Membrane-bound D-mannose isomerase of acetic acid bacteria: finding, characterization, and application

D-Mannose isomerase (EC 5.3.1.7) catalyzing reversible conversion between D-mannose and D-fructose was found in acetic acid bacteria. Cell fractionation confirmed the enzyme to be a typical membrane-bound enzyme, while all sugar isomerases so far reported are cytoplasmic. The optimal enzyme activity was found at pH 5.5, which was clear contrast to the cytoplasmic enzymes having alkaline optimal pH. The enzyme was heat stable and the optimal reaction temperature was observed at around 40 to 60˚C. Purified enzyme after solubilization from membrane fraction showed the total molecular mass of 196 kDa composing of identical four subunits of 48 kDa. Washed cells or immobilized cells were well functional at nearly 80% of conversion ratio from D-mannose to D-fructose and reversely 20-25% of D-fructose to D-mannose. Catalytic properties of the enzyme were discussed with respect to the biotechnological applications to high fructose syrup production from konjac taro.