The limited deglucosylation process of β-glucosidase in Bacillus cereus H62L for biotransforming secoisolariciresinol diglucoside into mammalian lignans

The amount of major lignans present in the alkaline extract of the defatted flaxseed (in mg/g) is as follows: secoisolariciresinol diglucoside (SDG, 109.80), p-coumaric acid glucoside (CAG, 70.21), ferulic acid glucoside (FAG, 68.81), secoisolariciresinol (SECO, 0.49), coumaric acid (CA, 0.37), and ferulic acid (FA, 0.85). When biotransformed, SDG yields two valuable mammalian lignans (MLG): enterodiol (END) and enterolactone (ENL). Bacillus cereus strain H62-L1 was isolated and tested for the process of bioconversion. Kinetic analysis revealed the sequential reaction SDG (A) → CAG (B) → END (C) → ENL (D) to be reversible at the first step with the generalized reaction type A^k2k1B^k3C^k4D. The obtained rate coefficients were as follows: k₁ = 0.001-0.028 h^-1, k ₂ = 0.006-0.077 h^-1, k₃ = 0.013 h^-1, and k₄ = 0.002 h^-1, and the overall kinetic parameter k₁/(k₂ + k₃) varied from 0.05 to 1.32, mostly not favoring the forward deglucosylation reaction. SDG with higher purity favored the forward reaction. In conclusion, to facilitate the production rate, we propose (i) using substrate SDG with high purity at a higher concentration; (ii) using an allosteric inhibitor to block the reverse reaction from SECO to SDG; or (iii) genetically modifying Bacillus cereus H62-L1.