RC-LH1 in Anoxygenic Phototrophs

Question 1

What is another way of explaining energy transfer from B800 to B850 in LH2 of BChl? (hint - soret 375 nm)

Answer 1

Bacteriochlorophyll absorb high energy photons (375 nm) with their soret band
- Relaxes down to Qx state and then to Qy state by vibrational relaxation

Qy is the lowest excited state and the only one that is different between the 2 BChl populations
There is energy transfer from B800 Qy to B850 Qy; 800 nm is higher energy
- Efficient energy transfer via spectral overlap and close proximity of molecules in LH2

Question 2

What happens to excitation energy delocalised around B850 ring?

Answer 2

Delocalised energy around B850 ring can migrate between B850 rings of LH2s until it reaches LH1 and then RC

Question 3

How far apart our our B850 BChls on neighbouring LH2?
Why is this important?

Answer 3

Just 3-4 nm apart

Close enough for efficient FRET between their B850 rings in just 2.7 ps

Question 4

How is excitation energy transferred from LH2 to LH1?
- Distance?
- Time?

Answer 4

Distance between LH2 B850 BChls and the B875 BChls of LH1 is approximately 3-4 nm as they are packed closely in the membrane

Downhill excitation energy transfer from LH2 to LH1 occurs in just 4.5 ps

Question 5

After LH2 energy transfer to LH1 what happens?
How is energy transfer from LH1 back to LH2 prevented?
- Energy loss?

Answer 5

FRET from LH1 back to LH2 is slower as energetically uphill; B875 to B800

Next step to the RC is faster so that is favoured
- Small loss of energy to gain this directionality

Question 6

Why is energy transfer from LH1 B875 to P870 special pair slower than other steps?
- Distance?
- Time?
How is this driven?

Answer 6

Energy transfer is slightly uphill; Lower to higher energy
- 35 ps

Distance between B875 and P870 special pair is 3.9-5 nm

Thermal energy from the environment dries the uphill energy transfer to the RC special pair

Question 7

Summarise energy transfer of LH2 to RC
- Time?
Why is it so fast and efficient?

Answer 7

Light absorption by LH2 and transfer of excitation energy from LH2 to LH1 and then the RC takes ~60 ps

Spectral overlap and proximity/orientation of donor and acceptor pigments mean not only is this process very fast, but it occurs with little loss of energy

Question 8

Describe structure of LH2 and RC-LH1 in terms of arrangement in the membrane
What does RC use light energy to do?
What does cytochrome bc1 complex do?

Answer 8

LH2 is peripheral antenna that neighbours LH1 antenna which surrounds type II RC

RC uses light energy to reduce quinone to quinol

Cytochrome bc1 complex (where quinols are oxidised to quinones; Reduces Cyt c2) generates PMF for ATP synthesis; Cyt c2 returns electron to the RC

Question 9

What is the structure of LH1 in Rba. Sphaeroides (purple bacteria)? (hint - pairs)
Dimers?
- Number and why? (hint - incomplete)
Extra pigments bound?

Answer 9

Formed of pairs of α and β transmembrane polypeptides

Each pair binds a BChl a dimer
Dimers are <1nm apart making them excitonically coupled

14 αβ pairs that incompletely surround the RC
Ring is held open by the PufX and protein Y polypeptides to allow quinones/quinols out

LH1 binds 2 carotenoids

Question 10

How is LH1 arranged so it can get closer to LH2?

Answer 10

Similar arrangement to LH2 BChl a pair; When LH2 and LH1 come close, their rings are arranged in the same way so they can get closer to one another

Question 12

How are RC-LH1 monomers arranged in Rba. Sphaeroides?
Subunit and pigment numbers?

Answer 12

2 RC-LH1 monomers combine to form a dimeric complex with an S-shape

28 subunit LH1 surrounding 2 RCs
Connected ring of 56 excitonically coupled B875 BChls spanning both monomers

Question 13

What holds together RC-LH1 dimers?
- Conserved residues?
- Lipids?

