title: The downfield resonances in the 1H NMR spectra of Azotobacter vinelandii and Pseudomonas putida seven-iron ferredoxins.
abstract: Pseudomonas putida and Azotobacter vinelandii ferredoxins each contain one [4Fe-4S] cluster and one [3Fe-4S] cluster. Their polypeptide chains are nearly identical, differing by only 15 residues out of a total of 106. T1 measurements and temperature dependence studies of the 1H NMR spectrum of each ferredoxin demonstrate that all six resolved downfield resonances are near an iron-sulfur center. The five most downfield resonances are shown to arise from protons on cysteinyl beta-carbons by incorporation of cysteine deuterated at the beta-carbon into cell protein. The sixth peak (10.5 ppm) is shown to be a non-cysteinyl proton. This peak resolves into two resonances of approximately equal intensity at temperatures below 15 degrees or above 25 degrees C. A nuclear Overhauser effect observed between the two downfield-most resonances of A. vinelandii ferredoxin indicates that they originate from a geminal pair of beta-cysteinyl protons. An Overhauser effect observed between the resonances at 22.3 and 15.7 ppm, in conjunction with other results, implies that the resonance at 22.3 ppm arises from a beta-proton on the 3Fe-center-bound Cys16, while the resonance at 15.7 ppm arises from Cys45 beta-proton, which is bound to the 4Fe center. The five most downfield resonances are pH-dependent. The sixth peak (10.5 ppm in P. putida ferredoxin) is pH-independent. Possible origins for the observed pH dependencies are discussed.
authors.affiliation: Department of Chemistry, Hunter College, New York, New York 10021.
authors.forename: H
authors.initials: H
authors.lastname: Cheng
authors.forename: K
authors.initials: K
authors.lastname: Grohmann
authors.forename: W
authors.initials: W
authors.lastname: Sweeney
mesh_terms: D001395:Azotobacter / Q000032:analysis*; D005288:Ferredoxins; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D011549:Pseudomonas / Q000032:analysis*
chemical_list: D005288:Ferredoxins