Lecture 10 C4 Metabolism.pdf

The Bifunctionality of Rubisco 30% of C fixed by photosynthesis can be reoxidized to CO2 CO2 carboxylase 2 x 3PGA sucrose + efficient RuBP +...

The Bifunctionality of Rubisco 30% of C fixed by photosynthesis can be reoxidized to CO2 CO2 carboxylase 2 x 3PGA sucrose + efficient RuBP + (2P6) O2 3PGA + glycolic acid CO2 oxygenase inefficient ▪ At normal atmospheric concentrations of CO2 and O2 both the carboxylase and oxygenase functions operate ▪ At high CO2:O2, only the carboxylase function operates “Nothing in biology makes sense except in the light of evolution” - Theodosius Dobzhansky (1973) (1973) 2  Evolutionary “Choices”………. Protein evolution; higher CO2 affinity but low catalytic efficiency (C3 plants) Modify gaseous micro-environment; develop a CO2 concentrating mechanism (cyanobacteria/microalgae/C4 plants) Carbon-concentrating mechanisms in bacteria, algae and plants Cyanobacteria C4 plants use concentrate CO2 in flagella phosphoenolpyruvate protein structures called carboxylase (PEPC) for the chloroplast carboxysomes (here primary carboxylation reaction, labeled with GFP) concentrating or storing CO2 nucleus upstream of Rubisco Carbonic anhydrase (CA) HCO2- CO2 py PEPC C4 acids pyrenoid CO2 Many algae concentrate Rubisco CO2 in pyrenoids within CB cycle their chloroplasts Sun, Y., Casella, S., Fang, Y., Huang, F., Faulkner, M., Barrett, S. and Liu, L. (2016). Light modulates the biosynthesis and organization of cyanobacterial carbon fixation machinery through photosynthetic electron flow. Plant Physiol. 171: 530-541. Engel, B.D., Schaffer, M., Kuhn Cuellar, L., Villa, E., Plitzko, J.M. and Baumeister, W. (2015). Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889. 4 Carbon-concentrating mechanisms in bacteria, algae and plants Cyanobacteria C4 plants use concentrate CO2 in flagella phosphoenolpyruvate protein structures called carboxylase (PEPC) for the chloroplast carboxysomes (here primary carboxylation reaction, labeled with GFP) concentrating or storing CO2 nucleus upstream of Rubisco Carbonic anhydrase (CA) HCO2- CO2 py PEPC C4 acids pyrenoid CO2 Many algae concentrate Rubisco CO2 in pyrenoids within CB cycle their chloroplasts Sun, Y., Casella, S., Fang, Y., Huang, F., Faulkner, M., Barrett, S. and Liu, L. (2016). Light modulates the biosynthesis and organization of cyanobacterial carbon fixation machinery through photosynthetic electron flow. Plant Physiol. 171: 530-541. Engel, B.D., Schaffer, M., Kuhn Cuellar, L., Villa, E., Plitzko, J.M. and Baumeister, W. (2015). Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889. 5  Cyanobacteria Evolved efficient CO2 concentrating mechanisms (CCM) Bicarbonate transporters Carboxysomes Why? Selective pressures Aquatic environments create problems with CO2 availability (diffusion 10,000X slower than air). CCM evolved when CO2 levels in atmosphere dropped (300Mya)  Carboxysomes (A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars indicate 100 nm.  The bacterial carboxysome sequesters Rubisco and concentrates CO2 Carbonic anhydrase (CA) Bacteria concentrate catalyzes the reversible reaction: CO2 and Rubisco in a microcompartment the CO2 + H2O HCO3- + H+ carboxysome. Rubisco Carboxysomes can CO2 concentrate CO2 CA 1000x above ambient. Inner membrane transporters for HCO3- CO2 HCO3- and thylakoid Carboxysomes have transporters for CO2 an icosahedral (20 HCO3- HCO3- bring inorganic carbon sided) protein shell into the cell (blue) that resists CO2 efflux, into which Rubisco (green) is packed Adapted from Price, G.D., Badger, M.R. and von Caemmerer, S. (2011). The prospect of using cyanobacterial bicarbonate transporters to improve leaf photosynthesis in C3 crop plants. Plant Physiol. 