Tertiary structure of oxidized flavodoxin from an eukaryotic red alga Chondrus crispus at 2.35-A resolution. Localization of charged residues and implication for interaction with electron transfer partners

The crystal structure of the oxidized form of a flavodoxin from an eukaryotic red alga, Chondrus crispus, has been determined by multiple isomorphous replacement and anomalous scattering methods. A model of the 173 residues and flavin mononucleotide (FMN) has been refined by a restrained least squares method to a crystallographic R-factor of 22.6% using 6236 reflections between 6.0 and 2.35 A with F greater than 3 sigma F. This molecule has a sheet consisting of five parallel beta-strands with two alpha-helices on one side of the sheet and three on the other side, and has a (beta alpha)5 structure. The molecule incorporates a substantial insertion in beta 5, as in Anacystis nidulans flavodoxin, which distinguishes these flavodoxins from the short-chain type. The isoalloxazine ring of FMN is sandwiched between the side chains of Trp-56 and Tyr-98, with its C-7 and C-8 methyl groups being exposed to solvent. The phosphate group of FMN is located at the N-terminal end of alpha 1, and forms extensive hydrogen bonds with the loop (T8-T13) between beta 1 and alpha 1 of the protein. Six of the total 11 lysine residues are clustered at the opposing face to the FMN-binding site, while about two-thirds of the total 35 acidic residues are located in the half of the molecule which includes the FMN-binding site. Such localization of charged residues produces a dipole within the molecule, which may be important in its recognition of the other proteins participating in electron transfer reactions.