Abstract
Direct, NAD(P)H-independent regeneration of Old Yellow Enzymes represents an interesting approach for simplified reaction schemes for the stereoselective reduction of conjugated C=C-double bonds. Simply by illuminating the reaction mixtures with blue light in the presence of sacrificial electron donors enables to circumvent the costly and unstable nicotinamide cofactors and a corresponding regeneration system.In the present study, we characterise the parameters determining the efficiency of this approach and outline the current limitations. Particularly, the photolability of the flavin photocatalyst and the (flavin-containing) biocatalyst represent the major limitation en route to preparative application.
Highlights
Ene-reductases from the Old Yellow Enzymes (OYEs) family have been known for close to a century [1]
OYEs contain a flavin prosthetic group – mostly a flavin mononucleotide (FMN) - which in the first reduction half-reaction of the catalytic mechanism is reduced by the nicotinamide adenine dinucleotide cofactor (NAD(P)H); in the second oxidation half-reaction of the mechanism, the reduced flavin transfers a hydride ion in a Michaeltype addition to the β‐carbon atom of the enzyme-bound unsaturated substrate followed by protonation of the resulting enolate anion in a trans fashion
We have confirmed that direct, NAD(P)H-independent regeneration of YqjM can productively be achieved using photoregenerated, reduced FMN photocatalysts/mediators
Summary
Ene-reductases from the Old Yellow Enzymes (OYEs) family have been known for close to a century [1]. We set out to investigate the scope and limitations of direct, photochemical regeneration of FMN to promote OYE-catalysed reduction reactions. As a model reaction for our investigations, we chose the stereospecific reduction of 2-methylcyclohexenone to (R)-2-methylcyclohexanone catalysed by the OYE homologue from Bacillus subtilis (YqjM) (Scheme 2) [32,33,34]. In situ regeneration of the reduced flavin prosthetic group is achieved using photoexcited FMN in the presence of EDTA as sacrificial electron donor.
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