Bert Vallee—A 20th Century Adventure(r) in Zincology

Bert Lester Vallee (1919–2019) has been among the most important biochemists of the 20th century, a pioneer in metalloproteins and discoverer of numerous zinc proteins and enzymes, such as carboxypeptidase, alcohol dehydrogenases and metallothioneins. His scientific achievements are condensed in over 600 publications, and articles relying on and citing his research are suited to fill entire bookshelves. Although Bert Vallee, as a scientist, has left a significant legacy on science, his more personal side and encounters have mostly escaped public observation. We deem this oversight rather unfortunate, as his personality, and indeed personal circumstances, have been truly turbulent and must have influenced his scientific career, from his birth as Bertold Blumenthal in the small village of Hemer in post-World War I Germany via Switzerland to New York and then Boston. Together with public records, the less obvious attributes and actions recommend a more holistic biography. On the occasion of Bert Vallee’s 100th birthday in 2019, we have attempted to provide such an inclusive and rounded résumé. We also propose that a similar rounded approach will add additional layers to the biographies of contemporary scientists, considering social, economic, political, and historical environments and their mutual interactions, which tend to shape the scientist embedded in them.

Cell-free biosynthesis of ω-hydroxy acids boosted by a synergistic combination of alcohol dehydrogenases

The activity orchestration of an unprecedented cell-free enzyme system with self-sufficient cofactor recycling enables the step-wise transformation of aliphatic diols into -hydroxy acids at the expense of molecular oxygen as electron acceptor. The efficiency of the biosynthetic route was maximized when two compatible alcohol dehydrogenases were selected as specialist biocatalysts for each one of the oxidative steps required for the oxidative lactonization of diols. The cell-free system reached up to 100% conversion using 100 mM of linear C5 diols, and performed the dessymetrization of prochiral branched diols into the corresponding -hydroxy acids with an exquisite enantioselectivity (ee > 99%). Green metrics demostrate a superior sustanability of this system compared to traditional metal catalysts and even to whole cells for the synthesis of 5-hydroxy petanoic acid. Finally, the cell-free system was assembled into a consortium of heterogeneous biocatalysts that allowed the enzyme reutilization. This cascade illustrates the potential of systems biocatalysis to access new heterofunctional molecules such as -hydroxy acids.

Efficient 1-Hydroxy-2-Butanone Production from 1,2-Butanediol by Whole Cells of Engineered E. coli

1-Hydroxy-2-butanone (HB) is a key intermediate for anti-tuberculosis pharmaceutical ethambutol. Commercially available HB is primarily obtained by the oxidation of 1,2-butanediol (1,2-BD) using chemical catalysts. In present study, seven enzymes including diol dehydrogenases, secondary alcohol dehydrogenases and glycerol dehydrogenase were chosen to evaluate their abilities in the conversion of 1,2-BD to HB. The results showed that (2R, 3R)- and (2S, 3S)-butanediol dehydrogenase (BDH) from Serratia sp. T241 could efficiently transform (R)- and (S)-1,2-BD into HB respectively. Furthermore, two biocatalysts co-expressing (2R, 3R)-/(2S, 3S)-BDH, NADH oxidase and hemoglobin protein in Escherichia coli were developed to convert 1,2-BD mixture into HB, and the transformation conditions were optimized. Maximum HB yield of 341.35 and 188.80 mM could be achieved from 440 mM (R)-1,2-BD and 360 mM (S)-1,2-BD by E. coli (pET-rrbdh-nox-vgb) and E. coli (pET-ssbdh-nox-vgb) under the optimized conditions. In addition, two biocatalysts showed the ability in chiral resolution of 1,2-BD isomers, and 135.68 mM (S)-1,2-BD and 112.43 mM (R)-1,2-BD with the purity of 100 % could be obtained from 300 and 200 mM 1,2-BD mixture by E. coli (pET-rrbdh-nox-vgb) and E. coli (pET-ssbdh-nox-vgb), respectively. These results provided potential application for HB production from 1,2-BD mixture and chiral resolution of (R)-1,2-BD and (S)-1,2-BD.