155: 20-26; Badger, M.R., and Price, G.D. (2003). CO2 concentrating mechanisms in cyanobacteria: molecular components, their diversity and evolution. J. Exp. Bot. 54: 609 – 622. Toyeates 8 © 2016 American Society of Plant Biologists Carbon-concentrating mechanisms in bacteria, algae and plants Cyanobacteria C4 plants use concentrate CO2 in flagella phosphoenolpyruvate protein structures called carboxylase (PEPC) for the chloroplast carboxysomes (here primary carboxylation reaction, labeled with GFP) concentrating or storing CO2 nucleus upstream of Rubisco Carbonic anhydrase (CA) HCO2- CO2 py PEPC C4 acids pyrenoid CO2 Many algae concentrate Rubisco CO2 in pyrenoids within CB cycle their chloroplasts Sun, Y., Casella, S., Fang, Y., Huang, F., Faulkner, M., Barrett, S. and Liu, L. (2016). Light modulates the biosynthesis and organization of cyanobacterial carbon fixation machinery through photosynthetic electron flow. Plant Physiol. 171: 530-541. Engel, B.D., Schaffer, M., Kuhn Cuellar, L., Villa, E., Plitzko, J.M. and Baumeister, W. (2015). Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889. 9  Pyrenoids are highly packaged Rubisco, interspersed with thylakoid membranes flagella chloroplast Thylakoids nucleus py Rubisco Rubisco Starch Engel, B.D., Schaffer, M., Kuhn Cuellar, L., Villa, E., Plitzko, J.M. and Baumeister, W. (2015). Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889. 10  CO2 is concentrated in pyrenoids via transporters and carbonic anhydrase to superf Starch layer: impermeable con cus HCO3- is poorly M Chloroplast wan CO2 diffusion HCO3- ↳ HCO3- HCO3- barrier membrane permeable, but specific transporters move it CA into chloroplasts and HCO3- CO2 thylakoids Bicarbonate Carbonic anhydrase transporter (CA) converts HCO3- to Chloroplast Pyrenoid CO2 (Rubisco) envelope CO2 is highly Plasma membrane permeable membrane but back-diffusion is Thylakoids slowed by the pyrenoid’s starch layer Adapted from Meyer, M. and Griffiths, H. (2013). Origins and diversity of eukaryotic CO 2-concentrating mechanisms: lessons for the future. J. Exp. Bot. 64: 769-786. 11  Pyrenoids and carboxysomes evolved long after plastid endosymbiosis Endosymbiotic origin of chloroplasts Adapted from Price, G.D., Badger, M.R. and von Caemmerer, S. (2011). The prospect of using cyanobacterial bicarbonate transporters to improve leaf photosynthesis in C3 crop plants. Plant Physiol. 155: 20-26; 12  Carbon-concentrating mechanisms in bacteria, algae and plants Cyanobacteria C4 plants use concentrate CO2 in flagella phosphoenolpyruvate protein structures called carboxylase (PEPC) for the chloroplast carboxysomes (here primary carboxylation reaction, labeled with GFP) concentrating or storing CO2 nucleus upstream of Rubisco Carbonic anhydrase (CA) HCO3- CO2 py PEPC ↓ C4 acids pyrenoid storage CO2 Many algae concentrate Rubisco CO2 in pyrenoids within CB cycle their chloroplasts Sun, Y., Casella, S., Fang, Y., Huang, F., Faulkner, M., Barrett, S. and Liu, L. (2016). Light modulates the biosynthesis and organization of cyanobacterial carbon fixation machinery through photosynthetic electron flow. Plant Physiol. 171: 530-541. Engel, B.D., Schaffer, M., Kuhn Cuellar, L., Villa, E., Plitzko, J.M. and Baumeister, W. (2015). Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography. eLife. 4: e04889. 