Membrane Proteins - Andrew Hitchcock Flashcards
1
Q
Why is photosynthesis a crucial area of study?
A
- O2 is essential for life on earth, oxygenation of atmosphere by cyanobacteria allowed for evolution of aerobic organisms and complex multicellular life.
- It helps maintain the CO2/O2 balance of the biosphere.
-Need for more food and energy with less CO2 emissions.
2
Q
What are the 4 main steps of photosynthesis?
A
- Light harvesting and energy transfer by antenna complexes within the antenna network
- Primary electron transfer at photosynthetic reaction centre, upon uptake of harvested excitation energy this drives the charge separation of a pair of redox active chlorophyll molecules causing an electron transfer that results in the reduction of a quinone to a quinol.
- The reduced quinones produced by the reaction centre migrate to Rieske/cytochrome B complex (BC1 or B6f) The reoxidising of the quinols is coupled with the generation of a PMF.
- The PMF is used to drive ATP synthesis by ATP synthase.
3
Q
What do the light dependent reactions of photosynthesis involve?
A
The light reactions of photosynthesis use light and H2O to phosphorylate ADP and reduce NADP+ to produce NADPH and ATP
4
Q
Where do the light dependent reactions take place?
A
The thylakoid membranes
5
Q
What are the light independent reactions?
A
Reactions of the Calvin cycle to produce a range of carbohydrates (fixing atmospheric CO2 to produce sugar molecules)
6
Q
Where do the light independent reactions take place?
A
The stroma
7
Q
Why are cyanobacteria thought to be evolutionary precursors to chloroplasts?
A
They perform photosynthesis in a very similar way, and-so are thought to have been incorporated into eukaryotes by endocytosis.
8
Q
How are light harvesting membrane systems specialised for light harvesting? (thylakoid)
A
Thylakoid membranes stake into Grana, these are connected by stromal lamellae (unstacked thylakoid membranes) -> these stacks maximise the surface area of the light harvesting membrane to maximise space for components of the ETC.
9
Q
Process of the linear electron transport chain following excitation of photosystem 2?
A
takes in light and H2O -> produced PQH2 (plastoquinol) are reoxidised by Cytochrome b6f, in turn Plastocyanin is reduced in the thylakoid lumen -> Pc donates an electron to PSI -> PSI uses this electron and harvest light to reduce ferredoxin -> which then via Fd-NADP+ reductase reduces NADP+ into NADPH which is fed into the Calvin cycle.
10
Q
What is the key side product of the light reaction of photosynthesis?
A
Oxygen, from the splitting of water by oxidation.
11
Q
How many bacterial phyla contain chloro-phototrophic members?
A
8
12
Q
What phylum are purple bacterium in?
A
Proteobacteria
13
Q
Advantages of using purple bacteria?
A
- Easy to grow in large volumes
-Most metabolically versatile organisms on the planet
14
Q
How do purple bacteria adapt to low oxygen conditions?
A
Under oxygen tension and the presence of light, they switch to photosynthetic metabolism, producing densely pigmented membranes (chromatophore) not dissimilar to thylakoids.
15
Q
What are the two main intracytoplasmic membrane architectures in purple bacteria?
A
Lamellar (concentric stacks) and Vesicular (spherical membranes formed from the invagination of the cytoplasmic membrane).
16
Q
What allows for the metabolic versatility of purple bacteria in aerobic conditions?
A
their extensive internal membrane system (known as intracytoplasmic membranes)
17
Q
What is the function of intracytoplasmic membranes (ICMs)?
A
To increase the membrane area housing photosynthetic apparatus to enhance the amount of solar energy that can be absorbed.
18
Q
Features of Oxygenic photosystems:
A
Two light driven reactions
Two types of reaction centre
Produces O2
Linear electron transfer
Cytochrome b6f
reduces plastocyanin
Generates PMF to drive ATP synthase
Directly reduces NADP+
19
Q
Features of Anoxygenic Photosystems:
A
One light driven reaction
One type of reaction centre
Does not produce O2
Cyclic electron transfer
Cytochrome bc1
reduces cytochrome c2
Generates PMF to drive ATP synthase
Doesn’t directly reduce NADPH+
20
Q
What feature of chlorophylls anchor them to complexes within the membrane?
