Surpassing thermodynamic, kinetic, and stability barriers to isomerization catalysis for tagatose biosynthesis

Abstract

3There are many enzymes that are relevant for making rare and valuable chemicals that 4 while active, are severely limited by thermodynamic, kinetic, or stability issues (e.g. 5 isomerases, lyases, transglycosidase etc.). In this work, we study an enzymatic reaction 6 system − Lactobacillus sakei L-arabinose isomerase (LsLAI) for D-galactose to D-tagatose 7 isomerizationthat is limited by all three reaction parameters. The enzyme has a low 8 catalytic efficiency for non-natural substrate galactose, has low thermal stability at 9 temperatures > 40 °C, and equilibrium conversion < 50%. After exploring several strategies 10 to overcome these limitations, we finally show that encapsulating the enzyme in a gram-11 positive bacterium (Lactobacillus plantarum) that is chemically permeabilized can enable 12 reactions at high rates, high conversion, and at high temperatures. The modified whole cell 13 system stabilizes the enzyme, differentially partitions substrate and product across the 14 membrane to shift the equilibrium toward product formation enables rapid transport of 15 substrate and product for fast kinetics. In a batch process, this system enables 16 approximately 50 % conversion in 4 h starting with 300 mM galactose (an average 17 productivity of 37 mM/h), and 85 % conversion in 48 h, which are the highest reported for 18 food-safe mesophilic tagatose synthesis. We suggest that such an approach may be 19 invaluable for other enzymatic processes that are similarly kinetically-, thermodynamically-20 , and/or stability-limited. 65We use the food-safe engineered probiotic bacterium Lactobacillus plantarum as the 66 expression host due to its increasing relevance to biochemical and biomedical research 30,31 . 67This approach enabled ~ 50 % conversion of galactose to tagatose in 4 h (productivity of ~ 68 38 mmol tagatose L -1 h -1 ) ultimately reaching ~ 85% conversion after 48 h at high galactose 69 loading (300 mM) in batch culture. This is among the highest conversions and 70 productivities reported to date for tagatose production using a mesophilic enzyme. Such an 71 approach is expected to be applicable to other biocatalytic systems where similar trade-offs 72 between kinetics, thermodynamics, and/or stability pose hurdles to process development. 73 74 Results: 75Characterizing L. sakei LAI (LsLAI) limitations. 76 At high substrate loading (400mM galactose) and 37 °C, the reported optimal temperature 77 for this enzyme 32 , LsLAI purified from L. plantarum exhibited an initial forward reaction 78 (turnover) rate (kinitial) of 9.3 ± 0.3 s -1 , which is lower than the maximum reaction rate 79 possible by this enzyme of 17.0 ± 1.3 s -1 min -1 with its preferred substrate, arabinose, at 400 80 mM. Increasing the reaction temperature to 50 °C increased the initial reaction rate to 11.2 81 s -1 ( Fig. 1a) but was accompanied by rapid enzyme inactivation, which is consistent with 82 previous reports of thermal instability of this enzyme 32 . The enzyme exhibited first-order 83 degradation with a half-life (t½) ...