Characterization of recombinantly expressed dihydroxy-acid dehydratase from Sulfobus solfataricus-A key enzyme for the conversion of carbohydrates into chemicals

Dihydroxyacid dehydratases (DHADs) are excellent biocatalysts for the defunctionalization of biomass. Here, we report on the recombinant production of DHAD from Sulfolobus solfataricus (SsDHAD) in E. coli and its characterization with special focus on activity toward non-natural substrates, thermo-stability, thermo-inactivation kinetics and activation capabilities and its application within multi-step cascades for chemicals production.Using a simple heat treatment of cell lysate as major purification step we achieved a specific activity of 4.4 units per gram cell mass toward the substrate d-gluconate. The optimal temperature and pH value for this reaction are 77°C and pH 6.2. The inhibitory concentration (IC₅₀, 50% residual activity) of different alcohols was determined to be 15% (v/v) for ethanol, 4.5% (v/v) for butanol and 4% (v/v) for isobutanol. Besides d-gluconate and the natural substrate 2,3-dihydroxyisovalerate (DHIV) SsDHAD is able to convert the C3-sugar-acid d-glycerate to pyruvate, a reaction, which does not occur in natural metabolic pathways, with a specific activity of 10.7±0.4mU/mg.The specific activity of the enzyme can be increased 3-fold by incubation with 2-mercaptoethanol. The activation has no impact on temperature dependence, but modulates the thermo-inactivation tolerance at 50. °C.The total turnover numbers for all of the three reactions was found to be 35.5×10³±1.0×10³ for the conversion of d-gluconate to 2-keto-3-deoxygluconate (KDG), 28.2×10³±0.8×10³ for DHIV to 2-ketovalerate (KIV) and 943±0.28×10² for d-glycerate to pyruvate. With activated SsDHAD these values could be further increased 5- and 4-fold for the d-gluconate and d-glycerate conversion, respectively.