Properties of L-arabinose isomerase from Escherichia coli as biocatalyst for tagatose production

L-arabinose isomerase (EC 5.3.1.4) mediates the isomerization of D-galactose into D-tagatose as well as the conversion of L-arabinose into L-ribulose. To investigate the properties of L-arabinose isomerase as a biocatalyst for the conversion of galactose to tagatose, the L-arabinose isomerase of Escherichia coli was characterized. The substrate specificity for L-arabinose was 166-fold higher than that for D-galactose. The optimal pH and temperature for the galactose isomerization reaction were 8.0 and 30 degreesC, respectively. The enzyme activity was stable for 1 h at temperatures below 35 degreesC and within a pH range of 8-10. The Michaelis constant, Km, for galactose was 1480 mM, which is 25-fold higher than that for arabinose. The addition of Fe2+ and Mn2+ ions enhanced the conversion of galactose to tagatose, whereas the addition of Cu2+, Zn2+, Hg2+, and Fe3+ ions inhibited the reaction completely. In the presence of 1 mM Fe2+ ions, the K-m for galactose was found to be 300 mM.