A
long hydrophobic isoprenoid tail
21
Q
What feature of chlorophylls is light sensitive?
A
They’re tetrapyrroles with a fifth ‘E’ ring. Upon light absorption, an electron within the conjugated pi electron system (formed by alternating double and single bonds) the tetrapyrrole is excited to a higher energy state.
22
Q
What types of diagrams are used to show electron energy states?
A
Jablonski diagrams.
23
Q
What is Forster Resonance Energy Transfer (FRET)
A
Non-radiative transfer of energy from an excited donor to an acceptor
24
Q
What are the two main requirements for FRET?
A
- There must be spectral overlap between the fluorescence emission of the donor and the absorbance of the acceptor.
- Small distance, the energy transfer efficiency is inversely proportional to the 6th power of the distance (must be within 5nm)
25
Why do purple bacteria absorb different wavelengths of light of other oxygenic autotrophs?
Their bacteriochlorophyll has adapted via changes in structure and scaffold proteins to absorb light not absorbed by the cyanobacteria closer to the water's surface.
26
Purpose of accessory light-harvesting pigments:
To increase the spectra of light which can be absorbed to excite the reaction centre.
27
What are carotenoids?
An example of an accessory light-harvesting pigment. They have a delocalised pi electron system, only capable of S0 to S2 excitation, very quickly transferring the excitation energy (due to their short excitation lifetime of 200ps)
28
What is meant by carotenoids being able to perform non-photochemical quenching (NPQ)?
Carotenoids perform a photoprotective function. Chlorophylls can form triplet excited states, these are long lived and can make reactive oxygen species and damage proteins -> carotenoids can accept the energy of the triplet, returning the chlorophyll’s electron to it’s ground state.
29
Why do phototrophs require an antenna system?
- The pigment density of the reaction centre is low, prioritising electron transfer over light harvesting. Antenna complexes allow for a much higher pigment density than the reaction centres alone, increasing the amount of light and wavelengths that can be absorbed.
- Without an antenna the RC would only act at ~1% of its theoretical efficiency
30
What are the properties of good antenna systems:
- Wide spectral Cross-section of absorbance
- High pigment concentration
- Wide Spatial Cross section
- Modular
- Directionality
31
How do antenna systems achieve a wide spectral cross-section?
They use a variety of pigments (or modified forms of the same pigment) to absorb a variety of wavelengths of light
32
How do modifications to pigments alter the wavelengths they absorb?
Modifications to the polypeptide environments surrounding pigments affect their pi-electron system, affecting their excited state properties and therefore their absorption maxima.
33
What is meant by antenna systems being modular?
Antenna systems have the ability to be modulated, increasing in number per RC in low light, a decreasing in number when there's high light intensity, where excess energy can cause damage.
34
What are the two broad classes of antenna complex?
Primary antenna complexes and Peripheral antenna complexes
35
Primary antenna complexes:
complexes directly associated with RC in a fixed arrangement and stoichiometry. Split into fused antenna and core antenna.
36
What is meant by antenna systems forming an 'energy trap' ?
The arrangement of pigments makes it unfavourable for energy to transfer from the reaction centre to the accessory pigments, thereby trapping the excitation energy.
37
Fused antenna complexes
Primary antenna complexes wherein RC pigments and the antenna are bound to the same polypeptide and cannot be biochemically separated
38
Core antenna complexes:
Primary antenna complexes wherein the RC and antenna pigments are on different polypeptides (coded by different genes) which then interact and associate with each other. These can be biochemically separated.
39
What antenna complex is an example of a core antenna?
LH1 in purple bacteria, wherein different constituent polypeptides associate to form a larger complex.
40
What is the role of peripheral antennas in light-harvesting systems?
To interact with RCs and primary antenna to increase the light-harvesting capacity of the system (in addition to the primary antenna)
41
What is the key difference between primary and peripheral antennas?
- They slightly differ by function
- Key difference is peripheral antennas don't have a fixed physical arrangement with other antenna or the RC, having the ability to be mobile within the membrane and interact with different RC-antenna complexes.
42
What are the two types of peripheral antennas?
