Human SLC2A5 is primarily a high affinity fructose transporter, whereas, hSLC2A9 is a very high affinity fructose and a high capacity urate transporter. Within the Class II human glucose transporters (hSLC2As), hSLC2A9 and hSLC2A5 have the highest sequence homology, with 29% identity and 59% similarity 8, 9, yet they appear to interact with substrates very differently. In addition to key amino acid residues forming a potential binding pocket, other structural features such as interactions between transmembrane helices, intracellular helices, disulfide bridges, and salt bridges have been implicated in governing substrate specificity in transporter function 4, 5, 6, 7. However, recent kinetic and substrate binding studies of glucose transporters reveal that a single binding pocket may not fully explain how a transporter protein selects and translocates its substrates. These data indicate that urate transport in hSLC2A9 involves several structural elements rather than just a unique substrate binding pocket.įor over half a century, structure-function studies have primarily focused on the identification of single substrate binding pockets within the aqueous channels of transporter proteins 1, 2, 3. Additional data from chimæric protein analysis illustrated that transmembrane helix 7 of hSLC2A9 is necessary for urate transport but not sufficient to allow urate transport to be induced in glucose transporter 5 (hSLC2A5). We have shown that cysteine residues, C181, C301 and C459 in hSLC2A9 are also essential elements for mediating urate transport. Our functional studies confirmed that N429 is a key residue for both urate binding and transport. Our in silico substrate docking study has revealed that urate and fructose bind within the same binding pocket in hSLC2A9, yet with distinct orientations, and allowed us to identify novel residues for urate binding. This ability is in contrast to other homologous sugar transporters such as glucose transporters 1 and 5 (SLC2A1 & SLC2A5) and the xylose transporter (XylE), despite the fact that these transporters have similar protein structures. Human SLC2A9 is unique in that it transports hexoses as well as the organic anion, urate. Human glucose transporter 9 (hSLC2A9) is critical in human urate homeostasis, for which very small deviations can lead to chronic or acute metabolic disorders.
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