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Y3 Membrane Proteins > Photosynthesis > Flashcards

Photosynthesis Flashcards

1
Q

What is primary electron transfer?

A

Where harvested excitation energy drives charge separation stabilised by secondary reactions

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2
Q

How does FRET happen between chlorophylls

A

They need to be <5nm apart
Non radiative transfer of energy from an excited donor to an acceptor
Spectral overlap between the donor and acceptor

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3
Q

How are gaps in chlorophyll absorbance accounted for?

A

Carotenoids plug gaps in absorbance
They move electrons from S0 to S2 to S1

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4
Q

How are chlorophylls structured?

A

Tetrapyrroles with a long hydrophobic isoprenoid tail
Alternate single and double bonds which forms π-electron system
Similar to hemes but less symmetrical and coordinate a central Mg2+
Absorbs blue and red light

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5
Q

How is proton motor force generated in chloroplast lumen?

A

Water is oxidised by PSII
Plastoquinol is oxidised by Cyt b6f
PQ/UQ has a relatively negative charge on the other side of the protons

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6
Q

Explain jablonski diagrams

A

S1 state is achieved by absorbing lower energy red photon
S2 state is achieved by absorbing higher energy blue photon
Electrons lose energy via internal conversions and fluorescence

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7
Q

What are billins and why are they useful?

A

Open chain tetrapyroles with conjugated π-electron system
in cyanobacteria antenna complexes
Quenches harmful excited states at 450-550nm and transfers remaining excitation energy to chlorophyll

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8
Q

Where does photosynthetic electron transport take place?

A

Thylakoid membrane. Folded up for high SA:V ratio
Stacked into grana connected by stronal lamellae

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9
Q

Why are antenna complexes necessary?

A

Despite pigment concentrations being low, increase electron transfer by 100 times.
Capture light and transfer it to the RC special pair
W=

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10
Q

What features make antenna complexes ideal for their function?

A

Wide spectral cross spectrum- contains lots of different pigments. π-electron system determines excited state.
High pigment concentration as these transfer energy to each other via FRET.
Modular- lots of antenna per RC in low light, vice versa

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11
Q

Why are purple bacteria favourable for studying photosynthesis?

A

Metabolically versatile
Easily genetically engineered
Easy to grow
Produce extensive internal membrane systems (intracytoplasmic membranes (ICMs)) to increase photosynthetic membrane under anaerobic conditions

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12
Q

How are intracytoplasmic membranes structured?

A

Lamellar (stacked discs like grana)
Vesicular

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13
Q

How can photosynthetic electron transfer become cyclic?

A

Proton can cycle from FNR to cyt B6f
Ferrodocxin can also donate an electron to reoxidise B6f

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14
Q

Describe anoxygenic photosynthesis

A

One type of reaction centre and light driven reaction
Cyclic electron transfer involving Cyt bc1 complex and Cyt C2
Uses ubiquinone and ubiquinol to generate pmf
NADPH is indirectly produced by reverse electron flow using external electron donors

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15
Q

Describe the antenna funnel system

A

Higher energy donor pigments transfer excitation energy to a lower energy acceptor via FRET
Carotenoids -> Chl b -> Chl a -> RC
No fixed physical arrangement and can be exchanged under certain light conditions

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16
Q

What type of antenna funnel systems are found in primary electron transfer systems?

A

Fused antenna: antenna and RCs bound to same polypeptide. Antenna not biochemically separated from RC

Core antenna: RC and antenna are formed from different polypeptides that interact with each other. Antenna and RC can biochemically separate

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17
Q

Describe Rba sphaeroides electron transfer chain?

A

LH2 surrounds the RC-LH1 which is ideal for energy transfer.
Electron moves from pigments with increasing wavelength, decreasing energy
PufX promotes dimerisation of LH1, preventing LH2 ring closing. Protein Y also prevents closing so UQ/UQH2 can move through

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18
Q

Describe Rba sphaeroides RC-LH1

A

BChls in LH1 are within 4nm of the special pair B870 which allows for efficient FRET
This is an energetically uphill transfer but it is driven by thermal energy in the environment
αβ are coupled such that the absorbance is red shifted and excitation energy can go from PSII to PSI
LH1 is formed by 14αβ pairs held open by protein Y and Puf X which dimerises it.
Bound to 2 carotenoids

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19
Q

What is present at the Rba sphaeroides LH1dimer interface?

A

PufX which creates a concave surface from TMs that have been pushed out
Lipids

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20
Q

How can RC-LH1 vary across species?

A

How open the LH1 rings are affects quinone/quinol diffusion
In complete rings, quinone/quinol moves through small pores in LH1 antenna
In open rings, such as Rba sphaeroides, this pore is covered by a carotenoid
14-17αβ pairs
3 or 4 RCs
Can bind Ca2+ to boost thermal stabiltiy

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21
Q

Describe T-tepidum LH1-RC

A

Monomeric
Forms completely closed ring
16 Ca2+ for thermal stability
This also redshifts LH1 for Qy transition
Has multiple pores as it has a carotenoid bound per αβ subunit

22
Q

What is Rba sphaeroides RC made up of?

A

3 polypeptide chains:
L and M which form a heterodimer with pseudo C2 symmetry and binds electron transfer cofactors
H
Special pair P870
Electron that moves from Cyt C2 to UQb creates a relatively regative charge that contributes to the pmf

23
Q

What makes electron transfer irreversible in the RC?

