PSII Flashcards
What is the difference in type I and type II RC in terms of how many electron transfer turnovers are needed to reduce quinone to quinol?
Type I only needs 1 turnover
Type II needs 2 electrons and 2 protons to reduce quinone; 2 turnovers
Is the electron transport chain of oxygenic photosynthesis linear or cyclic?
What is the first enzyme?
Linear electron transfer pathway
PSII is the first enzyme in the chain
What does PSII do?
How is cyt b6f similar and different to bc1?
Where does plastocyanin go?
PSII oxidises water to reduce plastoquinone into plastaquinol
Cyt b6f does same thing as bc1 but instead reduces plastocyanin which then goes into PSI
What is meant when we say PSII is a water:plastoquinone photo oxidoreductase?
Why is this impressive?
PSII oxidised water and reduces plastoquinone
Water is a very poor electron donor
What is the structure of Thermosynechococcus vulcanus PSII?
Key subunits? (hint - similarities with L and M subunits)
- RC
- Antenna
- Membrane extrinsic
Dimer with 2-fold axis symmetry; Mirror image monomers
D1 and D2 are the core RC subunits that bind electron transfer cofactors
These form heterodimeric core with 4 TM helices
CP43 and CP47 are the core antenna subunits that binds RC in each monomer and form a belt of pigment around RC
PsbO, PsbU and PsbV are the membrane extrinsic luminal subunits that stabilise oxygen evolving complex (OEC)
There is structural separation between belt of pigments in antenna and RC pigments in the middle of PSII
Why is this?
Allows energy into special pair but not out
What does PsbO stabilise?
Stabilises Mn-cluster
What are the special pair absorbances in RC-LH1 and PSII?
RC-LH1 - P870
PSII - P680
What is the structure of the PSII electron transport chain? (hint - branches)
Similarities with purple bacteria arrangement?
2 branches – A/D1 and B/D2
Similar arrangement of electron acceptors downstream of special pair P680
What are the important molecules of PSII electron transport chain and their function (not seen in RC-LH1)?
- Non-redox __
- Peripheral chlorophylls
Non-redox active Fe atom binds quinones
Bound bicarbonate ion for tuning of quinone redox potential
Peripheral ChlZ(D1) and ChlZ(D2)
2 β-carotenes involved in getting energy into RC and protecting complex if too much energy is absorbed
Important TyrZ and Mn-cluster on D1 side
What branch is travelled along in PSII electron transfer?
What happens after P680* formation?
Only branch A
Charge separation occurs and electron travels from P680* to PQ(B) to form semiquinone
Electron hole forms at P680 and it becomes very oxidising
What fills in P680+ electron hole and what is formed?
What does Mn-cluster do?
Electron hole is filled by donation from tyrosine; Forms tyrosine radical
Mn-cluster then donates electron to re-reduce tyrosine and reset the system
What happens after electron transfer system is reset?
What is taken up?
Why and how is system reset?
Same process occurs, and second electron travels to PQB
2H+ are taken up from cytoplasmic side of membrane to produce plastaquinol
Plastaquinol diffuses away and a new plastoquinone associates, resetting the system
Tyr again fill electron hole and Mn-cluster re-reduces tyrosine to reset system
How is speed and directionality of redox cofactors electron transfer achieved?
- Distance?
What is lost at each transfer?
Redox factors are scaffolded at <1.4 nm apart and precise orientations for quick transfer
Differences in redox potential are small and downhill for directionality
Lose a little bit of energy at each transfer
Why is forward reaction favoured over direct recombination to P680+?
Direct recombination is much slower due to large difference in redox potential (Marcus inverted region)
Smaller distance between redox cofactors than distance to special pair
How does cofactor midpoint potential in PSII RC compare to purple bacteria RC?
What is special about P680+/P680 midpoint potential that allows it to extract electrons from water?
Midpoint potential of P870+/P870 is not as oxidising/positive as P680+/P680 in PSII
P680+/P680 has exceptionally high oxidising potential
Why is there a greater risk of forming damaging triplets in PSII than there is purple bacteria?
PSII has smaller energy gap and recombination cannot occur
Why does PSII not afford such a large energy gap like purple bacteria? (hint - energy squeezed)
What happens if PSII receives too much energy?
PSII has to reduce PQ and oxidise water so it is ‘energy squeezed’
PQ pool is very reduced, damaging long lived triplets and singlet oxygen are formed
How does singlet oxygen cause damage? (hint - photoinhibition)
Singlet oxygen is highly reactive and damages the D1 protein such that it must be replaced approximately every 30 minutes; Slows down the rate of PSII (photoinhibition)
What is non-photochemical quenching (NPQ) and how is it mediated in plants and how in cyanobacteria?
NPQ of excited states of chlorophyll in antenna systems prevents excess excitation energy transfer into the RC to protect PSII
In plants, NPQ in LHCII is mediated by a process called the xanthophyll cycle where LH carotenoids are exchanged for energy quenching carotenoids
In cyanobacteria phycobilisomes , the orange carotenoid protein is involved in photoprotection
Where is the Oxygen Evolving Complex (OEC)?
OEC sits just outside the membrane on the luminal surface of PSII
Why are 4 charge separations required to split water into oxygen?
RC photochemistry is 1 photon/1 electron at a time
However, water oxidation is a 4-electron process as 4 electrons (and 4 protons) must be removed from 2 water molecules to make 1 oxygen molecule
Therefore, 4 charge separations at PSII are required to drive formation of 1 molecule of O2 from 2 water molecules
What is special about Mn as an electron donor?
Mn is a transition metal that can exist in a range of oxidation states (from +2 to +5) and so can take part in redox reactions; Mn ions in the cluster donate electrons to reduce P680+
The OEC contains a Mn-cluster
How does this help with water oxidation to form oxygen?
2 waters are bound to the OEC, and 4 successive charge separations result in 4 electrons being extracted from the Mn cluster by P680+
After the 4th turnover the Mn cluster has ‘built-up’ 4 positive charges, which allows it to oxidise the 2 waters and form di-oxygen (makes an oxygen-oxygen bond)
The 4 electrons from water oxidation ‘pay-back’ those lost from the Mn cluster and return the catalyst to its original state
The 4 protons liberated in this process are released into the lumen and contribute to the PMF
