Purple Photosynthesis L10 Flashcards
What is the difference b/w chlorophyll a and b and c
a and b have a phytyl side chain which make it lipid soluble
c lacks this phytol side chain
Chlorophyll is a substituted tetrapyrrole. The 4 pyrrole nitrogens coordinate a magnesium atom -magnesium porphyrin
The molecule has an extensive system of CONJUGATED DOUBLE BONDS. Electrons are delocalised over this system in molecular orbitals.
Name some light harvesting pigments
Carotenoids-linear polyenes, sometimes with one or two cyclic ends, -examples are beta carotene and spheroidenone
Bilins-linear tetrapyrrole that enable cyanobacteria and red algae to utilise YELLOW and GREEN light, -examples are phycocyanin and phycoerythrin
What is the photosystem design?
Antennae/light harvesting pigments surround a reaction centre and these collect light energy and pass the energy by RESONANCE transfer to the reaction centre where the ELECTRON transfer happens.
Once the excited state energy is funnelled by resonance energy transfer to the chlorophyll species inside the reaction centre that catalyses the photo-chemical reaction.
Once it arrives at the ‘primary donor’ a chlorophyll species, inside the reaction centre, the excited state energy triggers the PHOTO OXIDATION reaction-producing electron transfer
What is in the reaction centre? And what is in the antenna?
Rxn centre-proteins/chlorophylls/quinones/redox centres
Antenna-proteins/light harvesting or antenna pigments/chlorophylls/carotenoids
Resonance energy transfer
The Resonance energy transfer of an excited electron is passed to a neighbouring chlorophyll fuelling of energy towards the reaction centre, chlorophylls are activated because energy is passed progressively to RED SHIFT PIGMENTS
Anoxygenic photosynthetic bacteria
The most heavily studied are purple non-sulphur bacteria
These do not produce oxygen as a waste product of photosynthesis
CYCLIC electron transfer process, no water oxidation, no oxygen produced
Proteins inside these organisms are very oxygen sensitive
Rhodobacter are metabolically flexible organisms and grow through photosynthesis/anaerobic respiration/aerobic respiration. What colour are they in the absence and presence of oxygen?
In oxygen=bright red
anaerobically=brown/green
Their colour comes form the carotenoids not the chlorophyll pigments
The reaction centre in Rhodobacter is similar to PSII. What is it composed of?
3 polypeptide chains H L M
H is a single memb spanning helix sits on top of the other two polypeptides in the structure
L is composed of 5 TMS helices
pseudosymmetry to central part of structures in M
M is composed of 5 TMS helices
Structures purpose=structural support/scaffold for the pigment molecules that can either absorb light energy or move electrons around
Structure of Rhodobacter reaction centre
4 chlorophyll P870 2 bacteriopheophytins 2 ubiqinones 1 iron atom 1 carotenoid (long linear polyene)
Bacteriopheophytins-speciallised bacteriochlorophyll
no central metal (chlorophylls contain central Mg)
2 protons instead
free acid
TWO BRANCHES arranged around 2 fold symmetry (A=ACTIVE BRANCH)
NB/ ubiquinones and bacteriochlorophylls all have long hydrocarbon tails which make them lipid-soluble
Excitation of the rhodobacter energy
p870+ is reduced by by cytochrome c2 (water soluble) cytochrome docks onto the protein surface on the rxn centre and delivers an electron to reduce the cation.
Electron on QA moves laterally in the memb and arrives on the QB quinone. The system has been reset
Ubiquinol formation at QB
Double reduction of the UQ at QB is accompanied by the uptake of 2 protons to form ubiquinol. UQH2
Quinones can be either 1 electron redox carrier (if bound to a proton complex interior) or 2 electron/2 hydrogen carriers if its soluble-when free to interchange with the intermembrane quinone pool
Protons do not move across the memb as protons, they move across as hydrogen atoms connected to a quinone molecule-therefore this system is carrying electrons as hydrogen atoms
In photosynthetic bacteria where is proton uptake?
Proton uptake occurs in the cytoplasm and release in the periplasm
This generates a proton gradient
Free energy associated with the proton gradient drives protons back through ATP synthase to generate ATP
When ATP is hydrolysed you get lots of free energy out
Energy is locked up in a photon is stored in a high concentration of ATP
In purple bacteria what is the peripheral antenna complex like?