A

There is a large loss of redox potential as electron passes from P870* to quinone on Qb site
P870+ is too far from the Qa site for reverse electron flow
UQ to BPhe has such a large driving force it sits in the marcus inverted region, making it unfavourable

24
Q

Why are back reactions dangerous?

A

Form triplet species in chlorophylls which causes ROS to damage RC proteins

25
Q

Describe how an electron moves up a Rba sphaeroides electron transport chain

A

Electron moves from Cyt2 to P870, then up the A or B branch.
BChl to BPhe to Qa/b to Qb/a

26
Q

Describe homodimeric type I reaction centres

A

Simplest known RCs
Reduces Fd and quinones
Does not require tightly bound quinone for electron transfer
Duplication of RC subunit followed by divergence of two genes made RC heterodimeric

27
Q

What does PSI do?

A

Plastocyanin-ferrodoxin photooxidoreductase
Electron moves from cyt b6f to plastocyanin which then reduces ferrodoxin

28
Q

What RC subunits are in cyanobacteria PSI?

A

PsaA and PsaB are core subunits
PsaC forms an electrostatic surface with PsaD and E to bind ferrodoxin
PsaF interacts with plastocyanin
PsaL determines whether PSI forms trimers and tetramers

29
Q

What does PsaC do in cyanobacteria PSI?

A

Forms an electrostatic surface with PsaD and E to bind ferrodoxin

30
Q

What does PsaF do in cyanobacteria PSI?

A

Extends into lumen to interact with plastocyanin

31
Q

What does PsaL do in cyanobacteria PSI?

A

Determines whether PSI forms trimers or tetramers
Helps oligomers pack into PSI

32
Q

What does PsaH do in eukaryotic PSI?

A

Keeps it monomeric

33
Q

Describe eukaryotic PSI

A

PsaH keeps it monomeric
Forms a supercomplex with LHCI antenna
4-10 LHCI form a crescent at one half

34
Q

Describe plastocyanin

A

Electron acceptor from cty b6f then donor at PSI
Tetrahedrally coordinated by a copper atom that changes oxidation state
Substituted for cytochrome C6 in cyanobacteria and algae

35
Q

Describe ferrodoxin

A

2Fe-2S cluster coordinated by 4 Cys
Accepts electrons from plastocyanin
Oxidised by ferrodoxin-NADP+ reductase (FNR) which donates 2 electrons at a time

36
Q

How do PSI subunits interact with ferroxin/plastocyanin?

A

ferroxin/plastocyanin have negative residues that interact with positive residues on PSI subunits

37
Q

Describe PSII

A

Only known biological enzy,e that can oxidise water
Water:Plastoquinone photooxidoreductase
P680+/P680 special pair

38
Q

Describe plant and algae PSII-LHII

A

RCs in the middle
LHCII around the periphery
High pigment concentration around the antenna, lower in RC

39
Q

How do midpoint potentials of antenna funnels allow efficient electron transfer?

A

Differences are small and downhill

40
Q

Describe PSII structure?

A

Pseudo 2-fold structure means electron can only move up the A branch
Mn4O5Ca -> TyrZ -> Special pair P680 -> ChlD1 -> PheD1 -> PQa -> PQb

41
Q

Describe the Oxygen evolving complex of PSII

A

Mn4O5Ca
D1 side of PSII
The PSII subunits PsbO, U and V stabilise it and protect it from damage
1. 2 waters bind to it
2. 4 charge separations move electrons to P680+ and form O2
3. 4 electrons paid back to cluster
4. 4 protons released in lumen

42
Q

Where does plastoquinone move after being reduced in PSII?

A

From Qb site to Cyt b6f

43
Q

What are pros and cons of forward reactions and direct recombination?

A

Forward reactions- too much driving force then unfavourable due to marcus inverted region
Direct recombination- Safe but waste energy

44
Q

Why does the D1 branch of PSII need to be replaced every 30 minutes?

A

ROS damages it which slows rate of electron transfer

45
Q

Describe how quinols can be oxidised in the Q-cycle

A
  1. Electron bifurication happens from Quinol at the Qo site from Q pool
  2. Electron moves to Rieske subunit Fe-S (lumen side), Heme c1 and cyt c2, releasing a proton to the lumen and leaving a semiquinone at the Qo site.
  3. Another electron moves to Heme bc in Cytochrome b. This goes to Heme bh to a quinone. Another proton moves to the lumen
  4. Oxidised quinone leaves Qo site and new quinone arrives
46
Q

How do Cyt b6f and Cyt bc1 differ?

A

b6f has cyt f with a c-type heme
bc1 has cyt c with a c-type heme

cytb6f has Pet G, L, M and N which is involved in cyclic electron transfer

47
Q

What are chlorophyll and carotenoids doing in the cyt b6f Rieske subunit

A

They are too far apart for electron transfer so may act to gate Quinone/Quinol

48
Q

How can photosynthetic efficiency be improved?

A

Overproducing Rieske as Quinone turnober at Cyt b6f is rate limiting step

49
Q

Why are linear and cyclic electron transfer segregated at thylakoids and stromal lamellae respectively?

A

To allow for state transitions where LHII antenna move between PSII and PSI

50
Q

How does the Cyt b6f Rieske subunit allow for electron transfer?

A

Rotates the Fe-S closer to the c1 heme

Y3 Membrane Proteins (3 decks)

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