Characterization of a recombinant thermostable l-rhamnose isomerase from Thermotoga maritima ATCC 43589 and its application in the production of l-lyxose and l-mannose

A putative l-rhamnose isomerase (RhaA) from Thermotoga maritima was purified with a specific activity of 55 U/mg by His-Trap affinity chromatography. The native enzyme was estimated as a 46 kDa tetramer by gel filtration chromatography. The half-lives of the enzyme at 75, 80, 85, 90 and 95°C were 773, 347, 187, 118, and 65 h, respectively, indicating that it is the most thermostable of all RhaAs. Under the optimum conditions of pH 8.0, 85°C, and 1 mM Mn2+, RhaA with 100 U enzyme/ml converted 500 l-xylulose/l to 225 g/l l-lyxose after 3 h, and converted 500 l-fructose/l to 175 g/l l-mannose after 5 h.