Biology Reference
In-Depth Information
not play a role in photoactivation. When hECN and zeaxanthin are
absent, ECN (lacking the hydroxyl group) binds to the apo-protein and
the ECN-OCP is both photoactive and able to induce the photoprotec-
tive mechanism (
Punginelli, Wilson, et al., 2009
;
Wilson, Punginelli, et al.,
2011
). No differences in the rates of photoactivation or in the stability of
the red forms of hECN-OCP and ECN-OCP were detected. While the
hydroxyl group is not needed for photoactivation, the interaction of the
hECN hydroxyl ring with aromatic amino acids is known to be essential
for the formation and/or stabilization of OCP
r
. When Tyr 44 or Trp 110
are replaced by nonaromatic amino acids, the OCP remains orange under
illumination and does not induce photoprotection (
Wilson, Punginelli,
et al., 2008
;
Wilson, Punginelli, et al., 2011
).
Light absorption induces changes in the position and conformation of
the keto-carotenoid; its apparent conjugation length increases by about
one conjugated bond and adopts a less distorted, more planar structure
(
Wilson, Punginelli, et al., 2008
). Modifications of the excited states of
the carotenoid are associated with these conformational changes (
Berera,
van Stokkum, et al., 2012
). Binding of hECN to the protein is respon-
sible for shortening of the S1 lifetime (from 6.5 ps in solution to 3.3 ps)
in the OCP and for the stabilization of an intramolecular charge-transfer
(ICT) state, making the hECN-OCP a more effective energy dissipator
than free carotenoid (
Berera, van Stokkum, et al., 2012
;
Polivka, Kerfeld,
et al., 2005
). It was also shown that the hydrogen bonds between the car-
bonyl group of the carotenoid and the C-terminal domain of the OCP
play a crucial role in modulating and stabilizing the ICT state (
Berera,
van Stokkum, et al., 2012
;
Chábera, Durchan, et al., 2010
;
Polivka, Ker-
feld, et al., 2005
). In the excited state of OCP
r
, the contribution of the
ICT state is stronger than in OCP
o
(
Berera, van Stokkum, et al., 2012
),
increasing the potential for the carotenoid to be a mediator in the energy
dissipation process. The efficiency of energy quenching increases with
the strength of the ICT state (
Berera, Herrero, et al., 2006
). It was pro-
posed that OCP
r
accepts energy from an excited bilin of the phycobili-
some via its ICT state. The ICT state partially decays to the ground state
and partially to the S1 state, which in turn also decays to the ground
state (
Berera, van Stokkum, et al., 2012
). The lifetimes of the ICT and
S1 states (0.6 and 3.2 ps), which are about three orders of magnitude
shorter than that of the excited bilin, make the hECN a very efficient
quencher. Thus, energy absorbed by the phycobilisomes can be harm-
lessly dissipated as heat.
