Mechanism of Citrate Metabolism by an Oxaloacetate Decarboxylase-Deficient Mutant of Lactococcus lactis IL1403

Citrate metabolism in resting cells of Lactococcus lactis IL1403(pFL3) results in the formation of two end products from the intermediate pyruvate, acetoin and acetate (A. M. Pudlik and J. S. Lolkema, J. Bacteriol. 193:706-714, 2011). Pyruvate is formed from citrate following uptake by the transporter CitP through the subsequent actions of citrate lyase and oxaloacetate decarboxylase. The present study describes the metabolic response of L. lactis when oxaloacetate accumulates in the cytoplasm. The oxaloacetate decarboxylase-deficient mutant ILCitM(pFL3) showed nearly identical rates of citrate consumption, but the end product profile in the presence of glucose shifted from mainly acetoin to only acetate. In addition, in contrast to the parental strain, the mutant strain did not generate proton motive force. Citrate consumption by the mutant strain was coupled to the excretion of oxaloacetate, with a yield of 80 to 85%. Following citrate consumption, oxaloacetate was slowly taken up by the cells and converted to pyruvate by a cryptic decarboxylase and, subsequently, to acetate. The transport of oxaloacetate is catalyzed by CitP. The parental strain IL1403(pFL3) containing CitP consumed oxaloacetate, while the original strain, IL1403, not containing CitP, did not. Moreover, oxaloacetate consumption was enhanced in the presence of L-lactate, indicating exchange between oxaloacetate and L-lactate catalyzed by CitP. Hence, when oxaloacetate inadvertently accumulates in the cytoplasm, the physiological response of L. lactis is to excrete oxaloacetate in exchange with citrate by an electroneutral mechanism catalyzed by CitP. Subsequently, in a second step, oxaloacetate is taken up by CitP and metabolized to pyruvate and acetate.