Extension of l-8 A Resolution and Some Comparisons with the Oxidized State

As part of a series of comparisons of the structures of the three oxidation states of flavodoxin from Clostridium MP, phases for the semiquinonc form were determined to 2.0 A resolution by isomorphous replacement (<rn> = 0.725). Subsequently, the structure was refined at 1.8 A resolution by a combination of difference Fourier, real space and reciprocal space methods. After refining to an R of 0.194, we explored the conformation of the FMN binding site by real space refinement verau6 maps with Fourier coefficients of the form (2) F,I- 1 P,I) exp (in,). To minimize bias in the fitting, groups of atoms were systematically omitted from the structure factors used in computat.ion of the (212”,1- IF,\) maps. One-electron reduction of oxidized flavodoxin is accompanied by several changes at the FMN binding site: the conformation of residues in the reverse bend formed by Met56-Gly57-Asp58-Glu59 differs in the crystal structures of the oxidized and semiquinone species; further, backbone atoms in residues 55 and 89 shift by more than 0.5 A and the indole ring of Trp90 undergoes a significant displacement. The orientation of the peptide unit connecting Gly57 and Asp5