title: The iron-sulfur cluster composition of Escherichia coli nitrate reductase.
abstract: Nitrate reductase from Escherichia coli has been investigated by low-temperature magnetic circular dichroism and electron paramagnetic resonance (EPR) spectroscopies, as well as by Fe-S core extrusion, to determine the Fe-S cluster composition. The results indicate approximately one 3Fe and three or four [4Fe-4S]2+,1+ centers/molecule of isolated enzyme. The magnetic circular dichroism spectra and magnetization characteristics show the oxidized and reduced 3Fe and [4Fe-4S] centers to be electronically analogous to those in bacterial ferredoxins. The form and spin quantitation of the EPR spectra from [4Fe-4S]1+ centers in the reduced enzyme were found to vary with the conditions of reduction. For the fully reduced enzyme, the EPR spectrum accounted for between 2.9 and 3.5 spins/molecule, and comparison with partially reduced spectra indicates weak intercluster magnetic interactions between reduced paramagnetic centers. In common with other Fe-S proteins, the 3Fe center was not extruded intact under standard conditions. The results suggest that nitrate reductase is the first example of a metalloenzyme where enzymatic activity is associated with a form that contains an oxidized 3Fe center. However, experiments to determine whether or not the 3Fe center is present in vivo were inconclusive.
authors.forename: M K
authors.initials: MK
authors.lastname: Johnson
authors.forename: D E
authors.initials: DE
authors.lastname: Bennett
authors.forename: J E
authors.initials: JE
authors.lastname: Morningstar
authors.forename: M W
authors.initials: MW
authors.lastname: Adams
authors.forename: L E
authors.initials: LE
authors.lastname: Mortenson
mesh_terms: D002942:Circular Dichroism; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology*; D007506:Iron-Sulfur Proteins / Q000032:analysis*; D008667:Metalloproteins / Q000032:analysis*; D009565:Nitrate Reductases / Q000032:analysis*
chemical_list: D007506:Iron-Sulfur Proteins; D008667:Metalloproteins; D009565:Nitrate Reductases