Photosynthesis
Global carbon cycle
Biological energy cycle BIOMASS
Photosynthetic Reaction equation light CO 2 + 2H 2 O (CH 2 O) + O 2 + H 2 O 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! Light reactions ATP NADPH Carbon reactions H 2 O O 2 CO 2 CH 2 O 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 CO 2 into carbohydrates in the Calvin- cycle, and other assimilative reactions taking place in the chloroplast stroma and the cytosol.
Light reaction 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 b 6 -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
Protein Hydrophobic ( intrinsic ) D1 D2 CP47 CP43 Cytb 559 α Cytb 559 β psbh-psbn 22kDa Hydrophilic ( extrinsic ) 33kDa 23kDa 17kDa 10kDa Photosystem II Gene psba psbd psbb psbc psbe psbf psbh-psbn psbs psbo psbp psbq psbr Function Reaction centre protein Reaction centre protein Antenna binding Antenna binding Unknown Unknown Unknown Photoprotection Mn stabilizing protein Oxygen evolution Oxygen evolution Unknown
Plant PS II forms a dimeric complex O 2 evolving complex
Plant PS II forms a dimeric complex O 2 evolving complex extrinsic polypeptides
Plant PS II electron transfer chain O 2 evolving complex H 2 O [Mn 4 CaCl] Y z /Y z P680/P680 + Pheo a /Pheo a- Q A /Q A- Q B /Q B -
The electron transport chain Electron transport chain transfers electrons split from water onto NADP + PS II releases reduced PQH 2 (lipid) Cyt b6-f (transmembrane protein) accepts e - from PQH 2 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 1 O 2 Excess light overreduction of plastoquinone charge separation at P680 is blocked O 2 is excited by energy transfer to form 1 O 2 singlet oxygen ( 1 O 2 ) 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 c 6 -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 arrested chloroplasts. prolammelar bodies store lipids but lack proteins Plastic plastids 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