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 °C and 8.0. To produce d-tagatose from lactose, β-d-galactosidases from Lc. Lactis, Lactiplantibacillus plantarum, and Streptococcus thermophilus were further incorporated into E. coli by two strategies. These β-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 β-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.