8 research outputs found
Exploring the Active Site Structure of a Photoreceptor Protein by Raman Optical Activity
We have developed a near-infrared excited Raman optical
activity
(ROA) spectrometer and report the first measurement of near-infrared
ROA spectra of a light-driven proton pump, bacteriorhodopsin. Our
results demonstrate that a near-infrared excitation enables us to
measure the ROA spectra of the chromophore within a protein environment.
Furthermore, the ROA spectra of the <i>all</i>-<i>trans</i>, 15-<i>anti</i> and 13-<i>cis</i>, 15-<i>syn</i> isomers differ significantly, indicating a high structural
sensitivity of the ROA spectra. We therefore expect that future applications
of the near-infrared ROA will allow the experimental elucidation of
the active site structures in other proteins as well as reaction intermediates
N-Terminal Truncation Does Not Affect the Location of a Conserved Tryptophan in the BLUF Domain of AppA from <i>Rhodobacter sphaeroides</i>
The flavin-binding BLUF domains are a class of blue-light
receptors,
and AppA is a representative of this family. Although the crystal
and solution structures of several BLUF domains have already been
obtained, there is a key uncertainty regarding the position of a functionally
important tryptophan (Trp104 in AppA). In the first crystal structure
of an N-terminally truncated BLUF domain of AppA133 (residues 17–133),
Trp104 was found in close proximity to flavin (Trp<sub>in</sub>),
whereas in a subsequent structure with an intact N-terminus AppA126
(residues 1–126), Trp104 was exposed to the solvent (Trp<sub>out</sub>). A recent study compared spectroscopic properties of AppA126
and AppA133 and claimed that the Trp<sub>in</sub> conformation is
an artifact of N-terminal truncation in AppA133. In this study, we
compared the flavin vibrational spectra of AppA126 and AppA133 by
using near-infrared excited Raman spectroscopy. In addition, the conformations
as well as the environments of Trp104 were directly monitored by ultraviolet
resonance Raman spectroscopy. These studies demonstrate that the N-terminal
truncation does not induce the conformational switch between Trp<sub>in</sub> and Trp<sub>out</sub>
Raman Optical Activity Probing Structural Deformations of the 4‑Hydroxycinnamyl Chromophore in Photoactive Yellow Protein
Many biological cofactors, such as
light-absorbing chromophores
in photoreceptors, contain a π-electron system and are planar
molecules. These cofactors are, however, usually nonplanar within
a protein environment, and such structural distortions have been shown
to be functionally important. Because the nonplanar structure makes
the molecule chiral, Raman optical activity (ROA) provides a wealth
of stereochemical information about the structural and conformational
details of cofactors. The present study applied a near-infrared excited
ROA to photoactive yellow protein, a blue light receptor. We successfully
obtained the ROA spectra of the 4-hydroxycinnamyl chromophore embedded
in a protein environment. Furthermore, calculations of the ROA spectra
utilizing density functional theory provide detailed structural information,
such as data on out-of-plane distortions of the chromophore. The structural
information obtained from the ROA spectra includes the positions of
hydrogen atoms, which are usually not detected in the crystal structures
of biological samples
Raman Optical Activity Reveals Carotenoid Photoactivation Events in the Orange Carotenoid Protein in Solution
The
orange carotenoid protein (OCP) plays an important role in
photoprotection in cyanobacteria, which is achieved by the photoconversion
from the orange dark state (OCP<sup>O</sup>) to the red active state
(OCP<sup>R</sup>). Using Raman optical activity (ROA), we studied
the conformations of the carotenoid chromophore in the active sites
of OCP<sup>O</sup> and OCP<sup>R</sup>. This ROA measurement directly
observed the chromophore conformation of native OCP in solution, and
the measurement of OCP<sup>R</sup> first demonstrated the ROA spectroscopy
for the transient species. For OCP<sup>O</sup>, the spectral features
of ROA were mostly reproduced by the quantum chemical calculation
based on the crystal structure of the OCP. Within the spatial resolution
(∼2 Å), a slight modification of the polyene-chain distortion
improved the agreement between the observed and calculated ROA spectra.
While the crystal structure of OCP<sup>R</sup> is not available, the
ROA spectrum of OCP<sup>R</sup> was reproduced by using the crystal
structure of red carotenoid protein (RCP), an OCP<sup>R</sup> proxy.
