title: Substrate specificity of the citrate transporter CitP of Lactococcus lactis.
abstract: The citrate transporter CitP of lactic acid bacteria catalyzes electrogenic precursor-product exchange of citrate versus L-lactate during citrate-glucose cometabolism. In the absence of sugar, L-lactate is replaced by the metabolic intermediates/end products pyruvate, α-acetolactate, and acetate. In this study, the binding and translocation properties of CitP were analyzed systematically for a wide variety of mono- and dicarboxylates of the form X-CR(2)-COO(-), where X represents OH (2-hydroxy acid), O (2-keto acid), or H (acid) and R groups differ in size, hydrophobicity, and composition. It follows that CitP is a very promiscuous carboxylate transporter. A carboxylate group is both essential and sufficient for recognition by the transporter. A C-2 atom is not essential, formate is a substrate, and C-2 may be part of a ring structure, as in benzoate. The R group may be as bulky as an indole ring structure. For all monocarboxylates of the form X-CHR-COO(-), the hydroxy (X = OH) analogs were the preferred substrates. The preference for keto (X = O) or acid (X = H) analogs was dependent on the bulkiness of the R group, such that the acid was preferred for small R groups and the 2-ketoacid was preferred for more bulky R groups. The C(4) to C(6) dicarboxylates succinate, glutarate, and adipate were also substrates of CitP. The broad substrate specificity is discussed in the context of a model of the binding site of CitP. Many of the substrates of CitP are intermediates or products of amino acid metabolism, suggesting that CitP may have a broader physiological function than its role in citrate fermentation alone.
authors.affiliation: Top Institute Food and Nutrition, Wageningen, The Netherlands.
authors.forename: Agata M
authors.initials: AM
authors.lastname: Pudlik
authors.forename: Juke S
authors.initials: JS
authors.lastname: Lolkema
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001693:Biological Transport, Active; D002264:Carboxylic Acids / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D013294:Lactococcus lactis / Q000235:genetics / Q000378:metabolism*; D027361:Organic Anion Transporters / Q000235:genetics / Q000378:metabolism*; D013379:Substrate Specificity; D013997:Time Factors
chemical_list: D001426:Bacterial Proteins; D002264:Carboxylic Acids; D027361:Organic Anion Transporters; C065431:citP protein, Lactococcus lactis