Figure 4.

HPrK_C and phosphate binding. (A) Two phosphate ions are bound in the P-loop region, stabilized by invariant residues, most likely involved in catalysis (H136, K157, Ser-153, Glu-159, R202, E200) and/or metal binding (Asp-174/175) (see text). Phosphate 1 occupies the position of the β-phosphate of ATP and is additionally stabilized by water-mediated contacts with R267 from a neighboring subunit. Phosphate 2 (closest to S153) we propose to represent the position of the γ-phosphate of ATP after its transfer to Ser-46 of HPr. Major interactions stabilizing the phosphate ions are shown as blue dashed lines. H136 seems to adopt alternative conformations in the structure, both of which are displayed. (B) P-loop region of HPrK_C (yellow) superimposed on the AdK-related UK_yeast, bound to two ADP molecules (PDB code 1uky), shown in ball and stick. Electron density corresponding to the two phosphate ions found in HPrK_C has been included, matching nearly perfectly the β-phosphate positions of ADP bound to UK_yeast. Residues in UK_yeast that are structurally equivalent to those in the P-loop of HPrK_C are also displayed. K29 and R142 of UK_yeast, equivalent to K157 and R202, contact both phosphate ions and are believed to play a role in catalysis through stabilization of transition states. The putative adenine-binding loops are depicted in cyan; the corresponding loop (blue) from neighboring subunit of HPrK is included.