A novel ene-reductase from Halomonas elongata for flow biocatalytic synthesis of 3-phenylpropionaldehyde and sustainable indigo-carmine dyeing.

In order to broaden the toolbox of enzymes available for biocatalytic reductions of carbon-carbon double bonds, we investigated four promising ene-reductases (ERs) stemming from extremophilic organisms or showing homology with thermophilic ERs. The novel ene-reductase from the halophilic organism Halomonas elongata showed consistently high activity across a range of tested substrates. Upon immobilisation of the ERs, the flow biocatalytic ene-reduction of cinnamaldhehyde into 3-phenylpropionaldehyde was successfully achieved with an intensification of the process of 62-fold with respect to batch (2173.9 mg L-1 h-1 and 34.7 mg L-1 h-1, respectively). Additionally, to expand the scope of ERs applications, we describe a proof-of concept of a novel enzymatic cascade to convert indole to indigo using an unspecific peroxygenase, and its subsequent reduction to the water-soluble leuco-indigo. In addition, the co-production of the valuable pharmaceuticals precursor 2-oxindole was demonstrated.