title: Crystallographic evidence for the action of potassium, thallium, and lithium ions on fructose-1,6-bisphosphatase.
abstract: Fructose-1,6-bisphosphatase (Fru-1,6-Pase; D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) requires two divalent metal ions to hydrolyze alpha-D-fructose 1,6-bisphosphate. Although not required for catalysis, monovalent cations modify the enzyme activity; K+ and Tl+ ions are activators, whereas Li+ ions are inhibitors. Their mechanisms of action are still unknown. We report here crystallographic structures of pig kidney Fru-1,6-Pase complexed with K+, Tl+, or both Tl+ and Li+. In the T form Fru-1,6-Pase complexed with the substrate analogue 2,5-anhydro-D-glucitol 1,6-bisphosphate (AhG-1,6-P2) and Tl+ or K+ ions, three Tl+ or K+ binding sites are found. Site 1 is defined by Glu-97, Asp-118, Asp-121, Glu-280, and a 1-phosphate oxygen of AhG-1,6-P2; site 2 is defined by Glu-97, Glu-98, Asp-118, and Leu-120. Finally, site 3 is defined by Arg-276, Glu-280, and the 1-phosphate group of AhG-1,6-P2. The Tl+ or K+ ions at sites 1 and 2 are very close to the positions previously identified for the divalent metal ions. Site 3 is specific to K+ or Tl+. In the divalent metal ion complexes, site 3 is occupied by the guanidinium group of Arg-276. These observations suggest that Tl+ or K+ ions can substitute for Arg-276 in the active site and polarize the 1-phosphate group, thus facilitating nucleophilic attack on the phosphorus center. In the T form complexed with both Tl+ and Li+ ions, Li+ replaces Tl+ at metal site 1. Inhibition by lithium very likely occurs as it binds to this site, thus retarding turnover or phosphate release. The present study provides a structural basis for a similar mechanism of inhibition for inositol monophosphatase, one of the potential targets of lithium ions in the treatment of manic depression.
authors.affiliation: Gibbs Chemical Laboratory Harvard University, Cambridge, MA 02138, USA.
authors.forename: V
authors.initials: V
authors.lastname: Villeret
authors.forename: S
authors.initials: S
authors.lastname: Huang
authors.forename: H J
authors.initials: HJ
authors.lastname: Fromm
authors.forename: W N
authors.initials: WN
authors.lastname: Lipscomb
mesh_terms: D000494:Allosteric Regulation; D000818:Animals; D001665:Binding Sites; D018360:Crystallography, X-Ray; D006597:Fructose-Bisphosphatase / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000187:drug effects; D006600:Hexosephosphates / Q000737:chemistry; D007668:Kidney / Q000201:enzymology; D008094:Lithium / Q000737:chemistry* / Q000494:pharmacology; D008956:Models, Chemical; D008958:Models, Molecular; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors; D011188:Potassium / Q000737:chemistry* / Q000494:pharmacology; D011487:Protein Conformation; D013552:Swine; D013793:Thallium / Q000737:chemistry* / Q000494:pharmacology
chemical_list: C092888:2,5-anhydroglucitol-1,6-bisphosphate; D006600:Hexosephosphates; D008094:Lithium; D013793:Thallium; D006597:Fructose-Bisphosphatase; D010744:Phosphoric Monoester Hydrolases; C021822:myo-inositol-1 (or 4)-monophosphatase; D011188:Potassium