A two-dimensional 1H NMR study on <em>Megasphaera elsdenii</em> flavodoxin in the reduced state.

Assignments for the 137 amino acid residues of Megasphaera elsdenii flavodoxin in the reduced state have been made using the sequential resonance assignment procedure. Several hydroxyl and sulfhydryl protons were observed at 41 &#176;C at pH 8.3. Spin systems were sequentially assigned using phase-sensitive two-dimensional-correlated spectroscopy and phase-sensitive nuclear Overhauser enhancement spectroscopy. Spectra of the protein in H2O and of protein preparations either completely or partly exchanged against &#178;H2O were obtained. Use of the fast electron shuttle between the paramagnetic semiquinone and the diamagnetic hydroquinone state greatly simplified the NMR spectra, making it possible to assign easily the &#185;H resonances of amino acid residues located in the immediate neighbourhood of the isoalloxazine ring. The majority of the nuclear Overhauser effect contacts between the flavin and the apoprotein correspond to the crystal structure of the flavin domain of Clostridium MP flavodoxin, but differences are also observed. The assignments provide the basis for the structure determination of M. elsdenii flavodoxin in the reduced state as well as for assigning the resonances of the oxidized flavodoxin. [ABSTRACT FROM AUTHOR]