Role of Iron-Containing Alcohol Dehydrogenases in Acinetobacter baumannii ATCC 19606 Stress Resistance and Virulence

Most bacteria possess alcohol dehydrogenase (ADH) genes (Adh genes) to mitigate alcohol toxicity, but these genes have functions beyond alcohol degradation. Previous research has shown that ADH can modulate quorum sensing in Acinetobacter baumannii, a rising opportunistic pathogen. However, the number and nature of Adh genes in A. baumannii have not yet been fully characterized. We identified seven alcohol dehydrogenases (NAD+-ADHs) from A. baumannii ATCC 19606, and examined the roles of three iron-containing ADHs, ADH3, ADH4, and ADH6. Marker-less mutation was used to generate Adh3, Adh4, and Adh6 single, double, and triple mutants. Disrupted Adh4 mutants failed to grow in ethanol-, 1-butanol-, or 1-propanol-containing mediums, and recombinant ADH4 exhibited strongest activity against ethanol. Stress resistance assays with inorganic and organic hydroperoxides showed that Adh3 and Adh6 were key to oxidative stress resistance. Virulence assays performed on the Galleria mellonella model organism revealed that Adh4 mutants had comparable virulence to wild-type, while Adh3 and Adh6 mutants had reduced virulence. The results suggest that ADH4 is primarily involved in alcohol metabolism, while ADH3 and ADH6 are key to stress resistance and virulence. Further investigation into the roles of other ADHs in A. baumannii is warranted.

Identification of factors regulating the gene expression of alcohol dehydrogenases in hepatocellular carcinoma

Background Excessive alcohol consumption has been documented to increase the risk of liver hepatocellular carcinoma (HCC) development. Accordingly, a broad interest pointed to alcohol dehydrogenases (ADHs), which display essential roles in alcohol metabolism. Despite the relevance of ADHs expression and the prognosis of HCC has been estimated, so far, limited research concerning the factors that are responsible for the regulation of ADHs expression has been reported. Methods In this study, using The Cancer Genome Atlas (TCGA) and RegNetwork database, we predicted potential factors consisting of DNA methylation, gene copy number variations, transcription factors (TFs) and microRNAs (miRNAs) that might impact ADHs gene expression in HCC. Results We found that DNA methylation induced the down-regulated expression of ADH1B. Of note, our results implicated that gene copy number variation might not have effects on ADHs expression. Regarding TFs, we speculated that NFYA modulated ADH1C, E2F1 and TFAP2A regulated ADH6 expression based on their expression and prognostic value. Moreover, miR-185 and miR-561 might elicit the repression of ADH4, and miR-105 might impair ADH6 expression. Conclusion This study revealed that multiple factors, including DNA methylation, TFs and microRNAs, affect the expression of ADH family members, which provided new insights into discovering promising HCC-suppressive targets.

Transfer of a Rational Crystal Contact Engineering Strategy between Diverse Alcohol Dehydrogenases

Protein crystallization can serve as a purification step in biotechnological processes but is often limited by the non-crystallizability of proteins. Enabling or improving crystallization is mostly achieved by high-throughput screening of crystallization conditions and, more recently, by rational crystal contact engineering. Two selected rational crystal contact mutations, Q126K and T102E, were transferred from the alcohol dehydrogenases of Lactobacillus brevis (LbADH) to Lactobacillus kefir (LkADH). Proteins were expressed in E. coli and batch protein crystallization was performed in stirred crystallizers. Highly similar crystal packing of LkADH wild type compared to LbADH, which is necessary for the transfer of crystal contact engineering strategies, was achieved by aligning purification tag and crystallization conditions, as shown by X-ray diffraction. After comparing the crystal sizes after crystallization of LkADH mutants with the wild type, the mean protein crystal size of LkADH mutants was reduced by 40–70% in length with a concomitant increase in the total amount of crystals (higher number of nucleation events). Applying this measure to the LkADH variants studied results in an order of crystallizability T102E > Q126K > LkADH wild type, which corresponds to the results with LbADH mutants and shows, for the first time, the successful transfer of crystal contact engineering strategies.

An ADH toolbox for raspberry ketone production from natural resources via a biocatalytic cascade

Raspberry ketone is a widely used flavor compound in food and cosmetic industry. Several processes for its biocatalytic production have already been described, but either with the use of genetically modified organisms (GMOs) or incomplete conversion of the variety of precursors that are available in nature. Such natural precursors are rhododendrol glycosides with different proportions of (R)- and (S)-rhododendrol depending on the origin. After hydrolysis of these rhododendrol glycosides, the formed rhododendrol enantiomers have to be oxidized to obtain the final product raspberry ketone. To be able to achieve a high conversion with different starting material, we assembled an alcohol dehydrogenase toolbox that can be accessed depending on the optical purity of the intermediate rhododendrol. This is demonstrated by converting racemic rhododendrol using a combination of (R)- and (S)-selective alcohol dehydrogenases together with a universal cofactor recycling system. Furthermore, we conducted a biocatalytic cascade reaction starting from naturally derived rhododendrol glycosides by the use of a glucosidase and an alcohol dehydrogenase to produce raspberry ketone in high yield. Key points • LB-ADH, LK-ADH and LS-ADH oxidize (R)-rhododendrol • RR-ADH and ADH1E oxidize (S)-rhododendrol • Raspberry ketone production via glucosidase and alcohol dehydrogenases from a toolbox Graphical abstract