Membrane intrinsic (Non soluble e.g. LH2) and membrane extrinsic (water soluble e.g. phycobilisomes in cyanobacteria)
43
What model organism is used to study the light harvesting systems of purple bacteria?
Rba. sphaeroides
44
What are chromatophores?
Internal membrane vesicles produced by purple bacteria under anaerobic conditions, that increase the amount of membrane available for accommodating anoxygenic photosynthetic machinery. Each estimated to hold 2000 bacteriochlorophyll pigments.
45
What anoxygenic photosynthetic machinery are found in chromatophore membranes?
RC-LH1 core antenna complexes, LH2 peripheral antenna, Cytochrome bc1, ATP synthase
46
What are the structural effects of the addition of LH1 to RCs to form RC-LH1 complexes?
Increases the number of bacteriochlorophylls (BChl) from 4 to 32, and increases the area from 132nm2 to 490nm2, the pigment density increasing from 1 BChl per 33nm2 to 1 BChl per 15nm2.
47
What is the effect of LH1 association with RC in terms of functioning?
The physical area for light absorption is increased and pigment density is doubled, and-so the system becomes much more effective at absorbing energy from light.
48
How can the effects of antenna complexes be visualised?
By deconvoluting the absorption spectrum of a model organism, to visualise the contributions of different complexes to absorbance. They can be found to increase both the amount of light and breadth of the wavelengths absorbed.
49
Describe the structure of LH2 in purple bacteria
Basic unit: heterodimer of a and b polypeptides = ab pair
- Each ab pair binds: Bchla, Bchla dimer, and carotenoid
- 9 of these assemble to form LH2
50
How does each ab pair and associated pigments act as one single pigment?
The interactions between the Bchla and Bchla dimer allow energy transfer between them
51
What is the absorbance spectrum of LH2 (each pigment)
Bchla -> B850 ring (lowest energy)
Bchla dimer -> B800 ring
Carotenoid -> 400-550nM
52
Can energy be transferred between all of them?
B800 and B850 are close enough (1.8nm) for almost 100% efficient FRET
B850 are close enough to each other for delocalisation (0.9nm)
B800 are too far apart for this (2.1nm)
Carotenoid can transfer to both (0.5nm)
53
How is energy transferred?
Carotenoid -> B800 -> B850 (energetically downhill)
Carotenoid excited to S2, rapid internal conversion to S1
Transfers to B800 S1 in 1.6ps
Transfers to B850 S1 in 0.7ps
Rapidly delocalises around B850 in 0.1ps
54
Describe LH2 and RC-LH1 arrangement in chromatophores
LH2 surround RC-LH1 complexes, allowing energy to be trapped at the RC
Allows even very peripheral LH2 to transfer energy to RC (~60ps)
55
Why does Rba.Sphaeroides need to tune LH2?
Without tuning, Bchla absorbs at 771nm (needs to be at 800 or 850) so coordinate binding site
Mainly though: H-bonds to Ring E and C3-acetyl, acting as Mg ligand
56
How is it tuned to B800 ring
B800 tuning:
- H-bonds with C3-acetyl group and b-Arg30
- Carboxyl O of N-terminal carboxy Met acts as a ligand for central Mg
- H2O H-bonds to ester O on Ring E
His22 and Asn3 form H-bond network
57
How is it tuned to B850 ring
- His31 and His40 forming ligands for Mg
- Ser27 H-bonded to keto on Ring E
- Tyr45 forms H-binds to C3-acetyl carbonyl of Bchl and with the adjacent Bchl (2x)
2 C3 acetyl H-bonds = more red shifted
58
What does H-bonding to C3 acetyl groups cause?
