title: Heme-coordinating inhibitors of neuronal nitric oxide synthase. Iron-thioether coordination is stabilized by hydrophobic contacts without increased inhibitor potency.
abstract: The heme-thioether ligand interaction often occurs between heme iron and native methionine ligands, but thioether-based heme-coordinating (type II) inhibitors are uncommon due to the difficulty in stabilizing the Fe-S bond. Here, a thioether-based inhibitor (3) of neuronal nitric oxide synthase (nNOS) was designed, and its binding was characterized by spectrophotometry and crystallography. A crystal structure of inhibitor 3 coordinated to heme iron was obtained, representing, to our knowledge, the first crystal structure of a thioether inhibitor complexed to any heme enzyme. A series of related potential inhibitors (4-8) also were evaluated. Compounds 4-8 were all found to be type I (non-heme-coordinating) inhibitors of ferric nNOS, but 4 and 6-8 were found to switch to type II upon heme reduction to the ferrous state, reflecting the higher affinity of thioethers for ferrous heme than for ferric heme. Contrary to what has been widely thought, thioether-heme ligation was found not to increase inhibitor potency, illustrating the intrinsic weakness of the thioether-ferric heme linkage. Subtle changes in the alkyl groups attached to the thioether sulfur caused drastic changes in the binding conformation, indicating that hydrophobic contacts play a crucial role in stabilizing the thioether-heme coordination.
authors.affiliation: Department of Chemistry, Center for Molecular Innovation and Drug Discovery, and Chemistry of Life Processes Institute, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Martell
authors.forename: Huiying
authors.initials: H
authors.lastname: Li
authors.forename: Tzanko
authors.initials: T
authors.lastname: Doukov
authors.forename: Pavel
authors.initials: P
authors.lastname: Martásek
authors.forename: Linda J
authors.initials: LJ
authors.lastname: Roman
authors.forename: Michael
authors.initials: M
authors.lastname: Soltis
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Poulos
authors.forename: Richard B
authors.initials: RB
authors.lastname: Silverman
mesh_terms: D018360:Crystallography, X-Ray; D004791:Enzyme Inhibitors / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology*; D006418:Heme / Q000737:chemistry / Q000494:pharmacology*; D057927:Hydrophobic and Hydrophilic Interactions; D007501:Iron / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D052248:Nitric Oxide Synthase Type I / Q000037:antagonists & inhibitors*; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013440:Sulfides / Q000737:chemistry*
chemical_list: D004791:Enzyme Inhibitors; D013440:Sulfides; D006418:Heme; D007501:Iron; D052248:Nitric Oxide Synthase Type I