THE 3-DIMENSIONAL STRUCTURE IN SOLUTION OF THE PARAMAGNETIC HIGH-POTENTIAL IRON-SULFUR PROTEIN-I FROM ECTOTHIORHODOSPIRA-HALOPHILA THROUGH NUCLEAR-MAGNETIC-RESONANCE

The three-dimensional structure in solution of reduced recombinant hig h-potential iron-sulfur protein iso-I from Ectothiorhodospira halophil a was determined using 948 relevant interproton NOEs out of the 1246 o bserved NOEs. The determination was accomplished using the XEASY progr am for spectral analysis and the distance geometry (DG) program DIANA for generation of the structure as described by Wuthrich [Wuthrich, K. (1989) Ace. Chem. Res. 22, 36-44]. The FeS cluster was simulated usin g an amino acid residue constructed for the present work from a cystei nyl residue with an iron and a sulfur atom attached to the terminal th iol. The family of structures obtained from distance geometry were sub jected to energy minimization and molecular dynamics simulations using previously defined force field parameters. The quality of these struc tures at each stage of the refinement process is discussed with respec t to the dihedral angle order parameter and the root-mean-square devia tion of the atomic coordinates. The latter values for the backbone ato ms vary from 67 pm for the distance-geometry structures to 60 pm for t he energy-minimized structures to 51 pm for the structures subjected t o restrained molecular dynamics. Finally, the structure in best agreem ent with the NOE constraints has been further treated with extensive r estrained molecular dynamics in water. The solution structure is well defined and is very similar to the available X-ray structure. We do no t know of any previous determination of the structure of a paramagneti c protein in solution by NMR. The effect of paramagnetism on the quali ty of the structure determination is discussed.