More H-bonds to C3 acetyl means more red-shifted (increased nm absorbance)
59
What is different about the arrangement of m.purpuratum
Has 7 subunits instead of 9. This increases the distance and angle between pigments
Reduces excitonic coupling and lowers H-bonding = less red-shifted at 828nm
60
Describe the LHII PLANT peripheral antenna structure and energy transfer
Single polypeptide
Each polypeptide binds: 8 ChlA, 6 ChlB and 4 Carotenoids
Form trimers and monomers. Positioned: trimer -> monomer -> PSII core antenna -> PSII RC
61
Describe energy transfer from antenna to RC in anoxygenic PSS
- LH2 absorbs light energy, and internal transfer occurs
- LH2 transfers this energy to other LH2 in 2.7ps
- LH2 transfers to LH1 in 4.5ps which then transfers to RC in 50ps = slightly uphill so slower
62
How does it make sure energy doesn't transfer back
LH1 to LH2 transfer is slower making it energetically more uphill
63
Describe the LH1-RC complex from Rba.sphaeroides
Tuned at 875nm
Each ab pair binds a Bchla dimer and are faced perpendicular to each other to allow excitonic coupling and delocalisation
14 ab pairs assemble
LH1 complex binds 2 carotenoids
Incomplete ring around RC
Held open by PufX
64
what is excitonic coupling
If pigments are within 1nm of each other, they can acts as 1 pigment and energy delocalises around their ring in 0.1ps
65
Describe the RC-LH1 dimer complex
S-shaped 28 subunit surrounding 2 RCs
PufX essential for dimerisation, particularly its Arg45 and Arg53 residues (mutations stop dimers)
PufX forms salt bridge at dimer interface
66
What is the advantage of dimerisation
Curvature of 152 degrees imposed by PufX pushes on membranes, facilitating chromatophore formation
Also allows more efficient trapping - if one is busy the other can accept the energy
67
How are Bacteriochlorophyll of Rba.sphaeroides coordinated for excitonic coupling
His from each subunit coordinating central Mg ions
H-bonds from Tryp to C3 acetyl carbonyls
These red shift absorbance (to 875) and orient them for efficient energy transfer and directionality as 875 is lower than LH1 850
68
Difference between Type 1 and Type 2 RCs
Type 1 --> reduce Fd
Type 2 --> Reduce Quinones
69
What effect does BchlB and the gamma subunit have on B.viridus
BchlB used over BchlA = absorb lower wavelengths (red shifts)
Extra 16 gamma subunits shift this more, from useless 972nm (wavelength of water vapour) to 1018nm
Suggests gamma was recruited by bacteria to absorb useful wavelengths
70
How does an Open or Closed RC-LH1 complex affect quinone/quinol diffusion
Closed = diffuse through small pores in LH1 antenna. Possible as only 1 carotenoid per ab pair
Open = Some species have 2 carotenoids per pair that blocks usual pores, so structure needs to be open to allow diffusion
Shown -> PufX mutants can only grow once 2nd carotenoid removed
71
Describe the structure of the Rba.sphaeroides Reaction Centre
3 subunits = L, M and H
L and M form heterodimer with pseudo symmetry. These bind all the cofactors. Structurally similar (like D1 and D2 of PSII RC)
H subunit has a soluble cytoplasmic domain that insulates the quinone binding sites to stop them performing unwanted redox
72
Describe the ETC cofactors
BchlA dimer is P870 special pair
A branch (L):
BchlL--> BpheL --> Qa
B branch (B):
BchlM --> BPheM --> Qb
Carotenoid for photoprotection
Fe at the top for stabilisation
Asymmetrical so e- transfer up L branch only
73
Describe the electron transfer chain
1. P870 excited and becomes very reducing
2. Moves e- to BChlL ad BPheL in small downhill energy drops (favoured)
3. e- to Qa in big energy drop then to Qb in more favourable one
4. This separates e- from P870 sink on another side of the membrane
5. during this, reduced Cyt2 donates an e- to RC to reset it
6. This repeats until Q fully reduced to QH2
74
How is recombination prevented
FRET efficiency decays to 1/6th distance so cannot transfer straight back
Electron transfer efficiency decays with the square of the distance
Forward reactions are faster than recombination due to Marcus Inverted Theory = many small steps faster than one fewer big steps
75
Why is distance between RC and LH1 seen as a compromise
Distance created to make transfer: Fast enough for general efficiency but purposefully slower so RC doesn't get over excited and form toxic triplets
76
How and why is triplet formation prevented
If e- stalls at BPhe then it recombines with excited P870* and forms triplet state. Relaxation of this forms singlet O that damages RC in 30 mins
1. Big uphill energy gap from Qa to BPhe means more likely to safely recombine with P870+ instead
2. Carotenoids help photoprotection through NPQ (non-photochemical quenching) -> quench BChl triplets that form
77
What is the role of PSII
Using light energy to couple oxidation of 2 H2O to reduction of 2 PQ
78
What is the Oxygen Evolving Complex. Where is it and how does it work
Catalyses oxidation of water
Sits outside membrane on lumenal surface of PSII
D1 side
Sits next to TyrZ to funnel e- back into special pair
79
How does P680 and H2O/O2 potential change?
