Refined structures of oxidized flavodoxin from Anacystis nidulans

Flavodoxin from Anacystis nidulans (Synechococcus PCC 7942) was the first m ember of the flavodoxin family to be characterized, and is the structural p rototype for the "long-chain" flavodoxins that have molecular masses of sim ilar to 20 kDa. Crystal structure analyses and refinements of three orthorh ombic forms of oxidized A. nidulans flavodoxin are reported, and salient fe atures of the fold and the FMN binding site are compared with other flavodo xins. The structure of form I (wild-type: P2(1)2(1)2(1), a = 57.08 Angstrom , b = 69.24 Angstrom, c = 45.55 Angstrom), determined initially by multiple isomorphous replacement, has been refined to R = 0.183 and R-free = 0.211 for data from 10.0 to 1.7 Angstrom resolution. Structures of form II (wild- type: P2(1)2(1)2(1), a = 60.05 Angstrom, b = 65.85 Angstrom, c = 51.36 Angs trom) and form III (Asn58Gly: P2(1)2(1)2(1), a = 51.30 Angstrom, b = 59.15 Angstrom, c = 94.44 Angstrom) have been determined by molecular replacement and refined versus data to 2.0 Angstrom and 1.85 Angstrom, respectively; t he X values for forms II and III are 0.147 and 0.150. Changes in the molecu lar contacts that produce the alternative packings in these crystalline for ms are analyzed. Deletion of the Asn side-chain in the mutant Asn58Gly remo ves an intermolecular stacking interaction and allows the alternative packi ng found in form III crystals. The functionally important 50's loop of the FMN binding site is less restrained by intermolecular contacts in these cry stals but maintains the same conformation as in oxidized wild type protein. The structures reported here provide the starting point for structure-func tion studies of the reduced states and of mutants, described in the accompa nying paper. (C) 1999 Academic Press.