Characterization of a novel d-arabinose isomerase from Thermanaeromonas toyohensis and its application for the production of d-ribulose and l-fuculose.

d-Ribulose and l-fuculose are potentially valuable rare sugars useful for anticancer and antiviral drugs in the agriculture and medicine industries. These rare sugars are usually produced by chemical methods, which are generally expensive, complicated and do not meet the increasing demands. Furthermore, the isomerization of d-arabinose and l-fucose byDd-arabinose and l-fucose by d-arabinose isomerase from bacterial sources for the production of d-ribulose and l-fuculose have not yet become industrial due to the shortage of biocatalysts, resulting in poor yield and high cost of production. In this study, a thermostable d-ribulose- and l-fuculose producing d-arabinose isomerase from the bacterium Thermanaeromonas toyohensis was characterized. The recombinant d-arabinose isomerase from T. toyohensis (Thto-DaIase) was purified with a single band at 66 kDa using His-trap affinity chromatography. The native enzyme existed as a homotetramer with a molecular weight of 310 kDa, and the specific activities for both d-arabinose and l-fucose were observed to be 98.08 and 85.52 U mg ^-1 , respectively. The thermostable recombinant Thto-DaIase was activated when 1 mM Mn ²⁺ was added to the reactions at an optimum pH of 9.0 at 75 °C and showed approximately 50% activity for both d-arabinose and l-fucose at 75 °C after 10 h. The Michaelis-Menten constant (K _m ), the turnover number (k _cat ) and catalytic efficiency (k _cat /K _m ) for d-arabinose/l-fucose were 111/81.24 mM, 18,466/10,688 min ^-1 , and 166/132 mM ^-1  min ^-1 , respectively. When the reaction reached to equilibrium, the conversion rates of d-ribulose from d-arabinose and l-fuculose from l-fucose were almost 27% (21 g L ^-1 ) and 24.88% (19.92 g L ^-1 ) from 80 g L ^-1 of d-arabinose and l-fucose, respectively.
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