P680 becomes very oxidising (+1200mV to -630mV)
H2O becomes oxidised (+820mV to ??)
80
Describe PSII structure from Cyanobacteria T.vulcanus
Symmetrical dimer
Much more complex than PSI
- D1 and D2 subunits form heterodimeric RC core
- CP43 and CP47 are core antenna (bind RC and introduce carotenoids and chlorophyll in)
- PsbO ( and PsbU and PsbV) is extrinsic in membrane, and stabilises OEC and Mn cluster
81
Describe the PSII ETC structure in cyanobacteria T.vulcanus
Same pseudo symmetry to purple bacteria
A branch: D1 side
B branch: D2 side
ChlA dimer special pair (P680)
A --> ChlD1 --> PheD1 --> PQA
B --> ChlD2 --> PheD2 --> PQB
Non-heme Fe at the top
Carotenoids and peripheral Chlorphylls on each branch to help get energy in
TyrZ and Mn cluster om D1 side to reconcile charge separation
82
Describe the electron transfer steps
((Only branch A active, like purple bacteria))
1. P680 excited, e- moves to PQA then PQB = substrate binding site
2. Forms semiquinone (only 1 e- bound)
3. Electron hole at P680 filled by TyrZ donating e-
4. TyrZ re-reduced by Mn cluster (many oxidation states)
5. Reset to repeat again
The PQH2 moves to Cytb6f
83
Describe PSII energetcics
Redox cofactors are very close together (less than 1.4nm) with small downhill steps --> recombination much much slower than forward
84
Why is PSII energy squeezed?
Needs to reduce PQ and oxidise water together so it cannot afford the big energy gap between Phe and QA
85
Why can PSII not prevent triplet formation
No big energy drop means no safety mechanism
some electrons transfers back meaning PSIII RC needs to be replaced every 30 mins
Carotenoids perform NPQ to photoprotection
86
What's the midpoint difference of Redox potential between PSII and RC in purple bacteria? what's the importance
P680 (PSII) special pair starts with more (+) oxidising redox than PB P870 as it needs to oxidise H2O (PB cannot as P870 is less (+) than H2O)
But P870* reaches a more (-) reducing redox than P680*. This makes its potential difference between UQ 0.8V instead of 0.5V for P680* and PQ
This bigger difference makes it harder for back reaction to occur
87
Give general Z scheme of PSII -> Ctyb6f --> PSI
PSII generates reducing P680* to take e- from water
This donates 2e- to PQ to reduce it.
PQH2 moves to Cytb6f to be regenerated into PQ
PQ oxidation coupled to PC reduction
PSI oxidises PC to generate very reducing P700* to reduce Fd
FNR couples Fd oxidation to NADP+ reduction -> NADPH
88
Describe PC structure and redox reactions
Small, soluble
Has a Cu atom that can be +1 or +2 to change oxidation state
Reduced by Cytb6f, oxidised by PSI
89
Describe Fd structure and redox reactions
2Fe2S cluster coordinated by 4 Cys residues
Fd reduced by PSI then oxidised by FNR
90
How does FNR link Fd oxidation with NADPH generation
Serves as a converter --> Fd only transfers 1e- at a time, but reducing NADP+ needs 2e-
Uses Flavin cofactor that can be in different reduced states (like quinones forming semi quinones)
91
Describe the structure of PSI monomer in Cyanobacteria
- PsA and PsaB --> core RC subunits form asymmetrical heterodimer. Core antenna fused to core RC
- PsaC --> binds remaining e- acceptors. mediates electrostatic interactions with Fd
- PsaF --> subunit extends into lumen and mediates PC interactions
- PsaL --> important for oligomerisation
Spatial segregation = allows energy transfer in but not electron transfer out
92
Why and how do some cyanobacteria species form oligomers?
