TERTIARY STRUCTURE OF [2FE-2S] FERREDOXIN FROM SPIRULINA-PLATENSIS REFINED AT 2.5 ANGSTROM RESOLUTION - STRUCTURAL COMPARISONS OF PLANT-TYPE FERREDOXINS AND AN ELECTROSTATIC POTENTIAL ANALYSIS

The structure of plant-type [2Fe-2S] ferredoxin isolated from Spirulin a platensis has been refined using diffraction data to 2.5 Angstrom re solution by alternate cycles of simulated annealing and manual revisio n of the model, The final R factor is 19.9% for 2,912 reflections with F > 2(sigma F) between 8.0 and 2.5 Angstrom resolution, S, platensis ferredoxin, like other plant-type [2Fe-2S] ferredoxins, has a major al pha-helix flanking a sheet consisting of four beta strands, The presen t refinement revises the conformation of residues 56-71, in which a on e-turn helix was identified. Superposition of the Spirulina ferredoxin structure on the structures of other ferredoxins that have been well refined showed structural perturbation at a few residues on the amino and carboxyl termini and the turn between the first and second beta-st rands, The root-mean-square deviations of the corresponding C-alpha at oms of the pairs of ferredoxins range from 0.90 to 1.17 Angstrom for a ll the residues, but from 0.64 to 0.70 Angstrom if the few perturbed r esidues are excluded, Therefore, it may be concluded that the main-cha in foldings of all the plant-type [2Fe-2S] ferredoxins are essentially the same, Electrostatic potential analysis showed that the molecular surface around the cluster is negatively charged, whereas that of the beta-sheet of the other side is positively charged, The interaction be tween ferredoxin and ferredoxin-NADP(+) reductase is discussed on the basis of the charge distributions of these molecules and biochemical d ata.