J. Mot. Biol. (1983)165, 737-755 Structure of Oxidized Flavodoxin from Anacystis nidulans WARD W. SMITHt, KATHERINE A. PATTRIDGE, MARTHA L. LUDWIG Department of Biological Chemistry and Biophysics Research Division The University of Michigan Ann Arbor, Mich. 48109, U.S.A. GREGORY A. PETSKO:~, DEMETRIUS TSERNOGLOU Department of Biochemistry Wayne State University Detroit, Mich. 48202, U.S.A. MASARU TANAKA AND KERRY T. YASUNOBU Department of Biochemistry and Biophysics University of Hawaii Honolulu, Hawaii 96822, U.S.A. (Received 17 April 1981, and in revised form 6 December 1982) The structure of oxidized flavodoxin from the cyanobacterium Anacystis nidulans has been determined at 2"5 A resolution with phases calculated from ethylmercury phosphate and dimercuriacetate derivatives. The determination of partial sequences, including a total of 85 residues, has assisted in the interpretation of the electron density. Preliminary refinement of a partial model (1072 atoms) has reduced R to 0"349 for the 10,997 reflections between 2"0 and 5"0 A with I > 2a. The polypeptide backbone, which comprises 167 residues in the current model, adopts the familiar fl-c~-fl conformation found in other flavodoxins and in the nucleotide-binding domains of the pyridine-nucleotide dehydrogenases, with five parallel strands in the central sheet. Comparison with flavodoxin from Clostridium MP (138 residues) shows that extra residues of A. nidulans flavodoxin are accommodated in a major insertion about 20 residues in length, which" forms a lobe adjacent to the fifth strand of parallel sheet, and in additions to several external segments. Residues added between the fourth sheet strand and the start of the third helix alter the environment of the pyrimidine end of the fiavin mononucleotide ring. The flavin mononucleotide phosphate binds to the start of helix 1, interacting with hydroxyamino acids and with main-chain amide groups. Two hydrophobic residues, both tentatively identified as Trp, enclose the isoalloxazine ring; the solvent-exposed Trp is nearly parallel to the fiavin ring. The hydrophobic t Present address: Molecular Biology Institute, University of California, Los Angeles, Calif. 90024, U.S.A. ~:Present address: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Mass. 02139, U.S.A. 737 0022-2836/83/120737-19 $03.00/0 © 1983 Academic P~'ess Inc. (London) Ltd.