Monomers, trimers, tetramers, oligomers ...
Difference in bulkiness of PsaL sidechains allow different oligomers to form
Form to fit more membrane in and help with excitation sharing most likely
93
What PSI form do plants/Eukaryotes usually take.
monomeric PSI. Usually forms supercomplex with LHC1
94
Why can they not form oligomers
Cannot oligomerise due to extra subunit: PsaH that prevents it.
Tend to like to coat PSI in more antenna than anoxygenic does, meaning monomeric structure better for them
95
Describe the PSI-Fd complex that forms
Interactions mediated by electrostatic interactions:
PSa,c,d,e, and f subunits of PSI have (+) patches corresponding to Fd's (-) patches
96
Describe the PSI-PC complex that forms
Interaction relies on hydrophobic interactions with PsA and PsB on PSI
97
Why do these complexes need to form?
To allow quick docking and dissociation to allow PC and Fd to move between complexes
98
Describe the PSI electron transfer system
Have B and A branch (other way round)
P700 special pair --> chlorophyll --> chlorophyll --> Plastoquinone --> Fx --> Fa --> Fb (FeS clusters)
Electrons donated from Pc docking (fills e- hole)
Reduced Fd
99
What is different about this pathway?
e- transfer can proceed up either branch because electron doesn't need to be held somewhere = no 2e- gate needed (Fd only takes 1e-)
Both pathways intersect at Fx
100
How are the A and B branches tuned to give slightly different redox potentials. Why?
PQ potentials are tuned slightly different. A branch tuned to minimise triplet formation
B branch given slightly less driving force = step from Chlb --> PQb is quicker than A (smaller difference) but the next step to Fx is slower (bigger difference)
This means if e- stuck at Fx, it will travel down the A branch more readily as it is energetically downhill = quicker. A-branch is less likely to form triplet as recombination now is quicker than the back reaction.
101
Why is cyclic electron transfer needed in oxygenic PSS
Linear chain only makes 1.28:1 ATP:NADPH ratio but needs to make this 1.5:1 for Calvin cycle
Need to make ATP without making NADPH
102
Describe the 2 mechanisms of cyclic e- transfer
1. PGR5 docks e- from Fd/FNR on to Cytb6f to generated PQH2 and make PC
2. Photosynthetic complex 1 (added into membrane when needed). Uses e- from Fd/FNR to generate PQH2 and make PC
103
Give an example of a homo-dimeric Type 1 RC
Heliobacteria, green sulphur
104
Describe their structure
P800 special pair
Uses Bacteriochlorophyll G
Single Fx FeS cluster
No Quinone/Plastoquinone (e- goes straight from Chl to FeS)
105
How are these thought of evolutionarily?
Thought that duplication of ancestral homo-dimeric genes, then divergence of these genes from one another lead to formation of heterodimeric RCs
106
What are the key components of the mitochondrial ETC?
Complex I --> NADH ubiquinone oxidoreductase
Complex II --> succinate ubiquinone oxidoreductase
Complex III --> cytochrome bc1
Complex IV--> cytochrome c oxidase
Complex V --> ATP synthase
Succinate to Fumarate oxidation
Cytochrome C
Quinones/quinols
107
Describe the role and structure of mitochondrial complex I
Feeds NADH into Krebs cycle and adds to PMF
- Large 1000kDa
- 45 subunits
- 2 major domains:
1. hydrophilic arm extending into matrix that binds NADH and cofactors
2. Long membrane domain anchoring complex containing 4H+ pumps
108
What are MCI's electron transfer cofactors? How and where do they act
1 FMN and a wire of 7xFeS clusters on the arm domain
FMN accepts 2e- from NADH and transfers them down the clusters in the arm to Ubiquinone reduction site
UQ --> UQH2
Pumps 4H+
109
What is Photosynthetic complex 1?
PhS1 = NADPH dehydrogenase added into membrane when cyclic transfer is needed (making ATP not NADPH)
Re-oxidises 2 Fd (using 4xFeS clusters) by reducing PQ and pumps 4H+ across membrane
110
How do PhS1 and MC1 compare?
