A Novel 3,6-anhydro-L-galactose Dehydrogenase Produced by a Newly Isolated Raoultella ornithinolytica B6-JMP12

Despite an increasing potential of red algal biomass as a feedstock, biological conversion of red algal biomass has been limited by lack of feasible microorganisms which can convert structured AHG, which is a main component of red algal carbohydrate, into a common metabolite. In the AHG uptake pathway, AHG dehydrogenase (AHGD) is known to be a key step, therefore it is important to find an efficient dehydrogenase to break down 3,6- anhydro-L-galactose (AHG) for practical use of red macroalgae biomass in biorefineries requires. In this study, we isolate a novel AHG dehydrogenase (AHGD) with high activity produced by a newly isolated bacteria strain, Raoultella ornithinolytica B6–JMP12. The stability and compatibility of the enzyme were evaluated under various conditions to achieve high enzyme production. The AHGD was partially purified using conventional protein purification techniques such as ammonium sulfate precipitation and ion exchange followed by gel filtration chromatography, 37.24 fold with a final specific activity of 5.47 U/mg of protein with 32% yield recovery. SDS-PAGE was used to determine the molecular weight of the partially purified AHGD and its molecular weight was found to be around ~34 kDa. The optimal pH and temperature for the partially purified AHGD were 7.0 and 35°C, respectively. The K_m and V_max for 3,6-anhydro-L-galactose are 0.63 mg/mL and 0.38 μM/mL/min, respectively.