Biology Reference
In-Depth Information
In 2003, before any functional studies had been initiated on the OCP,
Kerfeld, Sawaya, et al. (2003)
reported the crystal structure of the
A. maxima
OCP isolated by Krogmann et al. More recently, Kerfeld's group has struc-
turally characterized the
Synechocystis
OCP (
Wilson, Kinney, et al. 2010
).
The OCP structures (
Fig. 1.1
), presumably both in the resting form, are
essentially identical, composed of two domains: an α-helical N-terminal
domain (residues ∼19-165) that is unique to cyanobacteria (Pfam 09150)
and an α/β C-terminal domain (residues ∼190-317) that is a member of
the widely distributed nuclear transport factor 2 (NTF2) fold superfamily
(Pfam 02136). The two domains are joined by a long (∼25 amino acids) flex-
ible linker. The multiple structures now available also suggest that there are
structurally conserved water molecules between domains and surrounding
the carotenoid. The functional significance of these is unknown, but they
could facilitate protein conformational changes. The carotenoid, 3-hECN,
spans both domains of the protein (
Kerfeld, Sawaya, et al., 2003
;
Wilson,
Kinney, et al., 2010
). hECN is a keto-carotenoid with a conjugated chain
of 11 carbon-carbon double bonds; in the OCP structures, these are in
all-trans configuration (
Kerfeld, Sawaya, et al., 2003
;
Polivka, Kerfeld, et al.,
2005
;
Wilson, Punginelli, et al., 2008
). The hECN keto- (carbonyl) group
is hydrogen bonded to absolutely conserved (
Fig. 1.2
) Tyr 201 and Trp 288
residues within an otherwise hydrophobic pocket of the C-terminal domain
(
Kerfeld, Sawaya, et al., 2003
;
Wilson, Kinney, et al., 2010
;
Wilson, Pungi-
nelli, et al., 2011
). The hydroxyl ring of hECN in the N-terminal domain is
nestled within a group of conserved aromatic residues (Trp 41, Tyr 44 and
Trp 110) (
Kerfeld, Sawaya, et al., 2003
;
Wilson, Kinney, et al., 2010
;
Wilson,
Punginelli, et al., 2011
).
3.2. Clues to the Structural Basis of Photoactivity
In the crystal structures of the OCP, the carotenoid is only sparingly surface
exposed, suggesting that some degree of structural rearrangement would be
necessary for it to be competent for energy transfer from the phycobilisome.
This hypothesis is supported by light-minus-dark differential FTIR spectra
showing that light absorption the by OCP induces conformational changes
in the protein. These changes were interpreted to correspond to a less-rigid
helical structure (loosening of α-helices) and a compaction (strengthening)
of the β-sheet domain with additional loop changes (
Wilson, Gwizdala,
et al., 2012
;
Wilson, Punginelli, et al., 2008
). These changes are essential for
OCP interaction with the phycobilisome (
Gwizdala, Wilson, et al., 2011
;
Wilson, Gwizdala, et al., 2012
).
