Figure 6

The sodium ion binding sites in KpCitS. (a) The outward-facing protomer in the asymmetric conformation. (b) The inward-facing protomer in the asymmetric conformation. (c) The protomer in the outward-facing conformation. (d) The protomer in the inward-facing conformation. Citrate is shown as an orange ball-and-stick model, sodium ions (Na1) as black spheres, and water molecules (W, W1, W2) as yellow spheres. The empty Na2 sites are shown as dashed spheres. Hydrogen bonds and ionic interactions are drawn as dashed lines. The simulated annealing omit maps superimposed on the refined citrates, sodium ions, and water molecules were contoured at 2.8 σ level. (e,f) WebLogo representations of the sequence alignments and conservation in the HP1 and HP2 loops of 2-HCT proteins^{54, 55}. The amino acid positions in the multiple sequence alignment are: KpCitS (182–189, 399–406), SeCitS (182–189, 399–406), BsCimH (191–198, 403–410), LmCitP (186–193, 396–403), and LlMelp (169–176, 378–385) for the HP1 and HP2 loops, respectively.