THE SOLUTION STRUCTURE OF OXIDIZED HIPIPI FROM ECTOTHIORHODOSPIRA-HALOPHILA - CAN NMR-SPECTROSCOPY BE USED TO PROBE REARRANGEMENTS ASSOCIATED WITH ELECTRON-TRANSFER PROCESSES

In the H-1 NMR spectrum of the oxidized form of the high-potential iro n-sulfur protein (HiPIPI) from Ectothiorhodospira halophila, 91% of th e total proton resonances and 100% of the residues have been assigned. The standard COSY, NOESY, and TOCSY sequences have been optimized for the paramagnetism of the molecule. Extensive assignment of the N-15 N MR spectrum has been obtained through HMQC spectra. With 1437 dipolar connectivities, of which about 10% involved fast-relaxing protons, a f amily of 18 structures was generated with an RMSD of 0.65 Angstrom, by using the programs developed by Wuthrich. The family of structures wa s further refined by various calculation steps; the final RMSD was 0.4 8 Angstrom. The structures appear to be very similar but not equal to the structures of the reduced protein. Despite the similarity in struc ture, significant variations in the chemical shifts are observed. A si milar behavior was observed for the homologous protein from Chromatium vinosum. It is concluded that NMR is a sensitive tool to monitor diff erences between oxidized and reduced proteins; however, the detailed s tructural variations should be evaluated with caution at the present l evel of resolution, which roughly corresponds to a resolution of 2.5 A ngstrom, in an X-ray structure.