Abstract
The use of biocatalysis for the synthesis of high value added chemical building blocks derived from biomass is becoming an increasingly important application for future sustainable technologies. The synthesis of a higher value chemical from l-arabinose, the predominant monosaccharide obtained from sugar beet pulp, is demonstrated here via a transketolase and transaminase coupled reaction. Thermostable transketolases derived from Deinococcus geothermalis and Deinococcus radiodurans catalysed the synthesis of l-gluco-heptulose from l-arabinose and β-hydroxypyruvate at elevated temperatures with high conversions. β-Hydroxypyruvate, a commercially expensive compound used in the transketolase reaction, was generated in situ from l-serine and α-ketoglutaric acid via a thermostable transaminase, also from Deinococcus geothermalis. The two steps were investigated and implemented in a one-pot system for the sustainable and efficient production of l-gluco-heptulose.
Highlights
The increasing production of bio-based chemicals and products is paving the way for a greener, sustainable future away from a fossilbased economy
To investigate the transaminase-transketolase coupled reaction, all reactions were performed at 50 °C in 50 mM buffer ranging from pH 7–8.5 containing 0.125 mM pyridoxal 5′-phosphate (PLP), 30 mM L-serine, 30 mM α-ketoglutaric acid, 10 mM L-arabinose, 4.5 mM MgCl2, 1.25 mM thiamine diphosphate (ThDP), 0.42 mg/mL TAm and 0.2 mg/mL TK
One equivalent of L-arabinose and three equivalents of L-serine were added to shift the equilibrium of the first step towards excess HPA generation for the subsequent TK reaction, which resulted in high conversion yields of 62% and 88% for L-gluco-heptulose in the TKs from D. geothermalis (TKDgeo) and TKDrad one-pot reaction respectively with purified enzyme
Summary
The increasing production of bio-based chemicals and products is paving the way for a greener, sustainable future away from a fossilbased economy. The upgrading of L-arabinose using a transketolase (TK) enzyme was previously performed to produce a higher value added compound, L-gluco-heptulose (Scheme 1) [4]. In many synthetic applications TKs are used with lithium β-hydroxypyruvate (Li-HPA) as the donor substrate, which renders the reaction irreversible due to the release of carbon dioxide (CO2) For this reason, TK catalysed carbon-carbon bond formation is considered to be a useful and desirable tool in ketose synthesis. Transaminases (TAm, EC 2.6.1-) are a highly diverse family of enzymes that have gained significant attention in applied organic synthesis due to their ability to produce a wide variety of single isomer chiral amines These pyridoxal 5′-phosphate (PLP) dependent enzymes catalyse the reversible transfer of an amino group from an amine donor to a keto acceptor, often from low cost substrates, to produce enantiopure amines [9]. In line with the extremophilic properties of D. radiodurans [14] and the thermophilic properties of D. geothermalis [15] and G. stearothermophilus [16], optimum temperature and pH of the TK and TAm enzymes were investigated
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