Visible radiation effects on flavocytochrome b(2) in dilute aqueous solution: a steady-state and laser flash photolysis study

Irradiation of flavocytochrome b(2) by visible radiation at 450 nm in dilut e aqueous solution is found to have a devastating effect not only on its ac tivity but also on the important flavin mononucleotide (FMN) constituents. The active site and the substrate binding site are also found to be largely modified on exposure to visible radiation. This has a telling effect on th e constituent aromatic amino acids, tryptophan and tyrosine, and therefore justifies the role of FMN as a very potent photosensitizer. Partial unfoldi ng of the irradiated enzyme molecule is also observed. Damage is much great er in deaerated conditions, which indicates that molecular oxygen plays a p rotecting role in this particular system. The inactivation is mediated thro ugh rapid electron transfer from tryptophan and tyrosine to excited flavin, forming flavin semiquinone and tryptophanyl and tyrosinyl radicals, which in turn cause permanent damage at the molecular level. (C) 1998 Elsevier Sc ience S.A. All rights reserved.