STRUCTURE OF THE TRIGONAL FORM OF RECOMBINANT OXIDIZED FLAVODOXIN FROM ANABAENA-7120 AT 1.40 ANGSTROM RESOLUTION

The oxidized recombinant flavodoxin from the cyanobacterium Anabaena 7 120 has been crystallized in a trigonal form. The recombinant protein has an identical primary structure to that purified directly from Anab aena, which functions as a substitute for ferredoxin in an iron-defici ent environment for electron transfer from photosystem I to ferredoxin -NADP(+) reductase. X-ray data to 1.40 Angstrom were collected on a Si emens area detector. Of the 311 379 reflections collected, 36 069 refl ections were unique in space group P3(1)21 (a = 55.36, c = 102.59 Angs trom) with an R(merge) Of 3.8%. The structure was solved by molecular replacement using coordinates from the wild-type monoclinic structure previously solved in this laboratory [Rao, Shaffie, Yu, Satyshur, Stoc kman and Markley (1992). Protein Sci. 1, 1413-1427]. The structure was refined with X-PLOR and SHELXL93 to a crystallographic R factor of 13 .9% for 32 963 reflections with I > 2 sigma(I). The final structure co ntains 2767 atoms including 31 flavin mononucleotide (FMN) atoms, 299 water molecules, and one sulfate ion. The protein is comprised of a ce ntral five-stranded beta-sheet surrounded by five helices and binds a single molecule of FMN at the C-terminus of the sheet. The trigonal pr otein structure and the crystal packing are compared with the monoclin ic wild-type protein. Helix alpha 3 in this structure is less distorte d than in the monoclinic structure and shows additional hydrogen bonds in the N-terminal portion of the helix. The trigonal structure is ext ensively hydrogen bonded in three major areas with neighboring molecul es compared with five regions in the monoclinic structure, but using s ignificantly fewer hydrogen bonds to stabilize the lattice. There are several hydrogen bonds to the amide groups from water molecules severa l of which stabilize and extend the ends of the beta-sheet.