Metabolic Engineering of <i>Lactococcus lactis</i> for the Enhanced Production of Lactate by Manipulating Pyruvate Dehydrogenase Complex E2 and Lactate Dehydrogenase

The pyruvate dehydrogenase complex (PDHc) in <i>Lactococcus lactis</i>, composed of <i>pdh</i> E1α, E1β, and E2, links glycolysis to the tricarboxylic acid cycle (TCA cycle). This study investigated the effect of the <i>pdh</i> E2 knockout on metabolism. The knockout strain showed 76% biomass, 48% lower PDHc activity, and 13.4% reduced ATP versus wild type but a 1.5-fold higher NAD⁺/NADH ratio. Lactate production increased by 5.6% in the knockout and 45.3% in overexpressing strains. Next to this, the lactate dehydrogenase activity was induced, achieving the highest enzyme amount with the addition of 5 ng/mL nisin after 48 h of induction (28.7% higher than that of wild-type <i>L. lactis</i>). In addition, the transcript levels of genes related to the pyruvate metabolic pathway also changed: <i>pdh</i> E2 decreased by 67.62%, while <i>pdh</i> E1β was upregulated 15-fold. Our study provides insights for enhancing lactate production and understanding <i>pdh</i> E2 function.