LH2 antenna complex consists of concentric cylinders of alpha and beta polypeptides that each have a single transmembrane alpha helix
Eg Rhd sphaeroides LH2 is similar=9 pairs of alpha and beta polypeptides
18 B850 arranged perpendicular to plane of the memb.
9 B800 BChls (rings) -higher energy are arranged parallel to the memb
In purple bacteria what is the core antenna complex like?
The core LH1 antenna complex has similar structure to LH2
It is larger so it can surround the reaction centre
Sandwiched b/w 15 pairs of alpha and beta polypeptides is a ring of 32 875nm-absorbing BChls arranged perpendicular to the plane of the memb
BChls in the reaction centre are P870
Energy is funnelled to the reaction centre because LH1 BChls absorb at lower energy than the LH2 BChls
Drop in energy ensures the energy is funnelled in the direction nature needs it to go in, Energetically favourable process.
Size of energy gap gets progressively smaller as you go more RED SHIFTED (longer wavelength)
What makes ubiqinones and bacteriochlorophylls lipid soluble?
They have long Hydrocarbon tails
Reaction centre
The rxn centre potential difference is coupled to an external circuit
Electron potential difference arises b/w the 2 poles (+p870 -QA)
Used to drive useful external work
Drives subsequent electron transfer in the system
What reduces P870+ ?
cytochrome c2 (water soluble)
cyt c2 docks onto protein surface of rxn centre and gives an electron to reduce p870+ cation
electron on QA- moves laterally in the memb and arrives at QB quinone
(QB is reduced by QA-)
SYSTEM HAS BEEN RESET
Photosystem I
2 active sites connected by an ETC
It is an enzyme with 2 enzymatic activities:
Plastocyanin oxidation
Ferredoxin reduction
Plastocyanin:ferredoxin oxidoreductase
Protein in the membrane is trimeric-11 polypeptide subunits (many with multiple TM spanning regions)
100 Chl a
2 phylloquinones
3 Fe4S4 clusters
Carotenoids (127 cofactors in each monomer)
PsaA/PsaB -11 TM alpha helices (Cterminus-5/11TM helices) (heart of complex) PsaC (sticks out of memb) polypeptides bind the cofactors involved in the ETC, forming a rxn centre
PsaA/B N terminus=antenna (6/11 TM helices)
Each bind up to 45 core anntenna chlorophophylls
These domains show homologies to the CP43/47 core antenna complex of PSII
Purple Bacteria
LH1 and LH2 are packed closely together since need to move E through system relatively rapidly to get it into the rxn centre from where it is initially absorbed.
All pigments have an intrinsic fluorescence life time
Absorb the photon and after a particular moment in time they will emit that photon as fluorescence unless you do something with the energy.
Carotenoids are present in these structures too.
Differences b/w PSII and bacterial rxn centre
PSII is similar but much larger
It is DIMERIC
At heart of system: reaction centre, CORE antennae, ETC
PSII reaction centre
studied more in cyanobacteria than plants
Main subunits=D1 and D2 polypeptides
5TM alpha helices
D1/D2 heterodimer similar structure to L/M heterodimer in purple bacteria rxn centre
10 TM alpha-helices 2 fold symmetry axis
4 chlorophylls (2 demethylated Chls in middle of the membrane like the bacterial system)
2pheophytins
bound at interface of D1/D2 polypeptides
2 extra chlorophylls bound in periphery of D1/D2 structure
Mn cluster
2x Tyrosine (Yz/YD)
P680 not P870(bacteria)
33,23,17kDa proteins form oxygen evolving complex and stabilise Mn cluster
What is the Mn composed of?
Mn complex with Ca and O2 to which water binds
Why are plants green?
P680 from PSII is in the red part of the spectrum and this is why plants are green-they are full of this red absorbing chl.
How is the P680+ cation re reduced in PSII of plants?
Re-reduced by electrons delivered via a redox active tyrosine (Yz) from TWO water molecules bound to a cluster of 4Mn atoms and 1Calcium
Yz has high oxidising potential
Redox active
Can lose an electron and pick one up from a donor
Yz+ is sufficiently oxidising to pull an electron off a water molecule bound to the Mn cluster
The Mn cluster catalyses the oxidation of water (pulls FOUR electrons off 2 water molecules) generating molecular oxygen and releasing 4H+
