Metabolic Engineering of Lactobacillus plantarum for Production of L-Ribulose

L-Ribulose is a rare and expensive sugar that can be used as a precursor for the production of other rare sugars of high market value such as L-ribose. In this work we describe a production process for L-ribulose using L-arabinose, a common component of polymers of lignocellulosic materials, as the starting material. A ribulokinase-deficient mutant of the heterofermentative lactic acid bacterium Lactobacillus plantarum NCIMB8826 was constructed. Expression of araA, which encodes the critical enzyme L-arabinose isomerase, was repressed by high glucose concentrations in batch cultivations. A fed-batch cultivation strategy was therefore used to maximize L-arabinose isomerase production during growth. Resting cells of the ribulokinase-deficient mutant were used for the production of L-ribulose. The isomerization of L-arabinose to L-ribulose was very unfavorable for L-ribulose formation. However, high L-ribulose yields were obtained by complexing the produced L-ribulose with borate. The process for L-ribulose production in borate buffer by resting cells was optimized using central composite designs. The experiment design suggested that the process has an optimal operation point around an L-arabinose concentration of 100 g liter⁻¹, a borate concentration of 500 mM, and a temperature of 48°C, where the statistical software predicted an initial L-ribulose production rate of 29.1 g liter⁻¹ h⁻¹, a best-achievable process productivity of 14.8 g liter⁻¹ h⁻¹, and a conversion of L-arabinose to L-ribulose of 0.70 mol mol⁻¹.