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Photosystem II01:22

Photosystem II

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The multi-protein complex photosystem II (PS II) harvests photons and transfers their energy through its bound pigments to its reaction center, and ultimately to photosystem I (PSI) through the electron transport chain. The pigments responsible for caputirng the light energy in photosystems include chlorophyll a, chlorophyll b, and carotenoids.
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Photosystem I01:27

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Although structurally similar to photosystem II (PSII), photosystem I (PSI) is has a different electron supplier and electron acceptor.
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The Photochemical Reaction Center01:29

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Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center is a more appropriate term that describes these complexes. The Nobel laureates Robert Emerson and William Arnold provided the first experimental evidence of photochemical reaction centers by demonstrating the participation of nearly 2,500 chlorophyll molecules for the release of just one molecule of oxygen. Despite thousands of photosynthetic...
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Photosystems01:32

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Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in the membrane of tiny sac-like structures called thylakoids placed inside the chloroplast.
Functioning of Photosystems
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The Z-Scheme of Electron Transport in Photosynthesis01:34

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The light reactions of photosynthesis assume a linear flow of electrons from water to NADP+. During this process, light energy drives the splitting of water molecules to produce oxygen. However, oxidation of water molecules is a thermodynamically unfavorable reaction and requires a strong oxidizing agent. This is accomplished by the first product of light reactions: oxidized P680 (or P680+), the most powerful oxidizing agent known in biology. The oxidized P680 that acquires an electron from the...
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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Redox-State-Dependent Structural Changes within a Prokaryotic 6-4 Photolyase.

Po-Hsun Wang1,2, Yuhei Hosokawa3,4, Jessica C Soares5

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Summary
This summary is machine-generated.

Prokaryotic (6-4) photolyases use blue light to repair UV DNA damage. Structural studies reveal the [4Fe-4S] cluster

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Structural Biology

Background:

  • Photolyases are enzymes that repair UV DNA damage using blue light.
  • Prokaryotic (6-4) photolyases are a key subgroup within the photolyase/cryptochrome superfamily.
  • These enzymes utilize flavin adenine dinucleotide (FAD) and a [4Fe-4S] cluster.

Purpose of the Study:

  • To investigate the structural dynamics of prokaryotic (6-4) photolyase from *Caulobacter crescentus* in different redox states.
  • To elucidate the function of the [4Fe-4S] cluster in DNA repair mechanisms.

Main Methods:

  • Damage-free crystallography utilizing X-ray free-electron lasers.
  • Electron Paramagnetic Resonance (EPR) spectroscopy.
  • Optical spectroscopy.

Main Results:

  • Redox-dependent structural transitions were observed, including the formation of an oxidized [4Fe-4S]3+ cluster with iron-sulfur bond cleavage.
  • Photoreduction to the FADH- state induced changes in the flavin binding site involving aromatic residues Y390/F394.
  • Oxidation led to protein matrix structural changes around the [4Fe-4S] cluster, potentially impacting DNA binding.

Conclusions:

  • The [4Fe-4S] cluster undergoes significant structural changes correlating with redox states.
  • These dynamics suggest a role for the iron-sulfur cluster in DNA binding and electron hole quenching.
  • Findings provide insights into the long-debated function of iron-sulfur clusters in DNA-interacting proteins.