A novel Lactococcus lactis L-arabinose isomerase for D-tagatose production from lactose

A new strain of Lactococcus lactis producing D-tagatose was isolated and identified. Its L-arabinose isomerase coding gene was cloned and expressed in Escherichia coli BL21 (DE3). The optimal temperature and pH of the purified enzyme were 50 ? and 8.0. To produce D-tagatose from lactose, beta-D-galactosidases from Lc. Lactis, Lactiplantibacillus plantarum, and Streptococcus thermophilus were further incorporated into E. coli by two strategies. These beta-D-galactosidases were fused to L-arabinose isomerase coupled with a peptide linker (GGGGS)(3). Meanwhile, they were co-expressed with L-arabinose isomerase using pETDuet-1 vector. Among these recombinant strains, the cell co-expressing L-arabinose isomerase and S. thermophilus beta-D-galactosidase showed maximal activity. SDS-PAGE results confirmed that exogenous enzymes had the maximum soluble expression level in this strain. At the optimal condition, the conversion rate of D-tagatose from 300 g/L lactose achieved to 42.4%, and the volumetric productivity reached 4.28 g/L/h at 15 h. This research established a highly efficient biotransformation system to produce D-tagatose from lactose.