Answer 13

Numerous protein–protein and pigment–protein interactions

PufX proteins are essential for mediating dimerisation
- 2 highly conserved arginine

SQDG (lipid) from salt bridge to PufX holding it together

Question 14

Why are dimerised RCs better than monomeric? (hint - trap)

Answer 14

Thought to be more efficient energy traps than monomeric ones
If one RC is ‘busy’ during an electron transfer, energy can quickly transfer to other RC

Question 15

How does RC-LH1 impose curvature on the membrane? (hint - PufX and TM regions)
What is this good for? (2 things)

Answer 15

Dimer has a concave periplasmic surface with PufX imposing bent conformation

Transmembrane regions from each monomer are inclined towards eachother at the cytoplasmic and periplasmic face, causing them to push apart the central LH1 α and β subunits

Curvature creates favourable environment for assembly of LH2 arrays, further curving membrane
Curvature helps drive chromatophore formation

Question 16

How are B875 BChls in RC positioned?
What coordinates these pigments?
How does this tune the absorption?

Answer 16

Positioned for strong excitonic coupling and ultrafast energy transfer
- Distance between dimers is actually smaller than distance within dimer; Very close for coupling

Coordinated by:
- Histidine ligands to the central Mg ions
- H-bonds from Trp residues to the C3 acetyl carbonyls

Excitonic coupling and H-bonds red-shift the absorbance to 875 nm; Imparts directionality on LH2 to LH1 excitation energy transfer

Question 17

What is the difference between type I and type II RC?

Answer 17

Type I - Related to PSI and have different type of antenna (no LH1)

Type II - (related to PSII) and surrounded by LH1 antenna complex

Question 18

What variations are seen in RC-LH1 structures?

Answer 18

Open vs closed LH1 rings
14-17 LH1 αβ pairs
3 vs 4 subunit RCs

Question 19

How is absorption in Blastocholoris viridis BChl b-containing RC-LH1 complex red-shifting beyond 1000 nm?
Why and how does it do it?

Answer 19

16 γ subunits sit on outside of LH1 ring

Absorption is past the ~920-980 nm where less light reaches the Earth’s surfaces
γ is recruited to affect distance between BChl and LH1 ring and change excitonic coupling

Question 20

What is the different reasoning for having an open or closed LH1 ring? (hint - quinone/quinol)

Answer 20

Complexes which have only 1 carotenoid per αβ subunit allow for quinone/quinol exchange through small pores in LH1 antenna via ‘breathing motion’

Complexes with 2 carotenoids per αβ pair have their pores blocked by this 2nd carotenoid
Therefore PufX utilised to open LH1 ring and allow quinone/quinol exchange

Question 21

What happens in ΔpufX mutants with 2 carotenoids and why?

Answer 21

ΔpufX mutants form closed rings and cannot grow photosynthetically unless the 2nd carotenoid is removed (by mutagenesis)

Question 22

What are the 3 subunits that make up Rba. sphaeroides RC and their structures?
Function?

Answer 22

L and M subunits each have 5 TM α helices
L and M form a heterodimeric core that bind all the electron transfer cofactors

H subunit has 1 TM helix and a soluble cytoplasmic domain
Insulates the quinone binding sites from the cytoplasm
This prevents unwanted redox reactions

Question 23

What are the electron transport cofactors that each RC binds?

Answer 23

BChl a dimer (P870) – Special pair that are strongly coupled and act as one pigment

2 monomeric BChls either side

2 bacteriopheophytins either side (BPhe; These are BChls missing the central Mg2+)

2 Ubiquinone either side

1 Fe atom that stabilises structure

15-cis carotenoid for photoprotection

Question 24

How is the RC asymmetric?
Why is it?

Answer 24

Electron transfer occurs only along the ‘A-branch’ (L subunit)

RC is a heterodimer and the redox potentials of the cofactors are tuned differently

Question 25

What are the 2 different ubiquinone types in RC? How are they different?

Answer 25

1 fixed called QA 1 dissociable called QB; Acts as substrate and can leave RC when reduced to quinol

Question 26

How is directionality achieved in RC electron transfer?

