6 research outputs found
New insights into the photochemistry of carotenoid spheroidenone in light-harvesting complex 2 from the purple bacterium Rhodobacter sphaeroides
Light-harvesting complex 2 (LH2) from the
semi-aerobically grown purple phototrophic bacterium
Rhodobacter sphaeroides was studied using optical (static
and time-resolved) and resonance Raman spectroscopies.
This antenna complex comprises bacteriochlorophyll
(BChl) a and the carotenoid spheroidenone, a ketolated
derivative of spheroidene. The results indicate that the
spheroidenone-LH2 complex contains two spectral forms
of the carotenoid: (1) a minor, ‘‘blue’’ form with an S2
(11
Bu
?) spectral origin band at 522 nm, shifted from the
position in organic media simply by the high polarizability
of the binding site, and (2) the major, ‘‘red’’ form with the
origin band at 562 nm that is associated with a pool of
pigments that more strongly interact with protein residues,
most likely via hydrogen bonding. Application of targeted
modeling of excited-state decay pathways after carotenoid
excitation suggests that the high (92%) carotenoid-to-BChl
energy transfer efficiency in this LH2 system, relative to
LH2 complexes binding carotenoids with comparable
double-bond conjugation lengths, derives mainly from
resonance energy transfer from spheroidenone S2 (11
Bu
?)
state to BChl a via the Qx state of the latter, accounting for
60% of the total transfer. The elevated S2 (11
Bu
?) ? Qx
transfer efficiency is apparently associated with substantially
decreased energy gap (increased spectral overlap)
between the virtual S2 (11
Bu
?) ? S0 (11
Ag
-) carotenoid
emission and Qx absorption of BChl a. This reduced
energetic gap is the ultimate consequence of strong carotenoid–protein
interactions, including the inferred hydrogen
bondin
Participation of Glutamate-333 of the D1 Polypeptide in the Ligation of the Mn 4 CaO 5 Cluster in Photosystem II
In the 1.9 Ã… structural model of photosystem II (PDB: 3ARC), the amino acid residue Glu333 of the D1 polypeptide coordinates to the oxygen-evolving Mn4CaO5 cluster. This residue appears to be highly significant in that it bridges the two Mn ions (MnB3 a
The D1-D61N Mutation in <i>Synechocystis</i> sp. PCC 6803 Allows the Observation of pH-Sensitive Intermediates in the Formation and Release of O<sub>2</sub> from Photosystem II
The active site of photosynthetic water oxidation by
Photosystem
II (PSII) is a manganese–calcium cluster (Mn<sub>4</sub>CaO<sub>5</sub>). A postulated catalytic base is assumed to be crucial. CP43-Arg357,
which is a candidate for the identity of this base, is a second-sphere
ligand of the Mn<sub>4</sub>–Ca cluster and is located near
a putative proton exit pathway, which begins with residue D1-D61.
Transient absorption spectroscopy and time-resolved O<sub>2</sub> polarography
reveal that in the D1-D61N mutant, the transfer of an electron from
the Mn<sub>4</sub>CaO<sub>5</sub> cluster to Y<sub>Z</sub><sup>OX</sup> and O<sub>2</sub> release during the final step of the catalytic
cycle, the S<sub>3</sub>–S<sub>0</sub> transition, proceed
simultaneously but are more dramatically decelerated than previously
thought (<i>t</i><sub>1/2</sub> of up to ∼50 ms vs
a <i>t</i><sub>1/2</sub> of 1.5 ms in the wild type). Using
a bare platinum electrode to record the flash-dependent yields of
O<sub>2</sub> from mutant and wild-type PSII has allowed the observation
of the kinetics of release of O<sub>2</sub> from extracted thylakoid
membranes at various pH values and in the presence of deuterated water.
In the mutant, it was possible to resolve a clear lag phase prior
to the appearance of O<sub>2</sub>, indicating formation of an intermediate
before the onset of O<sub>2</sub> formation. The lag phase and the
photochemical miss factor were more sensitive to isotope substitution
in the mutant, indicating that proton efflux in the mutant proceeds
via an alternative pathway. The results are discussed in comparison
with earlier results obtained from the substitution of CP43-Arg357
with lysine and in regard to hypotheses concerning the nature of the
final steps in photosynthetic water oxidation. These considerations
led to the conclusion that proton expulsion during the initial phase
of the S<sub>3</sub>–S<sub>0</sub> transition starts with the
deprotonation of the primary catalytic base, probably CP43-Arg357,
followed by efficient proton egress involving the carboxyl group of
D1-D61 in a process that constitutes the lag phase immediately prior
to O<sub>2</sub> formation chemistry