Flashcards in PSII Deck (79):
Describe PSII evolution.
PSII exists in all photosynthetic organisms and is highly conserved. However, the enzyme only evolved once and no other enzyme can perform the same chemistry.
What is the full name of PSII?
Water/plastoquinone photo oxidoreductase
What is the overall reaction catalysed by PSII?
2 H2O + 2 PQ -> 2 PQH2 + O2
Give the half reaction for the water oxidation.
2 H2O -> 4 H+ + 4e- +O2
Give the half reaction for the quinone reduction.
PQ + 2e- + 2H+ -> PQH2
What is the energy input for PSII?
Light energy - red photon
Give the wavelength and eV value for a red photon.
680 nm = 1.82 eV
Why isn't all of the energy in light collected by PSII?
Some will fluorescence away from the RC and there may be resonance transfer.
What type of chemistry is performed by PSII?
4 electron chemistry
Name the subunits found in PSII.
D1, D2, CP43, CP47, Cytb559 and other small regulatory subunits
Where are extrinsic polypeptides found and what is their role?
Found at the bottom of the structure. Involved in regulation, assembly and photoprotection of the enzyme.
Describe the D1 and D2 subunits.
Form a pseudohomodimer. Each consist of 5 TM domains and are symmetrical. Contain all of the redox cofactors needed for enzyme activity.
Where are the redox cofactors found in PSII?
In the D1 and D2 subunits.
What is the cuddling beans model?
Describes structure of D1 and D2 dimer. Evolved to have differently behaving quinones on each side of the enzyme.
What is the purpose of the CP43 subunits?
Acts as an antenna pigment to transfer energy from light to the reaction centre.
Describe the structure of the CP43 subunits.
6 TM domains. Contain chlorophyll and carotenoid.
Describe the structure of the Cytochrome b559 subunit.
Consists of 2 TM domains and contains heme b
What are the possible roles of the cytochrome b559 subunit?
Regulatory - it is possibly involved in a secondary electron transfer pathway to protect the complex from photodamage.
How were spectroscopic methods used to resolve the structure of PSII?
Used to follow colour changes of the cofactors during the reaction, this identified the location of the cofactors and could then be matched to the XC structure.
What is the major light harvesting pigment in PSII and what is the optimal wavelength absorbed?
P680 - absorbs at 680nm
Describe the structure of P680.
Consists of two chlorophyll a molecules
How is P680 excited?
Either by absorption of a photon or by exciton transfer from antennae pigments.
Where within the thylakoid membrane is PSII found?
Within the granal regions
Describe the structure of chlorophyll.
Porphorin ring, has 4 N atoms at the centre coordinated by a magnesium ion. Contains many conjugated double bonds and has a long hydrocarbon tail.
What happens following chlorophyll excitation?
Rapid electron transfer to nearby pheophytin
What is the difference in structure between pheophytin and chlorophyll and what advantage is this to pheophytin?
In pheophytin the central magnesium ion is replaced with 2 protons. This makes pheophytin easier to excite and able to remain in the reduced state for longer.
What is the result of the transfer of electrons to pheophytin?
Leaves a positive charge on P680, making it a strong oxidant. P680 is now able to remove 4 electrons from water.
Where in PSII are plastoquinones found?
Qa - tightly bound to a site in D2
Qb - found in D1
Describe the reduction of plastoquinone.
PQ -> PQH* -> PQH2
- 2 electron sequential reduction
- via semiquinone intermediae
Is plastoquinone a weak or strong oxidant?
Describe electron transfer from pheophytin to the plastoquinones.
Electron transfer from pheophytin to the plastoquinone at the Qa site in the D2 subunit. Electron is then transferred to the exchangeable plastoquinone at Qb site in D1 subunit, via a non-heme iron.
As a result of electron transfer to the quinone at Qb, what is produced?
Plastoquinol, with the uptake of 2 protons from the stroma.
Describe the semiquinone intermediate.
Unstable - acts as a strong oxidant and a strong reductant. Can be stabilised depending on its environment.
Why is quinone able to diffuse from PSII to Cytb6f?
Quinone is membrane soluble - has a long hydrocarbon tail with 9/10 isoprene groups.
Describe the structure of the oxygen evolving complex.
Consists of the Mn4CaO5 cluster, Tyr residue (that later forms a radical), a chloride ion and 3 extrinisic polypeptides (found at the bottom of PSII).
How is the Mn cluster coordinated?
By carboxylic acid groups and a his residue.
What is the oxygen evolving complex?
The active site for water oxidation
How is the Tyr residue coordinated and stabilised?
By hydrogen bonds to a his residue. Stabilised by hydrogen bonds to water molecules.
What is the role of the histidine residue?
Acts as a base to withdraw proton from Tyr - allows electron removal from Tyr.
What happens if chlorophyll if it absorbs a blue photon of light?
