2,3-Butanediol dehydrogenase is more efficient than acetoin reductase at metabolizing reserve carbon to improve carbon cycling pathways in Lactococcus lactis N8.

Acetoin (AC) and 2,3-butanediol (2,3-BDO) are metabolites produced by lactic acid bacteria using glucose as a carbon source. These two metabolites act as carbon reserves and can be reutilised by the cells. In this study, we investigated the enzymatic characteristics of acetoin reductase (ButA) and 2,3-butanediol dehydrogenase (ButB). The performance of butA or/and butB knockout mutants of Lactococcus lactis N8 was evaluated. ButA and ButB were heterologously expressed in E. coli, and their enzymatic characteristics were measured in vitro under different pH, temperature, and metal ion conditions. Kinetic parameters of the two enzymes indicated that ButA exhibited better catalytic efficiency with AC, whereas ButB performed better with 2,3-BDO. The dehydrogenase activity of ΔbutA, ΔbutB, and ΔbutBA strains were detected in vitro with AC or 2,3-BDO added medium. The ΔbutA mutant was found to metabolize both AC and 2,3-BDO more efficiently than the ΔbutB mutant. This study provides a comprehensive insight about the metabolic carbon reserve pool and cyclic pathways involving AC and 2,3-BDO in L. lactis N8.