Photosynthesis
Global carbon cycle
Biological energy cycle
BIOMASS
Photosynthetic “Reaction equation”
light
CO2 + 2H2O→(CH2O) + O2 + H2O
Beware! This is not what’s going on!
The crucial equation is: light →ATP, NADPH
Energy conversion and carbon fixation are
two completely separate sets of reactions!
→ATP→
→NADPH→
Light reactions
H2O
O2
Carbon reactions
CO2
CH2O
The different conversions are strictly compartmentalized
What is the maximum efficiency with which
photosynthesis can convert solar energy into
biomass? How does this compare to
photovoltaics or solar thermal energy?
Plants: Theoretical: In C4 plants 6%, C3 plants 4.6%,
Practical yield (e.g. seasonal loss of energy): ca. 1%
Photovoltaics: 8 to 24%, in the lab up to 40%.
Solar thermal energy: 60 to 75%
Ref for plants: Zhu et al (2008) Curr Opin Biotech 19:153ff
Carroll & Somerville (2009) Ann Rev Plant Biol 60:165ff
Ref for technical: Wikipedia
Photoinhibition
• In every plant, excessive light inhibits photosynthesis.
• This effect is further enhanced under suboptimal growth conditions (cold, heat,
ozone ...).
• This can even lead to the destruction of the photosynthetic machinery
Sections
• Light reactions
• Chloroplasts
The light reaction generates an electrochemical proton gradient at the thylakoid
membrane, driving ATP synthesis, and NADPH. These are required for the fixation
of CO2 into carbohydrates in the Calvin- cycle, and other assimilative reactions
taking place in the chloroplast stroma and the cytosol.
Light reaction
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Architecture of the electron transport chain
Photosynthetic pigments
Photochemical energy conversion
Structure of the light harvesting complex
Structure of Photosystem II
Photoinhibition
Proton-ATP synthase
The Light reaction
•Light reactions: PSII and PSI complexes
•Electron transport: plastoquinone, cytochrome b6-f, plastocyanin,
ferredoxin, ferredoxin NADP reductase
Pigment molecules in plants
• Chlorophyll a and b
• Carotenoids
-CHO in chl. b
Chlorophyll
• Most photosynthetic organisms contain chlorophyll
• Major light harvesting pigment
• Absorbs in violet-blue and orange-red range
Carotenoids
α
β
•The β form is common in photosynthesis
•Minor role as light harvesting pigments
•Absorption maxima at violet to blue
•Important structural role in the assembly of light harvesting
complex
•Photoprotection of photosynthetic apparatus
What does light do to Pigments ?
Excitation only if Ee-Eg=hν
Pigments absorb different wavelengths with different efficiencies
Fates of the excited state
Possible fates of the excited state
•
•
•
•
Relaxation →heat
Fluorescence→light
Resonance energy transfer→other pigment molecule
Charge separation→e-→acceptor molecule ( “photochemistry’)
The photosynthetic apparatus is known in great
structural detail
How photons are collected
Transmembrane
helices
• LHC II monomer positions 12 chlorophyll and 2 carotene molecules for
rapid energy transfer.
• LHC II is the major pigment-binding protein in chloroplasts and
represents 50% of the total thylakoid membrane protein
• LHC II is often organized in trimeric structures
How photons are collected
•Chlorophyll binding LHC complexes are tightly associated with
photosystem complexes
•The ratio of LHC trimer :PS monomer varies from 2 to 4,
depending on light intensity and quality
How photons are collected
• Photon energy is passed on by resonance energy transfer.
• The final energy acceptor is the ‘special pair’ of chlorophyll
molecules where initial charge separation takes place.
Photochemistry in a reaction centre
hν
• Chl A(cceptor)→Chl* A→Chl+ A• The reaction centre Chl is a dimer called the special pair.
• Depending on the wavelength of maximal absorption the
special pair chlorphylls are called P680 and P700
• The nature of the acceptor depends on the type of reaction
centre (other chlorophyll or Mg less chlorophyll i.e.
pheophytin).
Fates of the excited state
How electrons and protons are split
• Subunit D1 contains a highly excited chlorophyll P680
• The Mn-complex of PS II collects electrons from water in portions
of four.
