1. D1 and D2 2. CP43 and CP43 3. Cytb559 4. Extrinsic polypeptides 5. Dozens of other small subunits

1. Chlorophyll (4) 2. Pheophytin (2) 3. Quinone (2) 4. Manganese ions (4) 5. Calcium ion (1)

Photosystem II Flashcards by Zehra Rubbani

What is the enzyme name for PSII

Water/ plastaquinone photo oxidoreductase

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Overall reaction for PSII

2H2O + 2PQ (plastaquinone) —> 2PQH2 (plastaquinol) + O2

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Water oxidation half reaction

2H2O —> 4H+ + 4e- + O2 (Em +820mV)

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Quinone reduction half reaction

PQ + 2H+ + 2e- —> PQH2 (Em +100 mV)

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Delta E for PSII

-720 meV (delta G = 72kJ/mol) UNFAVOURABLE - energy input = light

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How many volts does a 680nm photon correspond to

1.82 eV

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Components of PSII

D1 and D2 2. CP43 and CP43 3. Cytb559 4. Extrinsic polypeptides 5. Dozens of other small subunits

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What are the two central proteins that are next to each other

D1 and D2

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D1 and D2

form pseudo homodimer each of 5 TM helices and symmetrical - location of all redox cofactors needed for enzyme function

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CP43 and CP43

peripheral antenna (light collection) - both 6 Tm helices and symmetrical - contain chlorophyll and carotenoid (also provide AAs to active site)

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Cytb559

2 TM helices with Hemeb

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Extrinsic polypeptides

insulate active site

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How many chlorophylls does PSII contain

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What absorbance for PSII

680 nm (nm at which it loses its colour - bleach)

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Main cofactors

Chlorophyll (4) 2. Pheophytin (2) 3. Quinone (2) 4. Manganese ions (4) 5. Calcium ion (1)

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What’s the redox active amino acid in PSII

Tyrosine

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Structure of pheophytin

Free base form of chlorophyll i.e. no metal - 2 H+ instead (easier to reduce)

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Chl/Chl(minus radical)

Em = -1V (very reducing)

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Chl/Chl (plus radical)

Em = +0.85V (very oxidising)

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PD1+/ PD1

Em = 1.25V

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Ph/Ph-

Em = -500mV

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Q/QH2

Em = 100mV

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Is semiquinone a strong oxidant or reductant

Both

1 e- chemistry at fairly low potential (no H+)

2 e- chemistry - stabilised by protonation of nearby amino acid and its electrostatic environment

Tyr(radical)/Tyr

Em = 1.2V (very oxidising)

How is Tyr radical stabilised

Histidine H-bonding partner and by H-bonds to waters

Manganese cluster

Mn4CaO5 Em = 1V

What are the ligands for the manganese cluser

Carboxylic acids and histidine

So how does PSII work

Excitation by light (ground state to excited)

What happens to the electron when there's excitation by blue light

Electron enters 2nd unoccupied molecular orbital (2nd excited singlet state)

What happens when there's excitation by light in light collecting proteins

Energy transfer (because the pigments are close together)

What happens in reaction centres when there's excitation by light

Charge separation (electron transfer)

Energy conversion

Wavelength (nm) = Plancks constant x speed of light / electron volts (eV)

What is hc

1240 nm/eV

Wavelength of blue light

460nm

Kinetic control

To avoid going back and recombination

What controls electron transfer rate

1. Distance (r) 2. Driving force (delta G or E) 3. Reorganisation energy (lambda) 4. Intervening medium (beta)

How much is lost due to back reactions

10%

In what step is there a big loss in energy

Electron transfer from Ph anion to quinone

What is the role of tyrosine

Electron carrier between chlorophyll (PD1) and the Mn4Ca active site

What is the role of His

To accept proton when tyrosyl radical formed

The S state cycle

Flash dependence of water splitting. Every fourth flash, electrons ripped out and O2 comes out. Move up oxidation each flash

What does the S state cycle show from is trend

Dampening effect (10% loss)

What is the most reduced S state

What is the confusion in the redox cycle for water oxidation

Whether Mn5+ is made or Tyr is oxidised AND where does water come in

Main features of redox cycle

1. In most oxidising state (S4) - 2 molecules of H2O are oxidised in a 4 electron process with no intermediates 2. Other steps in the cycle (S0-S1-S2-S3-S4) involve oxidation of active site components (Mn, Tyr) 3. Charge accumulation steps are mainly compensated by H+ release to the lumen - these are 'chemical protons' that came from the water that is oxidised

Why is water oxidised in one step

To prevent formation of reactive oxygen species

How many volts produced by PSII per photochemical turnover

1 volt

What is special about manganese cluster

Variation in position of oxygen 5 (2 states - Mn3+ or Mn4+) - acid base chemistry or radical chemistry

Overall how much is lost due to back reactions in PSII

600meV (but probably lose another 150-200 due to e- from 4 states and QH2)

What are the real life conditions

pH in lumen = 6. pH in stroma = 7.8

Real Em values

H20 half RX Em= 879mV not 820mV. PQ half RX Em=60mV not 100mV (total = -819meV)

Total energy used

600meV + 150/200meV + 820 meV = 1.62eV (1.82 in from red photon)

What could be used for the extra energy

Oxygen release and extra driving force