Citrate is best known as an intermediate in the tricarboxylic acid cycle of the cell. In addition to this essential role in energy metabolism, the tricarboxylate anion also acts as both a precursor and a regulator of fatty acid synthesis^1,2,3. Thus, the rate of fatty acid synthesis correlates directly with the cytosolic concentration of citrate^4,5. Liver cells import citrate through the sodium-dependent citrate transporter NaCT (encoded by SLC13A5) and, as a consequence, this protein is a potential target for anti-obesity drugs. Here, to understand the structural basis of its inhibition mechanism, we determined cryo-electron microscopy structures of human NaCT in complexes with citrate or a small-molecule inhibitor. These structures reveal how the inhibitor—which binds to the same site as citrate—arrests the transport cycle of NaCT. The NaCT–inhibitor structure also explains why the compound selectively inhibits NaCT over two homologous human dicarboxylate transporters, and suggests ways to further improve the affinity and selectivity. Finally, the NaCT structures provide a framework for understanding how various mutations abolish the transport activity of NaCT in the brain and thereby cause epilepsy associated with mutations in SLC13A5 in newborns (which is known as SLC13A5-epilepsy)^6,7,8.

柠檬酸盐最为人所知的是作为细胞三羧酸循环的中间体。除了在能量代谢中的重要作用外，三羧酸根阴离子还充当脂肪酸合成的前体和调节剂^1,2,3。因此，脂肪酸合成的速率与柠檬酸盐^4,5的胞质浓度直接相关。肝细胞通过钠依赖性柠檬酸转运蛋白 NaCT（由SLC13A5编码）输入柠檬酸)，因此，这种蛋白质是抗肥胖药物的潜在靶点。在这里，为了了解其抑制机制的结构基础，我们确定了与柠檬酸盐或小分子抑制剂复合的人 NaCT 的低温电子显微镜结构。这些结构揭示了抑制剂（与柠檬酸盐与同一位点结合）如何阻止 NaCT 的转运循环。NaCT 抑制剂结构也解释了为什么该化合物选择性地抑制 NaCT 超过两种同源的人二羧酸转运蛋白，并提出了进一步提高亲和力和选择性的方法。最后，NaCT 结构提供了一个框架，用于理解各种突变如何消除 NaCT 在大脑中的转运活性，从而导致与SLC13A5突变相关的癫痫新生儿（称为 SLC13A5-癫痫）^6,7,8。