Asymmetric synthesis of a fluoxetine precursor with an artificial fusion protein of a ketoreductase and a formate dehydrogenase

•A fusion protein consisting of a formate dehydrogenase and a ketoreductase was created.•A proteolytic decay during purification of the fusion protein was observed.•After elucidation of the cleavage site a proteolytically stable fusion protein was obtained.•The fusion protein performed equal or even better than the free enzymes in asymmetric reductions.•The better performance is due to an improved Km,EBA of the fused ketoreductase.

Herein we describe the kinetic characterization of a fusion protein from the 3-ketoacyl-[acyl-carrier-protein]-reductase (KR) from Synechococcus PCC 7942 and a mutant formate dehydrogenase from Mycobacterium vaccae N10 (MycFDH). Upon purification, a specific proteolytic cleavage of the MycFDH was observed. The cleavage site was elucidated, which is ubiquitously spread among prokaryotic FDHs. After depletion of the cleavage site the correct, full length fusion protein was obtained. In asymmetric reductions of ethylbenzoyl acetate (EBA) this fusion protein performed equal or even better than the free enzymes, yielding up to 39% more of the fluoxetine precursor ethyl-(S)-3-hydroxy-3-phenylpropanoate ((S)-HPPE). The rate acceleration is due to an improved Km,EBA of the KR subunit.