Type I Reaction Centres Flashcards
What are the dark reactions of photosynthesis responsible for?
How? (hint - cycle)
What is required and what provides it?
Converting CO2 into complex organic molecules (carbohydrates)
This is done via the calvin cycle which requires a large amount of ATP and reductant (NADPH), which are provided by the light reactions
Trait of NADPH and NADP+ (hint - electron donor and acceptor)
NADPH - Good electron donor that feeds electrons into the respiratory electron transport chain
NADP+ - Not a good electron acceptor, therefore its reduction requires and even more reducing donor
What does PSI oxidise?
What does PSI reduce? What does this then reduce?
PSI oxidises plastocyanin that has been reduced by quinol oxidation at cyt b6f complex
PSI reduces Fd which then reduces NADP+
Describe the Z scheme
P680 excited by light, causing plastoquinone reduction
PQ2 is oxidised at cyt b6f complex
plastocyanin (Pc) is reduced by cyt b6f
PSI oxidises Pc
PSI reduces Fd which then reduces NADP+
What is meant when we say PSI is a plastocyanin-Ferredoxin Photo-oxidoreductase?
What causes the +ve to -ve midpoint potential change?
PSI oxidises plastocyanin and reduces ferredoxin using light energy
Input of light energy causes +ve to -ve change
What is plastocyanin (Pc) and what does it do?
How can it accept electrons?
Small soluble electron transfer protein found in the thylakoid lumen
Pc accepts electrons from the cyt b6f complex and is oxidised by P700+ at PSI
Contains a single tetrahedrally coordinated copper atom that can change oxidation state (+1 to +2)
What is ferredoxin (Fd) and what does its structure?
What does it do?
Small soluble electron transfer protein found in the stroma/cytoplasm
2Fe-2S cluster coordinated by 4 cysteine residues and accepts electrons from PSI
What enzyme oxidises reduced Fd?
Ferredoxin-NADP+ reductase (FNR) enzyme
What is Ferredoxin-NADP+ Reductase (FNR)?
Why is it needed and what does it do? (hint - 1-electron)
Enzyme found in the cytoplasm/stroma
Ferredoxin transfers 1 electron at a time but reduction of NADP+ to NADPH requires 2 electrons (and 1 proton)
FNR serves as a ‘1-electron transfer’ to ‘2-electron transfer’ convertor; Takes 2Fd to reduce one NADP+
What does FNR contain that allows it to exist in the oxidised, 1-electron reduced and 2-electron reduced forms?
Flavin redox cofactor
What are the core RC subunits of cyanobacterial PSI monomer?
What are their structures?
PsaA and PsaB
Both are 11 TM helices proteins and together they form a heterodimeric complex that binds most of the electron transfer cofactors
The core antenna is fused to the core RC subunits in PSI
This antenna is structurally similar to CP43 and CP47 in PSII
How does the core antenna differ (hint - seperate) and why is it evolutionarily significant?
Core antenna is a part of a single polypeptide rather than CP43 and CP47 which are seperate proteins that associate with D1-D2 core
PSII must be replaced frequently due to photodamage so having fewer TM helices to replace is better (5 TMHs vs 11)
What does PsaC subunit do in PSI?
- PsaD and PsaE?
Binds the remaining (terminal) electron acceptors
Mediates electrostatic interaction with ferredoxin, along with PsaD and PsaE
What does PsaF subunit do in PSI?
Extends into lumen and mediates interaction with plastocyanin
What does PsaL subunit do in PSI?
Important for oligomerisation of cyanobacterial PSI
Forms denser belt of pigments than in PSII
What is a key determinant of whether PSI forms trimers or tetramers in cyanobacteria?
How?
Why form tetramers or trimers? (3 reasons)
PsaL subunit
In tetrameric PSI, PsaL subunits have bulkier side chains to prevent trimer formation
Oligomers allows you to fit more in the membrane or to get some degree of excitonic sharing between monomers or more efficient photoprotectors when in this arrangement
Arrangement of PSI in eukaryotes?
Why? (hint - PsaH)
PSI is monomeric and forms a supercomplex with the peripheral LHCI antenna
Eukaryotes have PsaH subunit that imposes rigidity and prevents formations of dimers, trimers etc.
How does Fd interact with PSI in plants?
Fd-PSI interaction is based on electrostatic interactions between positively charged amino acids in PSI subunits and negatively charged amino acids on the surface of Fd
How does Pc interact with PSI in plants?
Pc-PSI interaction relies on hydrophobic interactions formed by PsaA and PsaB and electrostatic interactions between lysine residues on PsaF and a negatively charged helix on Pc
Why do Fd and Pc need to quickly dissociate upon successful electron donation?
Reactions occur very quickly and more Pc and Fd need to come in
How is the PSI RC arranged?
P700 special pair
Chlorophyll on either branch
Another chlorophyll (A0) on either branch
Phylloquinones (A1) on either branch
Fx held between PsaB and PsaA
Fa and Fb bound to PsaC
What is the sequence of electron transfer in PSI upon P700 stimulation?
What fills in electron hole?
Can go up either branch A or B
P700* –> ChlB –> A0B –> A1B –> FX –> FA –> FB
Electron is then transferred to oxidised ferredoxin to reduce it
Reduced plastocyanin fills in electron hole at P700
What is hole migration in PSI?
Primary electron transfer in PSI may occur from an accessory chlorophyll and then the positive charge ‘migrates’ to P700; Hole migration
What is the 1 difference between A and B branch in PSI?
What does this do?
B-branch phylloquinone has a more reducing potential than the A-branch phylloquinone
This increases the forward electron transfer time from the phyllo semiquinone(A) to Fx because the A branch electron transfer to Fx is slightly uphill
Why are back reactions especially bad in PSI?
How are they favoured in PSI?
In presence of O2, back reaction to chlorophyll A0 acceptors can result in triplet formation and production of damaging singlet oxygen
Not ideal as PSI can’t be replaced like PSII
Back reaction is favoured along the A branch as FX –> A1A is slightly downhill, whereas FX –> A1B is slightly uphill
Despite back reaction occurring, how is further back reaction prevented?
When at A1A, next back reaction to A0A is now uphill and is slower than direct recombination
A branch provides a back reaction/recombination route that minimises triplet formation
Linear electron transfer produces 1.28 ATP per NADPH
However calvin cycle requires 1.5 ATP per NADPH
What makes up the rest of the necessary ATP?
Cyclic electron transfer makes up the rest; Uses PSI, but not PSII
How does cyclic electron transfer work in PSI?
Electron downstream of PSI are recycled back to quinone pool; Prevents NADPH formation but allows more ATP to be made
What are the 2 cyclic electron transfer pathways? (hint - PGR5, Photosynthetic complex I)
Electrons from Fd-FNR are returned to cyt b6f via PGR5 protein; PGR5 picks up electrons and docks on stromal side of cyt b6f; More contributed to pmf
Photosynthetic complex I accepts electrons from ferredoxin which reduces PQ to PQH2 and cyt b6f re-oxidises them contributing to pmf
What is special about homodimeric Type I RCs from heliobacteria and green sulphur bacteria?
Difference in electron transfer cofactors?
Simplest known RC
Formed from 2 identical copes of 11 TM helices subunit
Don’t require tightly bound quinone (e.g. phylloquinone) to mediate electron transfer from A0 to Fx
Homodimeric RC is more ancient
What allowed for conversion of homodimeric RC to a heterodimer RC?
Duplication of the core RC subunit gene followed by divergence of the 2 genes allowed for conversion of a homodimeric RC to a heterodimeric RC