Answer 26

Thermodynamically favourable downhill redox transfer from negative to positive V

Question 27

How is RC electron transfer initiated? How is electron transfer made to be efficient? What are the first 2 transfer steps after initiation and how fast?

Answer 27

P870 is excited from the dark state to the very reducing P870*, increasing free energy in the system Acceptors are all very close for efficient transfer P870* --> BChl(L) --> BPhe(L) in 4 ps

Question 28

What is an electron hole and how is it formed? Redox potential of resulting factor?

Answer 28

Special pair loses an electron, forming this electron hole Special pair is now oxidising

Question 29

What is the next electron transfer step after BPhe(L) and how fast? Size of drop in free energy?

Answer 29

BPhe(L) --> Qa in 200 ps Large drop in free energy

Question 30

Electron transfer step after Qa and time? Where are the electron and electron hole now relative to eachother? What do we now have (that has received one electron)?

Answer 30

Qa --> Qb in 100 µs Electron and electron hole (P870+) are now separated at opposite sides of the membrane We now have semi-quinone

Question 31

How is cyt c2 involved in electron transfer? Where does it then go? What is now re-set and what is it ready for?

Answer 31

Reduced cyt C2 donates electron to P870+ to fill electron hole Oxidised cyt c2 dissociates from the RC and heads back to the cyt bc1 complex RC is now reset, ready for another round of charge separation

Question 32

What happens in electron transfer after RC is reset? What is needed to from what final result?

Answer 32

Electron transfer cycle repeats, providing second electron to semi-quinone 2 protons are taken up from cytoplasm to doubly reduce quinone to quinol (QH2)

Question 33

What happens after quinol (QH2) is formed? - Replacement?

Answer 33

Quinol dissociates and travels to cyt bc1 It is replaced by a new quinone (Q)

Question 34

How does FRET efficiency and electron transfer efficiency decay differently?

Answer 34

FRET efficiency decays with the 6th power of the distance Electron transfer efficiency decays with the square of the distance

Question 35

Why does P870* act as an electron donor rather than returning to the ground state by de-trapping (excitation energy transfer back to LH1)?

Answer 35

Distance between B875 and P870 is close enough for excitation energy into RC by FRET However once at P870*, reverse energy transfer from P870* to B875 (de-trapping) (25-50 ps) is much slower than electron transfer from P870* to the very close BPhe(L) (4 ps)

Question 36

Why is the distance from LH1 to RC a compromise?

Answer 36

Close enough that transfer is fast enough to get energy in (by FRET) Far away enough that energy stays in RC for electron transfer

Question 37

What Stops the Electron Returning to P870+ (recombination)? (hint - speed and distance)

Answer 37

Forward reactions are faster than recombination of the radical pair to the ground state - Early recombination reactions fall in the Marcus inverted region By the time the electron reaches Qa, the distance between the electron and the hole spans the membrane, making recombination very slow compared to the forward reaction

Question 38

What is the Marcus inverted region?

Answer 38

Electron transfer slows down when driving force is too big

Question 39

How does recombination provide a safe route out for electrons?

Answer 39

If there is no reduced cyt c2 or quinone available (the RC lacks its substrates), electrons get ‘stuck’ If an electron is ‘stuck’ on Qa or Qb, it can safely directly recombine with P870+ - Wasteful as energy is lost as heat

Question 40

What are back reactions? Why are they undesirable? What prevents back reaction from Qa- to BPhe(L)?

Answer 40

If the forward reactions are not possible electrons can travel back through the chain (energetically uphill) Undesirable because they can generate BChl triplet excited states Big uphill energy gap prevents the back reaction from QA- to BPhe(L)

Question 41

Why are BChl triplet excited states bad?

Answer 41

They can sensitise singlet oxygen which is damaging

Question 42

RC M subunit binds a single carotenoid close to the BChl and BPhe on the inactive B-branch What is its role?

Answer 42

Photoprotection - If BChl triplets are formed they are quenched by this carotenoid Carotenoid safely dissipates the triplet energy as heat to prevent singlet oxygen formation