Harnessing -arabinose isomerase for biological production of -tagatose: Recent advances and its applications

d-tagatose is regarded as one of the most valuable rare sugars in the current market and its biotechnological ways of production is a very hot topic as of today. As a ketohexose, d-tagatose having low-caloric value and 90% of sweetness with sucrose is a rare monosaccharide that exists extremely in low quantities in nature. Owing to distinct properties and beneficial functions on human health it has received a great interest and attained Generally Recognised as Safe (GRAS) status by the U.S. Food and Drug Administration (USFDA) in 2010. Due to scarcity in nature, production of d-tagatose either by chemical or biological methods is of utmost importance. In order to overcome the challenges that occur in chemical production, recently, biological process by using l-arabinose isomerase (L-AIs) for converting d-galactose to d-tagatose has allured researchers to find out more new ways to synthesize it in a cheap and eco-friendly manner. Herein, a brief summary on the scope of enzyme engineering methods like rational, semi-rational and directed evolution approaches to generate tailored L-AIs for improving d-tagatose yield and its function under harsh conditions are summarized. In addition, comparison of biochemical parameters and structural key aspects are also highlighted in this paper. Till date, L-AIs from various microbial sources are identified and characterized. Elucidating crystal structures and catalytic mechanism has elevated the researcher's interest to modify the L-AIs at molecular level and demonstrate its potential that is suited for industrial settings. However, works to produce d-tagatose from cheap renewable feedstocks and methods like immobilization and cell-surface display of L-AIs for efficient uptake and biotransformation of d-galactose are scarcely reported. In future, by employing the aforementioned strategies, the research studies must be focused more on shaping L-AIs as powerful biocatalysts to produce d-tagatose at industrial scale that necessitates for the human welfare.