The photocycles of biological blue-light photoreceptors.
Living organisms employ photoreactive proteins for two main purposes: energy storage, and sensory functions. Presently, six families of biological photosensors are known.  Three of them, the rhodopsins, phytochromes, and xanthopsins, make use of a cis/trans isomerization of the photoreactive cofactor (retinal, tetrapyrrole, or p-coumaric acid). Since 1997, three further classes of photoreceptor protein domains have been found. They all contain a flavin cofactor as chromophore, which makes them sensitive for blue light only. The cryptochromes (CRY) [2,3] are frequently associated with circadian rhythms and contain flavin adenine dinucleotide (FAD), as do the BLUF (blue light sensing using flavin) proteins.  The third class are the phototropins,  which are composed of (usually two) LOV (light oxygen voltage sensitive) domains and a kinase domain. Each LOV domain non-covalently binds a flavin mononucleotide (FMN). When we began working on these systems in 2001, the reaction mechanism of all these photoreceptors was unknown.
In the meantime, we have characterized in detail [7-18] the photocycle of two LOV domains, both from the phototropin of the green alga chlamydomonas reinhardtii (C.r.). The chemical nature of two short lived intermediates and the long lived signalling state were identified by transient absorption spectroscopy, employing several point mutants. Since flavoproteins are known to be involved in many redox reactions, a change of the redox state was expected as the signalling event. Quite unexpectedly, however, a reaction through the FMN triplet state was observed, leading to the covalent adduct of FMN to a cystein residue at C4a of the flavin. The details of the mechanism are still under debate.
Left: X-Ray structure of LOV1 of C.r. from ; Right: Photocycle of LOV1 of C.r. .
Formation of the FMN-cysteine adduct makes the beginning of the signaling chain, which involves activation of a kinase and autophosphorylation of the phototropin. The presence of two different LOV domains in the phototropin suggests that both play a (probably different) role in the signal transduction, and that the interaction between these domains is modified by the adduct formation. Biochemical studies indicate that LOV domains like to form dimers, and that the monomer/dimer ratio changes upon irradiation . When the photoactive cysteine is replaced by glycin, a photoreduction leading to the neutral FMNH* radical is observed as the first reaction following formation of the triplet. The electron donor can be EDTA  or an aliphatic mercaptane. With CH3-SH a further intermediate is observed that has the same absorption spectrum as the natural photoadduct [17,18]. Based on these findings, a model for the function of the full phototropin protein is proposed.
In addition to the studies on LOV domains we also performed studies on some cryptochromes [19,20], BLUF proteins, and rhodopsins.
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