Light-Harvesting Antenna Complexes Flashcards
Reaction centre (RC) energy absorption ability?
RC electron transfer ability?
Why are antenna needed?
Reaction centre (RC) prioritises electron transfer over light-harvesting, so its pigment density is low
Special pair has a small absorption cross-section, receiving only 1 photon/s
RCs are capable of over 100 electron transfers per second
Why antenna networks needed?
Without an antenna the RC would act at only 1% of theoretical efficiency
Antenna complexes capture light and transfer excitation energy to the RC special pair
What are the typical properties of a good antenna system? (4 properties) (hint - spectra, concentration, spatial, modularity)
Wide spectral cross-section – Use different types of pigments to give multiple, broad absorption bands
- Effect of the specific polypeptide environment on the pigment’s π-electron system determines the excited state properties and absorption maxima
High pigment concentration – Antenna polypeptides position pigments at distances and orientations that allow fast and efficient intra-molecular pigment-to-pigment energy transfer by FRET, minimising losses of excitation energy as heat and fluorescence
Wide spatial cross-section – Provide a large physical area for capturing light; Antenna networks can span 50-300 nm within photosynthetic membranes
- Requires efficient inter-complex energy transfer between antenna pigments mediated by protein-protein interactions between antenna complexes
Modularity – Build up in low light (increased number of antenna per RC) and reduce in high light where excess energy could overwhelm the RC and be damaging
- Some antenna complexes can quench excited states to safely dissipate excess energy as heat and protect the RC – Called non-photochemical quenching (NPQ)
Light energy is absorbed by the antenna quickly and efficiently transferred to RC ‘traps’
How does the antenna form an ‘energy trap’?
Inter- and intra-complex energy transfer?
Antenna system is arranged as an ‘energy funnel’
High energy absorbing donor pigments are far from the RC and transfer excitation energy to lower-energy absorbing acceptor pigments that are closer to RC by FRET
This is thermodynamically favourable
This is the case for intra-complex pigment-pigment energy transfer within a single antenna
Also the case for inter-complex energy transfer between different antenna complexes
What are the 2 broad classifications of antenna complexes?
Primary antenna which is directly associated with the RC in a fixed arrangement and stoichiometry
Peripheral antenna which interact with RCs and the primary antenna to increase light-harvesting capacity of the system
Types of primary antenna and their traits (2 types and 2 traits for each) (hint - biochemical separation)
Fused antenna
- Antenna and RC pigments bound to the same polypeptide
- Antenna cannot be biochemically separated from the RC
Core antenna
- RC and antenna are formed from different polypeptides that interact with eachother
- Antenna can biochemically separated from the RC
Traits of peripheral antenna (5 traits) (hint - more adaptable)
Present in addition to primary antenna; Not instead
Present in variable amounts depending on light intensity
Don’t have fixed physical arrangement with other antenna/RC and may be mobile; Can interact with different RC-primary antenna complexes
May be exchanged for alternative variants to adapt to specific light conditions
May be membrane intrinsic (embedded) or membrane extrinsic (water-soluble and associate with membrane)
Rba. spaeroides (purple bacterium) – Model Organism for Studying Energy Transfer in Antenna Complexes
What does it form under anaerobic conditions and what do they do? (3 things)
Forms internal membrane vesicles called chromatophores
Increase amount of membrane for accommodating the protein complexes required for anoxygenic photosynthesis e.g. LHI and LHII
Number of chromatophores varies depending on light intensity
LHII can absorb light directly; Different wavelengths
What do chromatophores consist of? (4 things)
Monomeric and dimeric RCs enclosed by RC-LHI complexes
LHII peripheral antenna; Number varies depending on light intensity
Cytochrome bc1 complex
ATP synthase
How does RC-LH1 compare to RC?
RC-LHI has many more bacteriochlorophylls compared to RC alone
This increases physical area for light absorbing and doubles pigment density
RC-only mutants of Rba. sphaeroides grow very slowly even with very high illumination
How do LHII and LHI complexes improve light capture?
Increase the physical area over which light is absorbed to capture more photons for each RC
Each antenna absorbs light at different wavelengths to increase the range of the solar spectrum that the cell can utilise for photosynthesis
What is the structure of LH2 in purple bacteria Rhodoblastus acidophilus? (hint - heterodimer)
How many units?
Basic unit is a heterodimer constructed from 1 α (inside) and 1 β (outside) polypeptide
- Both are single transmembrane spanning proteins with a central α-helix in the membrane
- Between each subunit, several pigments are sandwiched
9 αβ pairs self-associate to form a monomeric LH2 complex
What does each αβ pair in Rhodoblastus acidophilus bind?
