Oxidation-reduction properties of phytoflavin, a flavoprotein from blue-green algae

Abstract

Phytoflavin from the blue-green alga Anacystis nidulans is a protein which can replace the low potential electron carrier, ferredoxin, in photosynthetic electron transport by isolated chloroplasts. Phytoflavin contains one molecule of FMN per molecule of protein, and was found to accept two reducing equivalents during reduction. It forms a stable semiquinone and can be reduced in two stages. Molar extinction coefficients have been determined for the absorption spectra of the three oxidation states of the protein. Oxidation-reduction potentials have been determined for each stage in reduction over a range of pH values. At pH 7.0, the potentials are −0.221 V and −0.447 V. The second potential is very low for a flavoprotein, and lower than the potentials determined for ferredoxin from A. nidulans and higher plants. The potentials determined provide a quantitative measure of the stability of the semiquinone of phytoflavin. The results presented help to explain the mechanism of reduction and oxidation and how phytoflavin can substitute for ferredoxin in photosynthetic electron transport by utilization of the oxidation-reduction couple, phytoflavin semiquinone/hydroquinone.