It is excited to a higher molecular orbital than if it were to absorb a red photon, this gives singlet state oxygen. PSII dissipates the extra energy as heat (only uses the energy equivalent to red photon) in order to prevent singlet oxygen and damaging ROS.
Why do all the pigments in PSII need to transfer energy between each other?
Chlorophyll rapidly returns to the ground state and emits green fluorescence. Energy transfer between pigments ensures that all the energy is used before it is lost as chlorophyll returns to the ground state.
What is the advantage of charge separation?
Separation of radical pairs prevents energy loss. Prevents recombination of the radical pairs to produce a triple state - could react with oxygen to produce singlet oxygen.
What is the eV value for a blue photon of light?
Give an overview of the electron transfer events in PSII. (8 steps)
1. P680 excited by photon
2. Pheophytin accepts excited electron
3. Pheophytin donates e- to Qa
4. P680+ takes electron from Tyr (must have H+ nearby to stablise)
5. Tyr takes electron from Mn cluster
6. Water produced in the Mn cluster (S state model)
7. Non heme iron transports e from Qa to Qb
8. Qb diffuses from PSII to Cyt b6f
Describe the energy differences between the steps involved in electron transfer.
Large energy drop for charge separation, this is thermodynamically favourable. Small energy drops between other steps.
How are back reactions and recombination reactions prevented?
By the close proximity and small driving force between cofactors.
In particular, which step occurs most quickly to avoid back transfer?
Electron transfer to and from the Tyr (in OEC). This is because Tyr161 of the D1 subunit can act as an electron carrier between the chlorophyll and the manganese.
What 4 factors control the electron transfer rate?
2. Driving force (eventually becomes limiting)
3. Reorganisation energy (λ)
4. Intervening medium - cofactors/proteins connected by chemical bonds
What is meant by reorganisation energy?
The energy required to distort the nuclear configuration of the reactions into that of the products, without electron transfer.
When is the rate of electron transfer optimal?
What is the overall energy loss due to back reactions?
How was the S state model discovered?
By giving flashes of light (1 photon per flash) to algae and following when oxygen was produced.
What were the results of experiments used to determine the S state model?
Nothing was seen until the 3rd flash, and then seen on every 4th flash. Gave the first indication that the system worked only using a single enzyme.
Why was oxygen production first seen on the 3rd flash and not the 4th flash?
Because the resting state in chlorophyll is actually S1 not S0
What happens to the oscillation in oxygen production after repeated flashes of light?
There is damping and the level of oxygen production converges to an average value.
Describe the S4 state.
Transient - when formed, an oxygen molecule is immediately and a new water molecule enters the OEC active site.
What is the redox state of manganese when it is taken into the cluster?
Mn 2+ - taken in, then oxidised one at a time to give the ions needed for the S0 state
Which is most likely the high valence model or the low valence model?
High valence model
What is the low valence model?
Suggestion that Mn2+ is present in the S0 state
What is the S0 state in the high valence model?
Mn3+, Mn3+, Mn3+ and Mn4+
During which stages of the S state model are the water molecules most likely to enter? How was this showed?
Following formation of S0 and S3 (one water at each). Showed by using water labelled with heavier oxygen.
Which stage of the S state model is there no proton release in and what effect does this have?
Formation of S2 - means this step slows down the reaction and there is higher charge accumulation in this step.
Why must the Mn cluster be kept in the membrane?
The oxidations required to produce the S0 state mean that the cluster is out of equilbrium with its environment and must be kept within the membrane to avoid any unwanted reactions.
How are ROS intermediates avoided?
By removing all 4 electrons from water in one step
How is charge accumulation accommodated?
By proton release into the lumen
Why are the protons released considered chemical protons?
Because they came from the water
Upon formation of the S3 state, what has been achieved by the cycle?
Accumulation of all the oxidising power in one place
What was the S4 state first proposed to contain?
Mn5+, Mn4+, Mn4+ and Mn4+ (called mixed valence)
What is the alternative S4 state? And is this considered more likely?
yes this is considered more likely. There is tyr oxidation near the Mn cluster - meaning the redox states of Mn are all 4+
On average, how much energy is generated by each step in the S state model?
How much energy is required to move one proton across the membrane?
How much energy is lost when stabilising the charge separation?
How much energy is used to produce the free quinone?
How much energy is used in water oxidation and plastoquinol reduction?
Give the overall energy used by PSII
600 + 200 + 820 = 1620 meV = 1.62 eV
What is the extra driving force in the system and what is it used for?
- used for the production of ATP
- allows PSII to overcome thermodynamically unfavourable conditions
Why is the pH of the lumen actually around pH 6?
Due to proton accumulation during the redox cycle
Why is it desirable to mimic the way PSII stores energy for fossil fuel development?
Need ways to store energy in chemical bonds as this is longer lived and easier to store than electricity.