• The plant PSII structure is still unresolved but bacterial structures
are available
Photosystem II
Protein
Gene
Function
D1
psbA
Reaction centre protein
D2
psbD
Reaction centre protein
CP47
psbB
Antenna binding
CP43
psbC
Antenna binding
Cytb559α
psbE
Unknown
Cytb559β
psbF
Unknown
psbH-psbN
psbH-psbN
Unknown
22kDa
psbS
Photoprotection
33kDa
psbO
Mn stabilizing protein
23kDa
psbP
Oxygen evolution
17kDa
psbQ
Oxygen evolution
10kDa
psbR
Unknown
Hydrophobic (‘intrinsic’)
Hydrophilic (‘extrinsic’)
Plant PS II forms a dimeric complex
O2 evolving complex
Plant PS II forms a dimeric complex
O2 evolving complex
extrinsic polypeptides
Plant PS II electron transfer chain
O2 evolving complex
H2O→[Mn4CaCl]→Yz/Yz•→P680/P680+→Pheoa/Pheoa-→QA/QA-→QB/QB-
The electron transport chain
• Electron transport chain transfers electrons split from water onto
NADP+
• PS II releases reduced PQH2 (lipid)
• Cyt b6-f (transmembrane protein) accepts e- from PQH2
• e- are passed on to plasotcyanin (soluble copper containing protein)
• Plastocyanin is the e- donor for PS I
PS I and PS II work analogously
The electron transport chain
• Electron transport chain transfers electrons split from water onto NADP+
• The number of electrons that are released from water need to exactly match
the electrons that are transferred to NADP+.
• Many stress conditions (cold, salt, heat, excessive light) cause imbalance
between oxidation of water and reduction of NADP+ leading to damage of PSII
(and PSI) by excess electrons - photoinhibition.
PS II as a source of singlet 1O2
• Excess light→overreduction of plastoquinone
• charge separation at P680 is blocked
• O2 is excited by energy transfer to form 1O2
• singlet oxygen (1O2) damages D1 which has to be removed by
proteases and replaced by newly synthesized D1
•→photosynthetic rate is reduced at excess light: photoinhibition
Photoinhibition
• In principle photoinhibition always occurs
• In addition to the optimized photosynthetic chain (e.g. photoprotectant
components of PSII, carotenoids etc.), plants employ several preventive
strategies to prevent photoinhibition. (‘Prevention’ see Stress in Plants)
• Lateral heterogeneity (discussed later)
• Repair cycle of photodamaged PSII
PS II partially disassembles for D1 repair
FtsH proteases degrade D1
The electron transport chain
• Electron transport chain transfers electrons split from water onto
NADP+
• This process builds up a proton gradient between the inner and the
outer side of the photosynthetic membrane
• Under optimal conditions: ∆pH=3-3.5
• The world’s smallest turbine - ATP synthase - harnesses the proton
gradient to generate ATP
H+driven ATP Synthase
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Summary Light reaction
• Light and carbon reactions are separated
• Light excites chlorophyll
• Excitation energy is channeled by resonance energy transfer
• In the PSII reaction centre, electrons are split from highly excited
P680
• This creates a strong oxidant that abstracts electrons from water,
which releases the same amount of protons
• The electrons are passed through a chain of mobile compounds
(plastoquinone, cytochrome c6-f, plastocyanine) to PSI and ferredoxin
NADP reductase.
• The reaction chain generates a proton gradient that drives ATP
production using a molecular turbine
Where does photosynthesis happen in plants?
The thylakoid membrane is one of the world’s most extensive membrane system
Chloroplasts
• Architecture of plastids
• Lateral heterogeneity
• Plastid biosynthesis and conversions
• Chloroplast protein import and sorting
QuickTime™ and a
Animation decompressor
are needed to see this picture.
Architecture of chloroplasts
stroma
stromal thylakoid membranes
thylakoid lumen
granum
intermembrane space
inner chloroplast membrane
outer chloroplast membrane
Why is the chloroplast so complex?
• Larger surface
• Compartmentalization
• Regulatory Potential
Lateral heterogeneity
Compartmentalization of photosynthetic complexes in the thylakoid membrane
• Redox sensitive protein kinase/phosphatase
regulates partitioning of LHC between thylakoid
domains
• Distribution of radiant energy between PSII and PSI
has to be tightly regulated
• Inefficient electron transfer would cause photooxidative damage
How are Chloroplasts made?
• Plastids reproduce by division
• Most types of plastids divide, most commonly proplastids,
etioplasts and young chloroplasts
• Plastids are inherited maternally, except for gymnosperms
Plastic Plastids
Plastic plastids
•Starch storage
•Statoliths
How are Chloroplasts made?
