7 research outputs found
Color Tuning in Binding Pocket Models of the Chlamydomonas-Type Channelrhodopsins
We examined the shift of absorption maxima between the chlamydomonas-type channelrhodopsins (ChRs) and bacteriorhodopsin (BR). Starting from the BR X-ray structure, we modeled the color tuning in the binding pockets of the ChRs by mutating up to 28 amino acids in the vicinity of the chromophore. By applying the efficient self-consistent charge density functional tight binding (SCC-DFTB) method in a quantum mechanical/molecular mechanical (QM/MM) framework, including explicit polarization and calculating excitation energies with the semiempirical OM2/MRCI method and the ab initio SORCI method, we have shown that multiple mutations in the binding pocket of BR causes large hypsochromic shifts that are of the same order as the experimentally observed shifts of the absorption maxima between BR and the ChRs. This study further demonstrates that mutations in the proximity of the Schiff base and complex counterion lead to a stronger but more flexible interaction with the retinal, which could serve as a possible explanation for the spectral patterns found in the ChRs
Key Residues for the Light Regulation of the Blue Light-Activated Adenylyl Cyclase from <i>Beggiatoa</i> sp.
Photoactivated adenylyl cyclases
are powerful tools for optogenetics
and for investigating signal transduction mechanisms in biological
photoreceptors. Because of its large increase in enzyme activity in
the light, the BLUF (blue light sensor using flavin adenine dinucleotide)-activated
adenylyl cyclase (bPAC) from <i>Beggiatoa</i> sp. is a highly
attractive model system for studying BLUF domain signaling. In this
report, we studied the influence of site-directed mutations within
the BLUF domain on the light regulation of the cyclase domain and
determined key elements for signal transduction and color tuning.
Photoactivation of the cyclase domain is accomplished via strand β5
of the BLUF domain and involves the formation of helical structures
in the cyclase domain as assigned by vibrational spectroscopy. In
agreement with earlier studies, we observed severely impaired signaling
in mutations directly on strand β5 as well as in mutations affecting
the hydrogen bond network around the flavin. Moreover, we identified
a bPAC mutant with red-shifted absorbance and a decreased dark activity
that is highly valuable for long-term optogenetic experiments. Additionally,
we discovered a mutant that forms a stable neutral flavin semiquinone
radical in the BLUF domain and surprisingly exhibits an inversion
of light activation
Microbial and Animal Rhodopsins: Structures, Functions, and Molecular Mechanisms
Microbial
and Animal Rhodopsins: Structures, Functions,
and Molecular Mechanism
Unfolding of the C‑Terminal Jα Helix in the LOV2 Photoreceptor Domain Observed by Time-Resolved Vibrational Spectroscopy
Light-triggered
reactions of biological photoreceptors have gained
immense attention for their role as molecular switches in their native
organisms and for optogenetic application. The light, oxygen, and
voltage 2 (LOV2) sensing domain of plant phototropin binds a C-terminal
Jα helix that is docked on a β-sheet and unfolds upon
light absorption by the flavin mononucleotide (FMN) chromophore. In
this work, the signal transduction pathway of LOV2 from Avena sativa was investigated using time-resolved
infrared spectroscopy from picoseconds to microseconds. In D<sub>2</sub>O buffer, FMN singlet-to-triplet conversion occurs in 2 ns and formation
of the covalent cysteinyl-FMN adduct in 10 μs. We observe a
two-step unfolding of the Jα helix: The first phase occurs concomitantly
with Cys-FMN covalent adduct formation in 10 μs, along with
hydrogen-bond rupture of the FMN C4O with Gln-513, motion
of the β-sheet, and an additional helical element. The second
phase occurs in approximately 240 μs. The final spectrum at
500 μs is essentially identical to the steady-state light-minus-dark
Fourier transform infrared spectrum, indicating that Jα helix
unfolding is complete on that time scale
Photoadduct Formation from the FMN Singlet Excited State in the LOV2 Domain of Chlamydomonas reinhardtii Phototropin
The
two light, oxygen, and voltage domains of phototropin are blue-light
photoreceptor domains that control various functions in plants and
green algae. The key step of the light-driven reaction is the formation
of a photoadduct between its FMN chromophore and a conserved cysteine,
where the canonical reaction proceeds through the FMN triplet state.
Here, complete photoreaction mapping of CrLOV2 from Chlamydomonas reinhardtii phototropin and AsLOV2
from Avena sativa phototropin-1 was
realized by ultrafast broadband spectroscopy from femtoseconds to
microseconds. We demonstrate that in CrLOV2, a direct photoadduct
formation channel originates from the initially excited singlet state,
in addition to the canonical reaction through the triplet state. This
direct photoadduct reaction is coupled by a proton or hydrogen transfer
process, as indicated by a significant kinetic isotope effect of 1.4
on the fluorescence lifetime. Kinetic model analyses showed that 38%
of the photoadducts are generated from the singlet excited state
Hydrogen Bond Switching among Flavin and Amino Acids Determines the Nature of Proton-Coupled Electron Transfer in BLUF Photoreceptors
BLUF domains are flavin-binding photoreceptors that can
be reversibly
switched from a dark-adapted state to a light-adapted state. Proton-coupled
electron transfer (PCET) from a conserved tyrosine to the flavin that
results in a neutral flavin semiquinone/tyrosyl radical pair constitutes
the photoactivation mechanism of BLUF domains. Whereas in the dark-adapted
state PCET occurs in a sequential fashion where electron transfer
precedes proton transfer, in the light-adapted state the same radical
pair is formed by a concerted mechanism. We propose that the altered
nature of the PCET process results from a hydrogen bond switch between
the flavin and its surrounding amino acids that preconfigures the
system for proton transfer. Hence, BLUF domains represent an attractive
biological model system to investigate and understand PCET in great
detail
Light–Dark Adaptation of Channelrhodopsin Involves Photoconversion between the all-<i>trans</i> and 13-<i>cis</i> Retinal Isomers
Channelrhodopsins (ChR) are light-gated
ion channels of green algae
that are widely used to probe the function of neuronal cells with
light. Most ChRs show a substantial reduction in photocurrents during
illumination, a process named “light adaptation”. The
main objective of this spectroscopic study was to elucidate the molecular
processes associated with light–dark adaptation. Here we show
by liquid and solid-state nuclear magnetic resonance spectroscopy
that the retinal chromophore of fully dark-adapted ChR is exclusively
in an all<i>-trans</i> configuration. Resonance Raman (RR)
spectroscopy, however, revealed that already low light intensities
establish a photostationary equilibrium between all<i>-trans</i>,15<i>-anti</i> and 13<i>-cis</i>,15-<i>syn</i> configurations at a ratio of 3:1. The underlying photoreactions
involve simultaneous isomerization of the C(13)C(14) and C(15)N
bonds. Both isomers of this DA<sub>app</sub> state may run through
photoinduced reaction cycles initiated by photoisomerization of only
the C(13)C(14) bond. RR spectroscopic experiments further
demonstrated that photoinduced conversion of the apparent dark-adapted
(DA<sub>app</sub>) state to the photocycle intermediates P500 and
P390 is distinctly more efficient for the all-<i>trans</i> isomer than for the 13-<i>cis</i> isomer, possibly because
of different chromophore–water interactions. Our data demonstrating
two complementary photocycles of the DA<sub>app</sub> isomers are
fully consistent with the existence of two conducting states that
vary in quantitative relation during light–dark adaptation,
as suggested previously by electrical measurements