User:SmartBiocat/sandbox/Smart cosubstrate

Scheme 1. Comparison of the use of isopropanol (left) to the ‘smart cosubstrate’ approach using 1,4-butanediol (1,4-BD). The lactone coproduct renders the regeneration reaction irreversible (Kara et al. 2013)

In redox biocatalysis, the nicotinamide cofactor (NAD(P)H or NAD(P)+) can act as an electron donor or acceptor by releasing or accepting a hydride, respectively. The cofactor must be used in the reaction either at stoichiometric amounts, which would lead to inhibition and economic issues, or at catalytic amounts coupled with an in situ regeneration system. A common approach for the latter is the excess use of sacrificial organic molecules such as isopropanol or ethanol. However, this approach would then lead to stoichiometric amount of waste.

The ‘smart cosubstrate’ concept aims at using alternative cosubstrates such as lactonizable diols for cofactor regeneration, which yields in enhanced productivity and minimized environmental impact (=low E-factors). The use of 1,4-butanediol as a ‘smart cosubstrate’ for the cofactor regeneration was the next step towards more sustainable redox biocatalysis (Scheme 1) (Kara et al. 2013). The formation of thermodynamically stable co-product gamma-butyrolactone drives the reaction to completion and at the same time yields in higher reaction rates.

The use of 1,4-butanediol as a smart cosubstrate has also been validated in non-aqueous media using a commercial ADH (Kara et al. 2014; Zuhse et al. 2015).

Double-smart cosubstrate

Scheme 2. Synthesis of ɛ-caprolactone (ECL) through a convergent cascade system by coupling a Baeyer–Villiger monooxygenase (BVMO)-catalyzed oxidation of cyclohexanone (CHO) to ECL, promoted by an alcohol dehydrogenase (ADH)-catalysed oxidation of the ‘double-smart cosubstrate’ 1,6-hexanediol (1,6-HD) for regeneration of NAD(P)H, also yielding ECL ( Bornadel et al, 2015)

The biocatalytic cascade reactions reported so far fall in into four different categories: (i) linear, (ii) orthogonal, (iii) parallel, and (iv) cyclic (García-Junceda et al. 2015; Schrittwieser et al. 2011). However, only two designs among the redox-neutral cascades have been reported for the in situ regeneration of the cofactors: parallel cascades (i.e., bi substrate—no intermediate—bi- or tri-product) and linear cascades (i.e., single substrate—one intermediate—single product) (Kara et al. 2014; Kara et al. 2013; Hummel and Gröger 2014).

Bringing this novel ‘smart cosubstrate’ concept one step further, a new class of redox-neutral reactions, called a ‘convergent cascade’ was designed, involving bi-substrate and a single product without the formation of an intermediate (Scheme 2). The convergent cascade was developed for the production of epsilon-caprolactone (ECL), consisting of a Baeyer- Villiger monooxygenase (BVMO) for oxidation of cyclohexanone (CHO), as well as an alcohol dehydrogenase (ADH) for oxidation of the ‘double-smart cosubstrate’ 1,6-hexanediol (1,6-HD) and for simultaneous regeneration of the nicotinamide cofactor (Scheme 2) (Bornadel et al. 2015). Recently, a two-step optimization of the aforementioned convergent cascade by Design-of-Experiments (DoE) and a biphasic system was reported (Bornadel et al. 2016).

Smart cosubstrates concept provides an elegant solution for thermodynamically limited redox reactions with multi-fold advantages: (1) no need for excess amounts of conventional cosubstrates (e.g., isopropanol, ethanol), which would negatively affect the enzymes’ activities, (2) reduced amount of waste generated and (3) faster reactions, which might be attributed to the absence of acetone or acetaldehyde coproduct leading to reduced enzyme activities.