title: An investigation of Chromatium vinosum high-potential iron-sulfur protein by EPR and Mossbauer spectroscopy; evidence for a freezing-induced dimerization in NaCl solutions.
abstract: The high-potential iron-sulfur protein (HiPIP) from Chromatium vinosum contains a cubane prosthetic group that shuttles between the [4Fe-4S]3+,2+ states. We find that the EPR spectra from this protein can be explained as a sum of two components, a major one with g = 2.02; 2.04; 2.12, and a minor one with g = 2.04; 2.07; approximately 2.13. In the presence of 0.1-2.0 M NaCl, freezing induces polymerization of the protein (presumably dimers), which is detected as intercluster spin-spin interaction in the EPR. The observed spin-spin interactions are interpreted as being due to two very similar dimeric structures in an approx. 1:2 ratio. Computer simulation of the X- and Q-band EPR spectra shows that the z-components of the g-tensors in each dimer pair must be co-linear, with center-to-center distances between the clusters of approximately 13 A and approximately 16 A. Inspection of possible dimeric structures of C. vinosum HiPIP by standard molecular graphics procedures revealed that the Fe/S cluster is exposed toward a flattened surface and is accessible to solvent. Moreover, the Fe/S clusters in two HiPIP molecules can easily achieve a center-to-center distance of approximately 14 A when approaching along a common 3-fold axis that extends through the S4 sulfur atom of the cubane; the z-component of the EPR g-tensor is co-linear with this symmetry axis.
authors.affiliation: Biophysics Research Division, University of Michigan, Ann Arbor 48109.
authors.forename: W R
authors.initials: WR
authors.lastname: Dunham
authors.forename: W R
authors.initials: WR
authors.lastname: Hagen
authors.forename: J A
authors.initials: JA
authors.lastname: Fee
authors.forename: R H
authors.initials: RH
authors.lastname: Sands
authors.forename: J B
authors.initials: JB
authors.lastname: Dunbar
authors.forename: C
authors.initials: C
authors.lastname: Humblet
mesh_terms: D002844:Chromatium / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D005615:Freezing; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D046911:Macromolecular Substances; D008961:Models, Structural; D009994:Osmolar Concentration; D011487:Protein Conformation; D012965:Sodium Chloride; D015204:Spectroscopy, Mossbauer / Q000379:methods
chemical_list: D007506:Iron-Sulfur Proteins; D046911:Macromolecular Substances; D012965:Sodium Chloride