title: Binding of imidazole to the heme of cytochrome c1 and inhibition of the bc1 complex from Rhodobacter sphaeroides: I. Equilibrium and modeling studies.
abstract: We have used imidazole (Im) and N-methylimidazole (MeIm) as probes of the heme-binding cavity of membrane-bound cytochrome (cyt) c(1) in detergent-solubilized bc(1) complex from Rhodobacter sphaeroides. Imidazole binding to cyt c(1) substantially lowers the midpoint potential of the heme and fully inhibits bc(1) complex activity. Temperature dependences showed that binding of Im (K(d) approximately 330 microM, 25 degrees C, pH 8) is enthalpically driven (DeltaH(0) = -56 kJ/mol, DeltaS(0) = -121 J/mol/K), whereas binding of MeIm is 30 times weaker (K(d) approximately 9.3 mM) and is entropically driven (DeltaH(0) = 47 kJ/mol, DeltaS(0)(o) = 197 J/mol/K). The large enthalpic and entropic contributions suggest significant structural and solvation changes in cyt c(1) triggered by ligand binding. Comparison of these results with those obtained previously for soluble cyts c and c(2) suggested that Im binding to cyt c(1) is assisted by formation of hydrogen bonds within the heme cleft. This was strongly supported by molecular dynamics simulations of Im adducts of cyts c, c(2), and c(1), which showed hydrogen bonds formed between the N(delta)H of Im and the cyt c(1) protein, or with a water molecule sequestered with the ligand in the heme cleft.
authors.affiliation: Center for Biophysics and Computational Biology, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Oleksandr
authors.initials: O
authors.lastname: Kokhan
authors.forename: Vladimir P
authors.initials: VP
authors.lastname: Shinkarev
authors.forename: Colin A
authors.initials: CA
authors.lastname: Wraight
mesh_terms: D003575:Cytochromes c1 / Q000378:metabolism*; D014450:Electron Transport Complex III / Q000037:antagonists & inhibitors*; D006418:Heme / Q000378:metabolism*; D007093:Imidazoles / Q000378:metabolism* / Q000494:pharmacology*; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D010084:Oxidation-Reduction / Q000187:drug effects; D012242:Rhodobacter sphaeroides / Q000201:enzymology*; D013057:Spectrum Analysis; D013696:Temperature
chemical_list: D007093:Imidazoles; D008024:Ligands; D006418:Heme; C029899:imidazole; D003575:Cytochromes c1; D014450:Electron Transport Complex III; C018100:1-methylimidazole
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