The acid tolerant l-arabinose isomerase from the food grade Lactobacillus sakei 23K is an attractive d-tagatose producer

The araA gene encoding an l-arabinose isomerase (l-AI) from the psychrotrophic and food grade Lactobacillus sakei 23K was cloned, sequenced and over-expressed in Escherichia coli. The recombinant enzyme has an apparent molecular weight of nearly 220kDa, suggesting it is a tetramer of four 54kDa monomers. The enzyme is distinguishable from previously reported l-AIs by its high activity and stability at temperatures from 4 to 40°C, and pH from 3 to 8, and by its low metal requirement of only 0.8mM Mn²⁺ and 0.8mM Mg²⁺ for its maximal activity and thermostability. Enzyme kinetic studies showed that this enzyme displays a high catalytic efficiency allowing d-galactose bioconversion rates of 20% and 36% at 10 and 45°C, respectively, which are useful for commercial production of d-tagatose.