Redox properties of LOV domains: chemical versus photochemical reduction, and influence on the photocycle

Abstract

LOV (light-oxygen-voltage-sensitive) domains comprise the light-sensitive parts of many blue light photoreceptor proteins. Photo-excitation of the chromophore FMN in these LOV domains leads to formation of a covalent adduct between FMN and a cysteine residue. So far, the electronically excited singlet and triplet states of FMN have been identified as the only intermediates in the photocycles of LOV domains from several organisms. Since many flavoproteins are redox-active, however, the photocycles of LOV domains might involve other redox states of FMN, and might be controlled by the external redox potential. Here we report on the redox properties of the LOV1 domain from phototropin of the green alga Chlamydomonas reinhardtii. By equil.-redox spectropotentiometry a redox potential [Efq/fhq (flavoquinone/flavohydroquinone)] of -290 mV vs. the normal hydrogen electrode (NHE) was detd. for the wild-type domain (LOV1-wt). A similar value of -280 mV was found for the mutant LOV1-C57G, in which the photoreactive cysteine is replaced by glycine. The recovery kinetics (photoadduct->ground state) in the photocycle of LOV1-wt are not influenced by a redox potential in the range between +500 and -260 mV vs. NHE. No flavosemiquinone could be generated by chem. redn. with sodium dithionite. However, photoredn. of LOV1-C57G with EDTA leads exclusively to the flavosemiquinone. This semiquinone is stable against disproportionation, and the photoredn. is not mediated by free FMN.