<p>Early processes
following photon absorption by the photosynthetic pigment-protein complex
photosystem I (PS I) have been the subject of decades of research,
yet many questions remain in this area of study. Among the trickiest to
investigate is the role of the PS I reaction center’s (RC’s) two accessory
(A<sub>‑1</sub>) chlorophyll (Chl) cofactors as primary electron donors or
acceptors, oxidizing the special pair (P<sub>700</sub>) of Chls or reducing a
nominal primary electron acceptor (A<sub>0</sub>) Chl in the first electron
transfer step. Such processes, which occur on a picosecond timescale, have long
been studied via ultrafast spectroscopy, though difficulty lies in distinguishing
among signals from early processes, which have similar lifetimes and involve
many identical pigments. In this work, we used steady-state and ultrafast
optical pump-probe spectroscopies on PS I trimers from wildtype and mutant
strains of the cyanobacterium <i>Synechocystis</i> sp. PCC 6803 in
which an asparagine amino acid residue near A<sub>‑1</sub> had been replaced
with methionine on one or both sides of the RC. We also conducted an identical
set of experiments on mutants in which A<sub>0</sub> was similarly targeted, as
well as studied the effects on the A<sub>0</sub> absorption spectrum of a third
category of mutations in which a peripheral H‑bond to A<sub>0</sub> was lost. Steady-state
absorption spectroscopy revealed that many of these mutations caused mild Chl deficiencies
in the light-capturing antenna of PS I without necessarily preventing
organisms’ growth. More importantly, we determined that contrary to certain hypotheses,
A<sub>‑1</sub> is the most likely true first electron acceptor, as reasoned
from observing rapid triplet state formation in double A<sub>‑1</sub> mutants. We
also concluded from non-additive detrimental effects of single-side mutations that
if one RC branch is damaged at the level of A<sub>0</sub> or A<sub>‑1</sub>,
electron transfer may be redirected along the intact branch. This may help
explain the conservation of two functional RC branches in PS I over many
generations of natural selection, despite the additional cost to organisms of
manufacturing both.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/13356515 |
| Date | 14 December 2020 |
| Creators | Antoine P. Martin (5930030) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Light_Reactions_of_Photosynthesis_Exploring_Early_Energy_and_Electron_Transfers_in_Cyanobacterial_Photosystem_I_via_Optical_Spectroscopy/13356515 |
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