1 monomeric BChl a
1 BChl a dimer (positioned face-to-face); BChls are so close (<1nm) that their electronic structures interact making them function as a single pigment
1 carotenoid
How are the LHII spectral properties fine tuned?
What forms the B850 ring? (hint - excitonically coupled)
Protein modifies the properties of pigments to tune spectra and act as a scaffold to ideally position them for efficient energy transfer
BChl a dimers are stacked within 0.9 nm of eachother, so all 18 of them are strongly excitonically coupled (acts as 1 big pigment
This tunes the Qy band to 850nm
What forms the B800 ring in LHII?
Intra-complex energy transfer?
Which ring is terminal acceptor?
BChl a monomers from the B800 ring; Higher energy
These close distance between B800 and B850 ring for efficient intra-complex energy transfer
B850 have lowest energy making them the terminal acceptor
What are the distances between B800 rings and B850 rings and the effects this has on energy transfer?
B850 BChls are separated by 0.9 nm so excitation energy delocalises over entire 18-pigment ring
9 B800 BChls are too far apart for excitonic coupling (2.1 nm)
B800 and B850 are separated by 1.8 nm with spectral properties tuned for rapid energy transfer
Carotenoid wavelength absorbance?
Distance between carotenoid and B800, B850 rings and speed of energy transfer?
Broad absorption between 400 - 550 nm
Within 0.5 nm of both so very fast energy transfer
Explain a Jablonski diagram for energy transfer within LH2
Light absorbed in the 400-550nm region excites a carotenoid to the S2 state, which rapidly (<300 fs) dissipates energy by internal conversion to reach the S1 state
S1 excited state of adjacent B800 BChl is energetically downhill so energy is transferred rapidly
S1 excited state of the close by B850 BChls is energetically downhill so energy is transferred rapidly
Excitation is rapidly delocalised around B850 ring
How are LH2 and RC-LH1 arranged in chromatophore membranes?
Arranged for ideal energy transfer
Antenna complexes are densely packed in membrane to minimise distance of energy transfer
Light absorbed by the LH2 array migrates to LH1 and is trapped by the RC which is up to 50 nm away
Antenna arrangement allows fast and efficient transfer with minimal loss of energy
Isolated BChl a absorbs maximally at 771 nm in solvent
What does this mean the protein does to its Qy? (2 options)
Protein tunes the Qy absorbance to either:
- 800 nm in the case of the monomeric B800 BChls
- 850 nm in the case of the dimeric B850 BChls
How is Qy absorbance in BChl a tuned to either 800 nm or 850 nm? (broad statements)
Modifications to asymmetrical chlorophyll molecule affect distribution of electrons in the conjugated π system, and in turn the energetic and spectroscopic properties of the pigment
Interactions with protein scaffolds and proximity to other pigments also tune the absorbance properties of (bacterio)chlorophylls
How is Qy absorbance of 800 nm achieved in B800? (4 factors)
H-bond network holding BChl in defined orientation in binding site
- β-Arg30 forming H-bonds to the C3-acetyl carbonyl group of BChl
- A carboxyl oxygen of the modified N-terminal carboxy-Met of the α-subunit acts as a ligand to the central Mg
- A water molecule forms a H-bond to an ester oxygen on ring E
- β-His22 and α-Asn3 form H-bond network
These interactions and proximity of carotenoids cause red shifting of absorbance from 771 to 800 nm
How is Qy absorbance of 850 nm achieved in B850? (4 factors)
H-bond network
- α-His31 and β-His40 form ligands to Mg ions
- α-Ser27 is H-bonded to the keto group on ring E
- α-Tyr45 forms H-bond to the C3-acetyl carbonyl of one of the BChls in the same heterodimer
- α-Tyr44 forms H-bond to the C3-acetyl carbonyl of a BChl bound to the adjacent heterodimer
These interactions and excitonic coupling cause red shifting of the absorbance to 850 nm
Why is heptameric B828 LH2 from Marichromatium purpuratum less red shifted?
How does it compensate?
Fewer subunits; 7 αβ heterodimers – Increases distance and angle between BChl dimers in adjacent α/β heterodimers, which reduces excitonic coupling
Along with difference in H-bonding at the binding site, this results in a less red-shifted ‘B850’ BChl dimer with an absorbance maxima at 828 nm
Binds an extra carotenoid per α/β pair – Increases absorption in 450-550 nm region