Similar:
Both use FeS clusters to reduce a quinone
Share 11 core subunits
Different:
Have complexes specific to each other (oxygenic specific, Fd or NADH specific)
PsC1 cannot bind NADPH so binds Fd instead
PsC1 has a shorter arm domain
111
Describe the role of Mitochondrial Complex III
Key players = Rieske, CytC and CytB
Always dimeric
Re-oxidise quinols, move H+ and create soluble e- carriers
112
What are respirasomes
Complex III, I and IV can form this to make it easier to pass molecules between them, and helps with membrane packing
113
Describe the structure of MCIII from purple bacteria
Symmetrical dimer
3 key subunits:
- CytB
- Rieske FeS protein (ISP)
- CytC
Each monomer has 3 core subunits and 2 independent Quinone/Quinol binding sites
114
Describe the structure of Photosynthetic Complex III (Cytochrome b6f)
Dimeric with 4 core subunits
Each monomer has 2 PQ/Q binding sites
Main subunits:
- Cytb6
- Subunit 4
- Rieske ISP
- CytF
- Pet G,L,M,N
115
Compare both Complex III's
Cytb6 + subunit 4 (Photosynthetic) are homologous to Cytb (mitochondrial) with one less TM
Rieske ISPs are largely same but differ in domain interacting with Cytochrome subunit
-
Both have helix and extrinsic domains of subunits swapped between 2 monomers to allow Rieske to interact with CytB and CytC1
116
Compare the redox factors of Cytbc1 and Cytb6f
Cytb6f has an addition C haem
117
How does Cytb6f coordinate Rieske FeS clusters differently to Fd? Why?
Unusual cluster ligation = gives it a more (+) redox value
Coordinates with 2xHis and 2xCys instead of 4xCys
Changes redox from -430mV to +300mV to allow it to strip electrons from quinones
118
Describe the process of the modified Q cycle of CtyBc1
Showing e- transfer of just one subunit:
- 2xQH2 arrive at Qo (oxidation) site
- 4e- released (and 4H+ moved to lumen)--> 2e- go down high redox cofactor chain and 2e- go up low potential redox chain
- HPC = Qo --> FeS cluster in Rieske --> Cyt1 --> reduces 2xCytC2
- LPC = Qo --> CytbL --> CytbH --> Qi (inside) --> reforms 1xQH2 (takes up 2H+)
119
Describe the movement of H+ and how many protons moved per Q
Oxidising 2xQH2 moves 4H+ into the lumen
Reforming one molecule of QH2 takes out 2 H+ from cytoplasm
This means for each 2 quinols oxidised, one is remade = doubles the number of protons translocated per quinol oxidised
120
Why does the Rieske subunit move?
When Q is at Qo site it is close enough to FeS cluster for e- transfer but the FeS is too far away to C1 (heme) for e- transfer
Rieske subunit uses a hinge region to move move the FeS cluster closer to C1 for quick e- transfer
121
What are plant chloroplasts organised into in higher plants
Interconnected stacks of thylakoid membranes: grana lamellae
Connected by non-stacked membranes: stroma lamellae
122
What is the function of arginine residues in the alpha subunits in F0 of ATP synthase?
Has an amine froup that can bind to protons, when in proximity to a glutamate it stabilises it, allowing for it to deprotonate the glutamate, allowing for the release of protons in the low high pH region.
123
What process is coupled in the F0 region of the ATP synthase?
the resolving concentration gradient with the generation of torque.
124
What is the difference between a and b subunits on ATP synthase?
They are structurally similar however only B units are catalytically active.
125
What are the 3 conformations of Beta subunits on F1 head of ATP synthase?
Open, Tight, and Loose.
126
What subunits form the F1 head of atp synthase?
3 alpha and 3 beta subunits.
127
What is the gamma subunit of ATP synthase?
The central shaft that transmits toque from F0 to F1, changing the conformation of the beta subunits.
128
What is the difference between a and b subunits on ATP synthase?
They are structurally similar however only B units are catalytically active.
129
What residue of the C ring of ATP synthase bind to the H+ ions ?
Glutamate residues
130
What are the components of ATP synthase?
A subunits, 8-15 c subunits, Y subunit, 3 alpha, 3 beta subunit, Beta 2 and sigma subunits
