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Amine transaminases are versatile biocatalysts for the production of pharmaceutically and agrochemically relevant chiral amines. They represent an environmentally benign alternative to waste intensive transition metal catalysed synthesis strategies, especially because of their high stereoselectivity and robustness. Therefore, they have been frequently used in the (chemo)enzymatic synthesis of amines and/or became attractive targets for enzyme engineering especially in the last decade, mainly in order to enlarge their substrate scope. Certainly, one of the most notable examples of amine transaminase engineering is the
manufacturing of the anti-diabetic drug Sitagliptin in large scale after several rounds of protein engineering. Thereby, the target amine was produced in asymmetric synthesis mode which is the most convenient and favored route to a target chiral amine, starting from the corresponding ketone. The choice of the amine donor is highly relevant for reaction design in terms of economical and thermodynamic considerations. For instance, the use of alanine as the natural amine donor is one of the most common strategies for the amination of target ketones but needs the involvement of auxiliary enzymes to shift the reaction equilibrium towards product formation. In fact, isopropylamine is probably one of the most favored donor molecules since it is cheap and achiral but it is supposed to be accepted only by a limited number of amine transaminases.
This thesis focusses on the optimization and application of amine transaminases for asymmetric synthesis reactions en route to novel target chiral amines using isopropylamine as the preferred amine donor.
The synthesis of several bioactive compounds and active pharmaceutical ingredients relies on the development of general and efficient methods to prepare optically pure amines. Transaminases are industrially relevant enzymes and are useful for synthesizing a large number of compounds that contain a chiral amine functionality. Although the immense potential associated to the use of these biocatalysts, the equilibrium position is often unfavorable for amine synthesis. The use of an excess of amine donor, compared to the ketone substrate, combined with selective removal of the formed product, can help in overcoming this limitation. This work mainly focused on broadening the application of membrane-based in situ product recovery (ISPR) techniques for the transaminase-catalyzed synthesis of chiral amines. The
overall work was designed around the implementation of amine donors, possessing considerably larger molecular ‘size’ compared to commonly used amine donors. To clearly
distinguish these molecules from traditional donor amines, we designate them as High Molecular Weigh amine donors. With a molecular weight between 400 and 1500 g/mol, in contrast to traditional donor amines, HMW amine donors enable a size-based separation between amine donor and amine product molecules. HMW amines, provided in excess for thermodynamic equilibrium shifting can thus be simply retained by a size-exclusion mechanism by commercial membranes, while the smaller product amines are permeated. Therefore, a selective recovery of the desired chiral amine product is possible. The implementation of ISPR techniques using HMW amine donors can theoretically lead to (i) equilibrium shifting, (ii) alleviation of product inhibition, and (iii) a highly pure product stream.
The feasibility of using HMW amine donors in aqueous, organic solvent and solvent-free media for the transaminase-catalyzed synthesis of 1-methyl-3-phenylpropylamine (MPPA) was proven in this thesis. The latter two approaches were investigated with the aim to achieve higher product concentrations. Along with that, we demonstrated two membrane-assisted ISPR proof of concepts. Specifically, nanofiltration was coupled with the enzymatic reaction performed in aqueous media (Article I), while liquid-liquid (L-L) extraction in a contactor was applied for transamination in organic solvent media (Article II). As an alternative to membrane-based strategies we also designed a spinning reactor concept for the integrated chiral amine synthesis (in organic solvent) and recovery (Article III).