such as "Introduction", "Conclusion"..etc
generation of a light signal by photoreceptor proteins relies on the
formation of a long-lived signaling state, where initial local
structural changes resulting from light absorption by the chromophore
are relayed to the protein surface, which undergoes specific
alterations that may be sensed by signaling partner proteins. In
“traditional” photoreceptors such as the rhodopsins, phytochromes and
the xanthopsins, the initial local change is achieved by rapid E/Z
isomerization photochemistry of their chromophore, which in turn
initiates larger conformational changes by creating steric conflicts,
or by charge movements among chromophore and apoprotein (1).
In the absence of isomerizable groups, flavin-binding photoreceptors
rely on different modes of light activation and thus provide an
alternative window on how photon absorption may be coupled to
biological sensory function. In the phototropins, for instance,
signaling occurs through the light-induced formation of a covalent
adduct between the flavin chromophore and a conserved cysteine in
light, oxygen, or voltage (LOV) domains, which eventually leads to
autophosphorylation of a C-terminal Ser/Thr kinase (8, 9).
In analogy with the related photolyases, cryptochromes are thought to
function by means of light-induced electron transfer (ET) reactions (10), but at present their mode of action remains largely obscure.
Recently, crystal and solution structures of several BLUF domains were obtained (11–14).
The BLUF domain shows a ferredoxin-like fold consisting of a
five-stranded β-sheet with two α-helices packed on one side of the
sheet, with the noncovalently bound isoalloxazine ring of flavin
adenine dinucleotide (FAD) positioned between the two α-helices. Fig. 1 shows a close-up of the FAD-binding pocket of the AppA BLUF domain (11).
FAD is involved in an extensive hydrogen-bond network with residues
lining the FAD binding pocket, including a highly conserved tyrosine
(Tyr) and glutamine (Gln). Upon illumination with blue light, BLUF
domains show a characteristic spectral red-shift of their absorption
spectrum by ≈10 nm, which is a unique feature of this photoreceptor
family and is thought to correspond to the signaling state (5).
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