Interpretation of the Mössbauer Spectra of the High‐Potential Iron Protein from Chromatium

Abstract

The Mossbauer spectra of both reduced and oxidized high-potential iron protein (Hipip) from Chromatium have been analysed using computer fits to theoretical spectra derived from a spin Hamiltonian. Fits to spectra obtained over a range of temperatures between 4.2 and 195 K and in applied magnetic fields up to 10.0 T lead to a consistent set of hyperfine parameters.These results are interpreted in terms of a model of the four-iron four-sulphide active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the reduced centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with the spins being coupled antiferromagnetically to give the centre zero net spin. The oxidized centre has one less electron which at low temperatures appears to have come predominantly from one pair of iron atoms which thus become ferric with the other pair remaining substantially unchanged. It is clear from the Mossbauer hyperfine parameters obtained from the computer fits to the low-temperature spectra that a larger magnetic moment is associated with the ferric/ferrous pair of iron atoms than with the ferric pair of iron atoms. This also explains the g values with an average of greater than 2 which are observed in electron paramagnetic resonance (EPR) measurements. At higher temperatures the differences between the electron charge density at the different iron atoms in the oxidized centre appear to become smeared out.The high-potential iron protein (Hipip) from Chromatium belongs to the class of iron-sulphur proteins which contain one four-iron four-sulphide centre per molecule. X-ray crystallographic studies on this protein show that the active centre consists of four iron atoms, each in a distorted tetrahedral environment of three labile sulphur atoms and a fourth sulphur ligand from a cysteine residue of the amino acid chain [l]. The four-iron and eight-iron ferredoxins also contain active centres of this type. It has been proposed that the centre can exist in three oxidation states [2], which may be designated C3+, C2+ a nd C1+. C 2 + corresponds to the equivalent states of oxidized ferredoxin and reduced Hipip, with C 3

The role of a conserved tyrosine residue in high‐potential iron sulfur proteins

“…The oxidized HiPIP cluster, with an overall charge of + 3 and a net spin of 1/2, is made up of two antiferromagnetically coupled spin pairs. One spin pair consists of two Fe3+, whereas the other consists of two (Middleton et al, 1980). The chemical shift of the PCH2 protons of the cluster cysteine ligands and their temperature dependence are consistent with this interpretation (Bertini et al, 1991).…”

“…The cluster of the reduced HiPlP has an overall charge of +2 and a net spin of 0 (Carter et al, 1972;Antanaitis &Moss, 1975). Mossbauer spectra indicate that the iron atoms are in equivalent, oxidation states (Middleton et al, 1980). The oxidized HiPIP cluster, with an overall charge of + 3 and a net spin of 1/2, is made up of two antiferromagnetically coupled spin pairs.…”

EPR and M�ssbauer spectroscopic studies on the tetrameric, NAD-linked hydrogenase of Nocardia opaca 1b and its two dimers: 1. The ??-dimer?a prototype of a simple hydrogenase

“…Isomer shifts and quadrupole splittings were determined from least-squares fits with Lorentzians: subspectrum 1, 6 = 0.45 mm/s, AE o = 1.21 mm/s, F = 0.4 mm/s, 61% of total absorption area; subspectrum 2, (5= 0.35 mm/s, AE o = 0.61 mm/s, F = 0.37 mm/s, 39% of total absorption area. The parameters of subspectrum 1 are characteristic of [4Fe-4S] 2+ clusters (Middleton et al 1980), as mentioned above for the fl6-dimer, while those of subspectrum 2 are consistent with parameters of [3Fe-4S] 1+ clusters (Mtinck 1982, Orme-Johnson & Orme-Johnson 1982 and [2Fe-2S] 2+ clusters (Dunham et al 1971). The relative intensities of subspectra 1 and 2, 61% and 39% of the total absorption area, are in agreement with the values 61.5% and 38.5%, expected for two [4Fe-4S] clusters, accounting for subspectrum 1 as well as one [3Fe-4S] and one [2Fe-2S] cluster accounting for subspectrum 2.…”

Electronic structure of the unique [4Fe-3S] cluster in O 2 -tolerant hydrogenases characterized by 57 Fe Mössbauer and EPR spectroscopy

“…The UV-visible spectra of superoxidized Hase I prepared in air show broad bands around 400 nm, arising from S-to-Fe charge transfer transitions (16) (19)(20)(21)(22). This is not completely unexpected, because the 4Fe cores are formally isoelectronic and the modified [4Fe-3S] cores differ from classical cubanes only by an inorganic S 2− ligand.…”