The present results showed that the chromophore conformations in the
crystal structures of OCP and RCP hold true for OCP<sup>O</sup> and
OCP<sup>R</sup> in solution. Particularly, ROA spectroscopy of the
native OCP<sup>R</sup> provides a direct support for the 12 Ã…
translocation of chromophore in the photoactivation, which was proposed
by X-ray crystallography using RCP [R. L. Leverenz, M. Sutter, et
al. <i>Science</i> <b>2015</b>, 348, 1463–1466]
Active Site Structure of Photoactive Yellow Protein with a Locked Chromophore Analogue Revealed by Near-Infrared Raman Optical Activity
Many biological cofactors, such as
light-absorbing chromophores
in photoreceptors, are intrinsically planar molecules. A protein environment,
however, causes structural distortions of the cofactor, and such structural
changes can lead to a modulation of chemical properties of the cofactor
to maximize its biological activity. Here, we investigate the active
site structure of photoactive yellow protein (PYP), a blue light photoreceptor
that contains a <i>p</i>-coumaric acid (<i>p</i>CA) chromophore, by a near-infrared excited Raman optical activity
(ROA). Specifically, we measured the ROA spectra of PYP, whose chromophore
is replaced with a locked <i>p</i>CA analogue. Furthermore,
we show that a spectral analysis based on quantum mechanical/molecular
mechanical (QM/MM) calculations of the whole protein molecule is useful
to obtain structural information from the observed ROA spectra. The
use of the near-infrared ROA combined with QM/MM calculations is a
novel and generally applicable spectroscopic tool to study the chromophore
distortions within a protein environment
Spectroscopic Validation of Crystallographic Structures of a Protein Active Site by Chiroptical Spectroscopy
Out-of-plane distortions of a cofactor molecule in a
protein active
site are functionally important, and in photoreceptors, it has been
proposed that they are crucial for spectral tuning and energy storage
in photocycle intermediates. However, these subtle structural features
are often beyond the grasp of structural biology. This issue is strikingly
exemplified by photoactive yellow protein: its 14 independently determined
crystal structures exhibit considerable differences in the dihedral
angles defining the chromophore geometry, even though most of these
are at excellent resolution. Here we developed a strategy to verify
cofactor distortions in crystal structures by using quantum chemical
calculations and chiroptical spectroscopy, particularly Raman optical
activity and electronic circular dichroism spectroscopies. Based on
this approach, we identify seven crystal structures with the chromophore
geometries inconsistent with the experimentally observed data. The
strategy implemented here promises to be widely applicable to uncovering
cofactor distortions at active sites and to studies of reaction intermediates
Hydrogen Bonding Environment of the N3–H Group of Flavin Mononucleotide in the Light Oxygen Voltage Domains of Phototropins
The light oxygen
voltage (LOV) domain is a flavin-binding blue-light
receptor domain, originally found in a plant photoreceptor phototropin
(phot). Recently, LOV domains have been used in optogenetics as the
photosensory domain of fusion proteins. Therefore, it is important
to understand how LOV domains exhibit light-induced structural changes
for the kinase domain regulation, which enables the design of LOV-containing
optogenetics tools with higher photoactivation efficiency. In this
study, the hydrogen bonding environment of the N3–H group of
flavin mononucleotide (FMN) of the LOV2 domain from <i>Adiantum</i> neochrome (neo) 1 was investigated by low-temperature Fourier transform
infrared spectroscopy. Using specifically <sup>15</sup>N-labeled FMN,
[1,3-<sup>15</sup>N<sub>2</sub>]ÂFMN, the N3–H stretch was identified
at 2831 cm<sup>–1</sup> for the unphotolyzed state at 150 K,
indicating that the N3–H group forms a fairly strong hydrogen
bond. The N3–H stretch showed temperature dependence, with
a shift to lower frequencies at ≤200 K and to higher frequencies
at ≥250 K from the unphotolyzed to the intermediate states.
Similar trends were observed in the LOV2 domains from <i>Arabidopsis</i> phot1 and phot2. By contrast, the N3–H stretch of the Q1029L
mutant of neo1-LOV2 and neo1-LOV1 was not temperature dependent in
the intermediate state. These results seemed correlated with our previous
finding that the LOV2 domains show the structural changes in the β-sheet
region and/or the adjacent Jα helix of LOV2 domain, but that
such structural changes do not take place in the Q1029L mutant or
neo1-LOV1 domain. The environment around the N3–H group was
also investigated