Summary of residues critical for UapA function and specificity.

TMS: transmembrane segment.
B: beta sheet conformation within the TMS.
1“−”: no major effect on Vmax >50%, “+”: major effect on Vmax <10%, “+/−”: Vmax <30% and “+ or −“: depending on specific substitution.
2Evidence for involvement in substrate binding through peptide backbone interactions, as shown by docking. Consequently mutations with relatively small amino acids do not have an effect.
3Q408E, confers ability to bind hypoxanthine and guanine, but does not lead to their transport. Q408 in combination with gate mutations leads to high-medium affinity binding and transporter of all purines and uracil.