l -arabinose isomerases: Characteristics, modification, and application

d-Tagatose, a ketose isomer of d-galactose, is a kind of rare sugar existing in nature. The sweetness of d-tagatose is comparable to that of sucrose, but d-tagatose has a relatively lower energy content, indicating that it is an ideal substitute for sucrose in daily life. In addition, d-tagatose has some significant physiological functions and medicinal benefits, and it has been approved as a “generally recognized as safe “(GRAS)” material in food products, including dietary supplements. The enzymatic production of d-tagatose from d-galactose may be carried out using l-arabinose isomerase (L-AI, EC 5.3.1.4). In this article, the enzymatic performances of characterized L-AIs, as well as their applications in bioproduction of d-tagatose, were compared in detail, and molecular modifications of those L-AIs were also reviewed. At present, more than 30 microbial L-AIs have been reported and characterized. In addition, the crystal structures of seven L-AIs have been solved, and an ene-diol intermediate catalytic mechanism has also been proposed. Attempts to use the molecular modification strategies to decrease the optimum pH of L-AI or to enhance the d-tagatose-producing ability have been made. However, information about the crystal structure of L-AIs in complex with the substrates l-arabinose or d-galactose remains insufficient, and molecular modifications for improving enzyme activity and strengthening thermostability should be the next research topic.