Citrate metabolism in Leuconostoc mesenteroides subspecies mesenteroides is associated with the generation of a proton motive force by a secondary mechanism. The pathway consists of four steps: (i) uptake of citrate, (ii) splitting of citrate into acetate and oxaloacetate, (iii) pyruvate formation by decarboxylation of oxaloacetate, and (iv) reduction of pyruvate to lactate. Studies of citrate uptake and metabolism in resting cells of L. mesenteroides grown in the presence or absence of citrate show that the citrate transporter CitP and citrate lyase are constitutively expressed. On the other hand, oxaloacetate decarboxylase is under stringent control of the citrate in the medium and is not expressed in its absence, thereby blocking the pathway at the level of oxaloacetate. Under those conditions, the pathway is completely directed towards the formation of aspartate, which is formed from oxaloacetate by transaminase activity. The data indicate a role for citrate metabolism in amino acid biosynthesis. Internalized radiolabeled aspartate produced from citrate metabolism could be chased from the cells by addition of the amino acid precursors oxaloacetate, pyruvate, α-ketoglutarate, and α-ketoisocaproate to the cells, indicating a broad specificity of the transamination reaction. The α-ketocarboxylates are readily transported across the cytoplasmic membrane. α-Ketoglutarate uptake in resting cells of L. mesenteroides was dependent upon the presence of an energy source and was inhibited by inhibition of the proton motive force generating F0F1 ATPase and by selective dissipation of the membrane potential and the transmembrane pH gradient. It is concluded that in L. mesenteroides α-ketoglutarate is transported via a secondary transporter that may be a general α-ketocarboxylate carrier.
