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 Spirulina platensis has been refined using diffraction data to 2.5 Angstrom resolution by alternate cycles of simulated annealing and manual revision 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 alpha-helix flanking a sheet consisting of four beta strands, The present refinement revises the conformation of residues 56-71, in which a one-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-strands, The root-mean-square deviations of the corresponding C-alpha atoms of the pairs of ferredoxins range from 0.90 to 1.17 Angstrom for all the residues, but from 0.64 to 0.70 Angstrom if the few perturbed residues are excluded, Therefore, it may be concluded that the main-chain 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 between ferredoxin and ferredoxin-NADP(+) reductase is discussed on the basis of the charge distributions of these molecules and biochemical data.