L-Arabinose isomerase (AI) catalyzes the isomerization of L-arabinose to L-ribulose, the first reaction in its conversion into D-xylulose-5-phosphate, an intermediate in the pentose phosphate pathway, which allows L-arabinose to be used as a carbon source. AI can also convert D-galactose to D-tagatose at elevated temperatures in the presence of divalent metal ions. D-tagatose, rarely found in nature, is of commercial interest as a low-calorie sugar substitute.