Mechanism of chloroplast division is still unclear
are localized at poles and
might regulate FtsZ ring
assembly
forms a filamentous
contractile ring
GC: giant chloroplasts
ARC: accumulation and
replication of chloroplasts
Plastid fate depends on the set of imported proteins
Plastic plastids
• arrested chloroplasts.
• prolammelar bodies store
lipids but lack proteins
• light reaction
• synthetic reactions
.
• Coloration
• Carotene synthesis
• Monoterpene synth.
•Starch storage
•Statoliths
Chloroplast protein import and sorting
• Plastids contain ca. 3000 proteins.
• ±100 proteins are encoded in the plastic genome
• The majority is encoded in the nucleus and translated in
cytosolic ribosomes.
• Specific plastid protein import and intraplastidal sorting
mechanisms are required for appropriate protein targeting.
• The complement of imported proteins defines the developmental
fate of the plastid.
• Import is mediated by molecular machines in the inner and outer
plastid membrane called translocons.
Schematic Representation of Pathways Responsible for Targeting Proteins
to Their Proper Location within Chloroplasts
Keegstra, K., et al. Plant Cell 1999;11:557-570
Chloroplast protein import
•
Plastid import is a post-translational process.
•
Plastid import depends on amino terminal targeting signals.
•
Transit peptides are remarkably heterogeneous.
•
20->100aa, frequent OH-resid., rare COO- resid.
•
Programs can predict chloroplast targeting signals (e.g. PSORT:
http://wolfpsort.org/ )
•
Some signal peptides are also targeted to mitochondria.
•
Cytosolic chaperones hsp70 prevent premature folding
•
Translocon at the outer envelope of chloroplasts: Toc
•
Translocon at the outer envelope of chloroplasts: Tic
•
Upon emergence on the stromal side, the signal peptides are cleaved
off by the stromal processing peptidase (SPP).
Chloroplast protein import components
• Toc: ca 500kDa
• Toc159 and 34 control peptide recognition
(GTPases)
• Toc75 forms a pore through the outer envelope
• Toc12, Hsp70, Tic22 facilitate passage
• Tic110 and 20 form inner env. Channel (?)
• Cytosol Hsp70, 14-3-3, Toc64 give
additional guidance (?).
• Tic55, -63, -32 regulate import in response to
redox signals (?).
• Multiple arabidopsis genes encode isoforms
performing performing varying functions.
• Different Toc159 isoforms are required for
photosynthetic and non-photosynthetic proteins.
Schematic Representation of the Components and Stages Involved in the
Translocation of Precursors across Plastid Envelope Membranes
Keegstra, K., et al. Plant Cell 1999;11:557-570
Alternative mechanisms of GTPase function
How is import of specific proteins regulated?
• Initially components were identified biochemically using isolated
chloroplasts
• The sequencing of the Arabidopsis thaliana genome shows that many
component proteins are encoded by multiple genes
•The differential expression of isoforms with different specificities might
regulate selective protein import and plastid fate
Toc 159 isoforms are functionally diverged
chloroplast defect:
albino leaves
etioplast defect:
in roots
Ultrastructure of Plastids in the Toc159 Homolog Knockout Mutants at
Different Developmental Stages
Kubis, S., et al. Plant Cell 2004;16:2059-2077
Other mechanisms are involved in sorting to subcompartments
e.g. Toc34
Keegstra, K., et al. Plant Cell 1999;11:557-570
Sorting to chloroplast domains
• Envelope:
• 1: Small proteins without signal peptide are targeted to outer membrane
(targeting information in transmembrane domain).
• 2: Larger proteins with signal peptides are targeted to inner envelope membrane.
• Thylakoid lumen: two tandem signals
• 1: Sec system (ATP dept., ∆pH assisted, substrates unfolded, ca. 50% of
lumenal proteins
• 2: Tat system (recognizes Twin arginine translocase signal, ∆pH dependent,
substrates folded
• Thylakoid membrane:
• Signal recognition particle (SRP) pathway (SRP, FtsY [GTPases], Alb3 [integr.
membrane], for some LHC proteins)
• Major pathway: possibly spontaneous
Summary Chloroplasts
• Chloroplasts have three distinct membranes and three
separate aqueous compartments
• Lateral heterogeneity thylakoid protein composition is
part of photosythetic efficiency regulation
• Plastids arise by division
• Import and sorting of nuclear encoded proteins
determines plastid fate