title: Spectral properties of bacterial nitric-oxide reductase: resolution of pH-dependent forms of the active site heme b3.
abstract: Bacterial nitric-oxide reductase catalyzes the two electron reduction of nitric oxide to nitrous oxide. In the oxidized form the active site non-heme Fe(B) and high spin heme b(3) are mu-oxo bridged. The heme b(3) has a ligand-to-metal charge transfer band centered at 595 nm, which is insensitive to pH over the range of 6.0-8.5. Partial reduction of nitric-oxide reductase yields a three electron-reduced state where only the heme b(3) remains oxidized. This results in a shift of the heme b(3) charge transfer band lambda(max) to longer wavelengths. At pH 6.0 the charge transfer band lambda(max) is 605 nm, whereas at pH 8.5 it is 635 nm. At pH 6.5 and 7.5 the nitric-oxide reductase ferric heme b(3) population is a mixture of both 605- and 635-nm forms. Magnetic circular dichroism spectroscopy suggests that at all pH values examined the proximal ligand to the ferric heme b(3) in the three electron-reduced form is histidine. At pH 8.5 the distal ligand is hydroxide, whereas at pH 6.0, when the enzyme is most active, it is water.
authors.affiliation: Centre for Metalloprotein Spectroscopy and Biology, Schools of Biological Sciences and Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom.
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Field
authors.forename: Louise
authors.initials: L
authors.lastname: Prior
authors.forename: M Dolores
authors.initials: MD
authors.lastname: Roldan
authors.forename: Myles R
authors.initials: MR
authors.lastname: Cheesman
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Thomson
authors.forename: Stephen
authors.initials: S
authors.lastname: Spiro
authors.forename: Julea N
authors.initials: JN
authors.lastname: Butt
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Watmough
authors.forename: David J
authors.initials: DJ
authors.lastname: Richardson
mesh_terms: D001419:Bacteria / Q000201:enzymology*; D001665:Binding Sites; D006418:Heme / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism
chemical_list: D006418:Heme; D010088:Oxidoreductases; C034747:nitric-oxide reductase