Dependence of the proton magnetic resonance spectra on the oxidation state of flavodoxin from Clostridium MP and from Peptostreptococcus elsdenii.
The broadening of protein nuclear magnetic resonances in the spectra of the semiquinone forms of flavodoxins derived from Clostridium MP and Peptostreptococcus elsdenii relative to the resonances in the oxidized and reduced forms is highly selective. Spectra from both species of flavodoxin indicate that conformational differences between the oxidized and fully reduced states are minor and, consequently, the broadening in the semiquinone form is ascribed to the paramagnetic effect of the flavin free radical. The chemical shifts of the paramagnetically broadened lines are used in conjunction with x-ray crystallographic models to assign peaks to amino-acid residues in the proximity of the flavin mononucleotide. Species-dependent differences in the spectra can generally be attributed to differences in amino-acid composition and sequence. The spectra from both species of flavodoxin indicate that there is slow exchange between oxidized and semiquinone forms or reduced and semiquinone forms of the flavodoxins with a limit of k(ex) < 50 sec(-1) for the exchange rate.