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patterns could be one means to propagate the light-responsive signal to the surface of the OCP.
Hydrogen bonding in the OCP is extensive. There are two hydrogen bonds to the keto-oxygen
of the 3
-hydroxyechinenone via invariant C-terminal residues Tyr 203 and Trp 290, as shown in
Figure 1.4b. Tyr 203 is further hydrogen-bonded to the main chain atoms of Leu 207 and Thr 199;
the latter residue is conserved and surface exposed. Trp 290 is hydrogen-bonded to the invariant res-
idues Val 271 and Phe 292; these residues in the strands of the beta-sheet are also surface exposed.
The surface accessibility of the hydrogen-bonded residues poises them to possibly communicate the
status of the chromophore to the surface of the OCP. Similarly, at the hydroxyl terminus of the caro-
tenoid, where it is most solvent accessible, there is a potential for forming a weak hydrogen bond
to the conserved residue Leu 37 which is, in turn, hydrogen-bonded to the main chain of invariant
residues Ala33 and Trp 41. These residues are also surface exposed.
Likewise, in the LOV domains of plants and fungi, light-driven structural changes in the
chromophore result in a hydrogen-bond switch that causes beta-sheet motion and subsequent dis-
placement of a small segment of alpha-helix, which is packed against the beta-sheet in the resting
state (Harper et al. 2003, 2004, Nozaki et al. 2004, Halavaty and Moffat 2007). The hydrogen bond
that is altered is between the l avin mononucleotide chromophore and the side chain of a conserved
Gln, which belongs to the central strand of the LOV beta-sheet. An analogous mechanism is pos-
sible for the OCP via the hydrogen bond between the 3
′
-hydroxyechinenone carbonyl oxygen and
Trp 290; Trp 290 is part of the central strand of the beta-sheet of the OCP's C-terminal domain.
Light-triggered conformational changes of the 3
′
-hydroxyechinenone could alter the strength of this
hydrogen bond. This, as in LOV domains, could inl uence the conformation of the central beta-sheet,
affording signal propagation pathway from the carotenoid to the surface of the OCP. Furthermore,
as in the LOV domains, a short alpha-helix from the N-terminus of the protein interacts with the
central beta-sheet of the OCP, as shown in Figure 1.3a and c. In a mechanism analogous to the signal
triggering in the LOV domain caused by the displacement of this helix (Harper et al. 2003, 2004,
Halavaty and Moffat 2007) light-induced changes in the equilibrium of bound and unbound state of
this N-terminal helix in the OCP could underlie the signaling/quenching switch.
The photoresponse of PYP also involves an “arginine gateway”: altered hydrogen bonding to a
conserved Arg displaces the side chain allowing access to the chromophore (Genick et al. 1997).
The structure of a long-lived PYP
M
intermediate has been determined by millisecond time-resolved
crystallography (Genick et al. 1997). During the bleaching of the protein an arginine gateway opens,
allowing solvent exposure and protonation of the phenolic oxygen. In the OCP, invariant Arg 155
is found at the interface of the N- and C-terminal domains, as shown in Figures 1.3b and 1.4b,
occluding solvent access to the carotenoid. The alteration of the disposition of this residue in the
OCP would, as in PYP, increase substantially the solvent accessibility of the 3
′
′
-hydroxyechinenone
molecule.
At the time of its elucidation, one of the most intriguing features of the OCP structure was the
preponderance of Met residues with their thioether groups oriented toward the carotenoid. Many
of these are absolutely conserved among the primary structures of the OCP. There are several
potential roles for the Met side chains in the function of the OCP. The potential for the oxidation
of Met residues could confer a protective function for the carotenoid, by intercepting reactive oxy-
gen species (via oxidation to methioinine sulfoxide and methionine sulfone) that would otherwise
damage the pigment. All of the conserved Met residues make at least three hydrogen bonds to
residues that are surface exposed. Of the conserved N-terminal domain Met residues (47, 61, 74,
and 83), only Met 83 is buried within the protein. In contrast, Met 286, the single conserved Met
in the C-terminal domain, is entirely buried. Alternatively, the Met residues may function in signal
propagation, perhaps through bound water molecules. The polarizability of the sulfur atom and
the distinctive geometries of Met observed in its interaction with a nucleophile and an electrophile
provide structural versatility that could facilitate signaling. The structural basis of function in the
BLUF domain offers an example of the role of Met residues in signaling through the protein (Jung
et al. 2006). A comparison of the BLUF domain in both the dark adapted and the photoexcited,
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