title: Alternate modes of binding in two crystal structures of alkaline phosphatase-inhibitor complexes.
abstract: Two high resolution crystal structures of Escherichia coli alkaline phosphatase (AP) in the presence of phosphonate inhibitors are reported. The phosphonate compounds, phosphonoacetic acid (PAA) and mercaptomethylphosphonic acid (MMP), bind competitively to AP with dissociation constants of 5.5 and 0.6 mM, respectively. The structures of the complexes of AP with PAA and MMP were refined at high resolution to crystallographic R-values of 19.0 and 17.5%, respectively. Refinement of the AP-inhibitor complexes was carried out using X-PLOR. The final round of refinement was done using SHELXL-97. Crystallographic analyses of the inhibitor complexes reveal different binding modes for the two phosphonate compounds. The significant difference in binding constants can be attributed to these alternative binding modes observed in the high resolution X-ray structures. The phosphinyl group of PAA coordinates to the active site zinc ions in a manner similar to the competitive inhibitor and product inorganic phosphate. In contrast, MMP binds with its phosphonate moiety directed toward solvent. Both enzyme-inhibitor complexes exhibit close contacts, one of which has the chemical and geometrical potential to be considered an unconventional hydrogen bond of the type C-H...X.
authors.affiliation: Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, USA.
authors.forename: K M
authors.initials: KM
authors.lastname: Holtz
authors.forename: B
authors.initials: B
authors.lastname: Stec
authors.forename: J K
authors.initials: JK
authors.lastname: Myers
authors.forename: S M
authors.initials: SM
authors.lastname: Antonelli
authors.forename: T S
authors.initials: TS
authors.lastname: Widlanski
authors.forename: E R
authors.initials: ER
authors.lastname: Kantrowitz
mesh_terms: D000469:Alkaline Phosphatase / Q000037:antagonists & inhibitors* / Q000737:chemistry; D001665:Binding Sites; D002244:Carbon / Q000737:chemistry; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D006838:Hydrocarbons; D006860:Hydrogen Bonding; D007700:Kinetics; D008697:Methane / Q000031:analogs & derivatives / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D063065:Organophosphonates / Q000737:chemistry* / Q000378:metabolism; D010746:Phosphonoacetic Acid / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D055672:Static Electricity; D013438:Sulfhydryl Compounds / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics; D015032:Zinc / Q000737:chemistry
chemical_list: D006838:Hydrocarbons; D063065:Organophosphonates; D013438:Sulfhydryl Compounds; C415199:mercaptomethylphopshonic acid; C030011:carbene; D002244:Carbon; D000469:Alkaline Phosphatase; D015032:Zinc; D010746:Phosphonoacetic Acid; D008697:Methane