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Zusammenfassung
Phot photoreceptors make up an important protein family regulating biol. processes in response to blue light. They contain two light, oxygen, and voltage sensitive (LOV) domains and a serine/threonine kinase domain. Both LOV domains noncovalently bind a FMN. Upon absorption of blue light, the LOV domains undergo a photocycle, transiently forming a covalent adduct of a cysteine residue and the FMN ...
Zusammenfassung
Phot photoreceptors make up an important protein family regulating biol. processes in response to blue light. They contain two light, oxygen, and voltage sensitive (LOV) domains and a serine/threonine kinase domain. Both LOV domains noncovalently bind a FMN. Upon absorption of blue light, the LOV domains undergo a photocycle, transiently forming a covalent adduct of a cysteine residue and the FMN (LOV-390). The mechanism of formation of this flavin-thiol adduct is still unclear. The authors studied a mutant of the LOV1 domain from the green alga Chlamydomonas reinhardtii with a methionine replacing the reactive cysteine 57 (C57M). As in the wild type, irradn. leads to formation of a photoadduct, which, however, is irreversibly converted into a red absorbing species, C57M-675. On the basis of spectroscopic results and the 2.1 .ANG. resoln. crystal structure, this highly unusual FMN species was assigned to a neutral flavin radical covalently attached to the apoprotein at the N(5) position. In contrast to other flavoprotein neutral radicals, C57M-675 is stable even under aerobic or denaturing conditions. Pathways for the photoinduced formation of the adduct are discussed for the C57M mutant as well as the wild-type LOV1 domain.