faculty of Materials Science and Ceramics AGH University of Science and Technology and Philipps-Universität Marburg.

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Publications on the topic:

1. F.  Arndt , G. Schmitt, A.  Winiarska, M. Saft, A. Seubert, J. Kahnt,  J. Heider,   Characterization of an Aldehyde Oxidoreductase From the Mesophilic Bacterium Aromatoleum aromaticum EbN1, a Member of a New Subfamily of Tungsten-Containing Enzymes. Front. Microbiol., 10 (2019) 71. doi: 10.3389/fmicb.2019.00071

2. A. Winiarska, J. Heider, M. Szaleniec. D. Hege, F. Arndt „Sposób otrzymywania aldehydów poprzez enzymatyczną redukcję kwasów karboksylowych”, Polish Patent Application P.437445 (29.03.2021)

3. A. Winiarska, J. Heider, M. Szaleniec. D. Hege, F. Arndt „Sposób enzymatycznej redukcji formy utlenionej dinukleotydu nikotynowo-adeninowego.” Polish Patent Application P.437449 (29.03.2021)

4. A. Winiarska, J. Heider, M. Szaleniec. D. Hege, F. Arndt, A. Wojtkiewicz „A method of enzymatic reduction of the oxidized nicotinamide adenine dinucleotide and carboxylic acids”, European Patent Application EP22164459.4

5. A. Winiarska, D. Hege, Y. Gemmecker, J. Kryściak-Czerwenka, A. Seubert, J. Heider, M. Szaleniec, Tungsten Enzyme Using Hydrogen as an Electron Donor to Reduce Carboxylic Acids and NAD+, ACS Catalysis, 2022, 12 (14), 8707-8717, doi: 10.1021/acscatal.2c02147

6. P. Kalimuthu, D. Hege, A. Winiarska, Y. Gemmecker, M. Szaleniec, J. Heider, P. V. Bernhardt, Electrocatalytic Aldehyde Oxidation by a Tungsten Dependent Aldehyde Oxidoreductase from Aromatoleum Aromaticum, Chem. Eur. J. doi: 10.1002/chem.202203072

7. A. Winiarska, F. Ramrez-Amador, D. Hege, Y. Gemmecker, S. Prinz, G. Hochberg, J. Heider, M. Szaleniec, J. Michael Schuller, “A bacterial tungsten-containing aldehyde oxidoreductase forms an enzymatic decorated protein nanowire”, Science Adv. 9 (2023) eadg6689. DOI: 10.1126/sciadv.adg6689