13  History: Hatch and Slack, 1966. A new carboxylation reaction. Pulse-chase study: 14CO2 label accumulates first in malate and other C4 acids, then 3-PGA Ist product to be p fant (3) Hexose not (2) + Asp - phosphates malute 3-PGA wa not U3 PGA photosynthesis Republished with permission from Hatch, M.D., and Slack, C. R. (1966). Photosynthesis by sugar-cane leaves: A new carboxylation reaction and the pathway of sugar formation. Biochem. J. 101: 103–111. 14  History: Hatch and Slack, 1966. A new carboxylation reaction. Pulse-chase study: 14CO2 label accumulates The C4 cycle proposed by Hatch and Slack first in malate and other C4 acids, then 3-PGA (3) (3) Hexose (2) phosphates (2) 3-PGA Rubisco-catalyzed carboxylation Decarboxylation Novel Carboxylation - (1) C3 + CO2 C4 theLa Republished with permission from Hatch, M.D., and Slack, C. R. (1966). Photosynthesis by sugar-cane leaves: A new carboxylation reaction and the pathway of sugar formation. Biochem. J. 101: 103–111. 15  Kranz Anatomy in Sugarcane (Saccharum officinarum L.) In typical C4 grasses, mesophyll cells (arrows) are radially arranged around the bundle sheaths, which consist of large cells containing many large chloroplasts u ⑧ O 8 The C4 pathway occurs in the mesophyll cells; the Calvin cycle occurs in the bundle sheath cells 16  C4 Pathway C4 plants - spatial separation of CO2 requir s esation fixation from Rubisco PEPC P factor See psil reaction of rubisco ros cansuncion. with O2 13 Cu avoids costly glycolate pathway. CO2 concentrating are PSI a S mechanism oveelivation 17  Differentiation of chloroplasts in the C4 Pathway - PHOTOSYSTEM 1 Mesophyll ~ O2 Bundle sheath CPs lack appressed lamellae Only PSI No PSII = no O2 evol. > Decreased RuBP O2 -ase separate Bundle sheath Kept PS'sare photo preven respiration PHOTOSISTEM 118  C4 photosynthesis has evolved more than 60 times, mainly in hot, dry regions Centers of origin for some C4 plants C4 PS increases biomass Hot, dry conditions Strong advantage Selected repeatedly Corn (maize) Sugar cane Sorghum Pearl millet Sage R.F., Christin P.-A., and Edwards E.J. (2011). The C4 plant lineages of planet Earth. J. Exp. Bot. 62: 3155-3169 by permission of Oxford University Press 19  Why hasn’t C4 photosynthesis evolved everywhere? 10°C C3 plants - advantage at 35°C cool temperature (the additional carboxylation C4 plant Photosynthesis steps of C4 require energy) Photosynthesis C3 plant C3 plant Ofne available a temps C4 plant collection Photorespiration at high increases with temp. great at CO2 energy a sequest but requiresefficient less C4 plants have an ↳plants Or et Full sunlight Full sunlight advantage at higher Irradiance temperatures Irradiance C4 – dry, hot, sunny regions Adapted from C4 photosynthesis, Plants in Action C3 – cool, temperate climates 20  C3 – C4 intermediates (C2 photosynthesis): Metabolite transfer to specialized bundle sheath cells Glycine decarboxylase (GDC) in mesophyll : lacks P subunit (L,T,H present) – inactive ↳ unable to bly EL Ser Photorespiratory glycine (2C) transferred from mesophyll to up bundle sheath cells = two-celled photorespiratory CO2 pump. CO2- by Sequestering Decarboxylation of glycine by GDC : only in mitochondria of bundle sheath cells. mito Centripetal microanatomy – CPs “overlay” mitos - facilitates reassimilation of CO2. 21 Strategies to Engineer more efficient C3 plants – rice, wheat Raise [CO2] at Rubisco e.g. introduction of : HCO3- transporters Pyrenoids Carboxysomes Other strategies: Increasing Rubisco selectivity (?) Introduce synthetic or alternative carbon-fixation pathways Engineer more efficient photorespiration pathways (by-pass GDC) 22