초록
Chiral amino alcohols are structural motifs present in sphingolipids, antibiotics, and antiviral glycosidase inhibitors. Their chemical synthesis presents several challenges in establishing at least two chiral centres. Here a de novo metabolic pathway using a transketolase enzyme coupled with a transaminase enzyme has been assembled. To synthesise this motif one of the strategies to obtain high conversions from the transaminase/transketolase cascade is the use of hydroxypyruvate (HPA) as a two-carbon donor for the transketolase reaction; although commercially available it is relatively expensive limiting application of the pathway on an industrial scale. Alternately, HPA can be synthesised but this introduces a further synthetic step. In this study two different biocatalytic strategies were developed for the synthesis of (2S,3R)-2-amino-1,3,4-butanetriol (ABT) without adding HPA into the reaction. Firstly, a sequential cascade of three enzymatic steps (two transaminases and one transketolase) for the synthesis of ABT from serine, pyruvate and glycolaldehyde as substrates. Secondly, a two-step recycling cascade where serine is used as donor to aminate erythrulose (catalysed by a transketolase) for the simultaneous synthesis of ABT and HPA. In order to test the novel pathways, three new transaminases are described, two ω-transaminases able to accept a broad range of amine acceptors with serine as amine donor; and an α-transaminase, which showed high affinity towards serine (K<SUB>M</SUB>: 18mM) using pyruvate as amine acceptor. After implementation of the above enzymes in the biocatalytic pathways proposed in this paper, the two-step recycling pathway was found to be the most promising for its integration with E. coli metabolism. It was more efficient (10-fold higher conversion), more sustainable and cost-effective (use of low cost natural substrates and only two enzymes), and the reaction could be performed in a one-pot system.