Calcium, Ammonia, Redox-Active Tyrosine YZ, and Proton-Coupled
Electron Transfer in the Photosynthetic Oxygen-Evolving Complex
Version 2 2018-04-14, 13:13
Version 1 2017-04-14, 12:49
Posted on 2018-04-14 - 13:13
A redox-active tyrosine, YZ (Y161
in the D1 polypeptide), is essential
in photosystem II (PSII), which conducts photosynthetic oxygen evolution.
On each step of the light-driven oxygen evolving reaction, YZ radical
is formed by a chlorophyll cation radical. YZ radical is then reduced
by a Mn4CaO5 cluster in a proton coupled electron
transfer (PCET) reaction. YZ is hydrogen bonded to His190-D1 and to
water molecules in a hydrogen-bonding network, involving calcium.
This network is sensitive to disruption with ammonia and to removal
and replacement of calcium. Only strontium supports activity. Here,
we use electron paramagnetic resonance (EPR) spectroscopy to define
the influence of ammonia treatment, calcium removal, and strontium/barium
substitution on YZ radical PCET at two pH values. A defined oxidation
state of the metal cluster (S2) was trapped by illumination
at 190 K. The net reduction and protonation of YZ radical via PCET
were monitored by EPR transients collected after a 532 nm laser flash.
At 190 K, YZ radical cannot oxidize the Mn4CaO5 cluster and decays on the seconds time scale by recombination with
QA–. The overall decay half-time and
biexponential fits were used to analyze the results. The reaction
rate was independent of pH in control, calcium-reconstituted PSII
(Ca-PSII). At pH 7.5, the YZ radical decay rate decreased in calcium-depleted
(CD-PSII) and barium/strontium-reconstituted PSII (Ba-PSII, Sr-PSII),
relative to Ca-PSII. At pH 6.0, the YZ radical decay rate was not
significantly altered in CD-PSII and Sr-PSII but decreased in Ba-PSII.
A two-pathway model, involving two competing proton donors with different
pKa values, is proposed to explain these
results. Ammonia treatment decreased the YZ decay rate in Ca-PSII,
Sr-PSII, and CD-PSII, consistent with a reaction that is mediated
by the hydrogen-bonding network. However, ammonia treatment did not
alter the rate in Ba-PSII. This result is interpreted in terms of
the large ionic radius of barium and the elevated pKa of barium-bound water, which are expected to disrupt
hydrogen bonding. In addition, evidence for a functional interaction
between the S2 protonated water cluster (Wn+) and the YZ proton donation pathway is presented. This interaction
is proposed to increase the rate of the YZ PCET reaction.
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Guo, Zhanjun; Barry, Bridgette A. (2017). Calcium, Ammonia, Redox-Active Tyrosine YZ, and Proton-Coupled
Electron Transfer in the Photosynthetic Oxygen-Evolving Complex. ACS Publications. Collection. https://doi.org/10.1021/acs.jpcb.7b01802
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AUTHORS (2)
ZG
Zhanjun Guo
BB
Bridgette A. Barry
KEYWORDS
Photosynthetic Oxygen-Evolving Complexp KEPRYZ decay rateseconds time scale190 KRedox-Active Tyrosine YZdecay rate190-DCD-PSIIammonia treatmentS 2 protonated water clusterPSIIMn 4 CaO 5 clusterProton-Coupled Electron Transferhydrogen-bonding networkYZ proton donation pathwayphotosynthetic oxygen evolutionII532 nm laser flashYZ PCET reaction