title: Kinetics and equilibria of cyanide binding to prostaglandin H synthase.
abstract: Cyanide binding to prostaglandin H (PGH) synthase results in a spectral shift in the Soret region. This shift was exploited to determine equilibrium and kinetic parameters of the cyanide binding process. At pH 8.0, ionic strength 0.22 M, 4 degrees C, the cyanide dissociation constant, determined from equilibrium experiments, is (65 +/- 10) microM. The binding rate constant is (2.8 +/- 0.2) x 10(3) M-1 s-1, and the dissociation rate constant is zero within experimental error. Through a kinetic study of the binding process as a function of pH, from pH 3.96 to 8.00, it was possible to determine the pKa of a heme-linked acid group on the enzyme of 4.15 +/- 0.10 with citrate buffer. An apparent pKa of 4.75 +/- 0.03 was determined with acetate buffer; this different value is attributed to complexation of the enzyme with one of the components of the acetate buffer.
authors.affiliation: Department of Chemistry, University of Alberta, Edmonton, Canada.
authors.forename: I D
authors.initials: ID
authors.lastname: MacDonald
authors.forename: H B
authors.initials: HB
authors.lastname: Dunford
mesh_terms: D000818:Animals; D003486:Cyanides / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism*; D011485:Protein Binding; D012669:Seminal Vesicles / Q000201:enzymology; D012756:Sheep; D013053:Spectrophotometry; D013055:Spectrophotometry, Infrared
chemical_list: D003486:Cyanides; D011451:Prostaglandin-Endoperoxide Synthases