light reactions of photosynthesis 2 h 2 o + 2 nadp + + 8 photons → o 2 + 2 nadph + 2 h + animation
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Light Reactions of Photosynthesis
2 H2O + 2 NADP+ + 8 photons →
O2 + 2 NADPH + 2 H+
ANIMATION
Integration of photosystems I and II in chloroplasts. The "Z scheme“ evolved by combining 2 bacterial RCs.
Reaction center chlorophylls lie close to the exiton acceptor preventing internal conversion (fluorescence). This fixed orientation mimics the solid state.
Resembles that of green sSulfur bacteria
Resembles that of purple bacteria.
analogous to cyt c
Photosystem II of the cyanobacterium Synechococcus elongates
All electron carriers are bound to a
nearly symmetric
dimer. Participants are positioned like the bacterial RC in purple
bacteria.
Light Reactions of Photosynthesis PS II
4 P680 + 4 H+ + 2 PQB + 4 photons →
4 P680+ + 2 PQBH2
The supramolecular complex of PSI and its associated antenna chlorophylls
Light Reactions of Photosynthesis PS I
4 P700 + 2 H+ + 2 NADP+ + 4 photons →
4 P700+ + 2 NADPH
Electron and proton flow through the cytochrome b6f complex
Plastoquinol (PQH2) formed in PSII is oxidized
by the cytochrome b6f complex in a series of
steps like those of the Q cycle in the cytochrome
Complex III of mitochondria. One
electron from PQH2 passes to the Fe-S center of the Rieske protein, the
other to heme bL of cytochrome b6. The net
effect is passage of electrons from PQH2 to
the soluble protein plastocyanin, which carries them to PSI.
Heme groups
f for frons
Localization of PSI and PSII in thylakoid membranes
• Non cyclic electron flow (PSI + PS II) produces a proton gradient + NADPH
• Cyclic electron flow (PSI) produces a proton gradient only
• Calvin cycle requires ATP and NADPH in a ratio of 3:2
• The 2 PS are physically separated to prevent exitons from leaving P680 and transferring to P700. Why would this happen?– (longer wavelength, lower energy)
Localization of PSI and PSII in thylakoid membranes. PSII is present almost exclusively in the appressed regions (granal lamellae [stacks], in which several membranes are in contact) , and PSI almost exclusively in nonappressed (stromal lamellae) regions, exposed to the stroma. LHCII is the "adhesive" that holds appressed lamellae together.
Accumulation of plastoquinol stimulates a protein kinase that phosphorylates a Thr residue in the hydrophobic domain of LHCII, which reduces its affinity for the neighboring thylakoid membrane and converts appressed regions to nonappressed regions (state 2). A specific protein phosphatase reverses this regulatory phosphorylation when the [PQ]/[PQH2] ratio increases.
Balancing of electron flow in PSI and PSII by state transition
noncyclic cyclic
Water-splitting activity of the oxygen-evolving complex
Water splitting complex passes 4 electrons, 1 at a time, back to P680+. The electrons lost from the multinuclear Mn center pass one at a time to an oxidized Tyr residue in a PSII protein, then to P680+
H+ released into lumen.
Light-Induced Redox Reactions and Electron Transfer Cause Acidification of Lumen
Because the volume of the lumen is small, a few hydrogen ions dramatically change the pH: lumen = pH 5, stroma pH = 8!!!The proton-motive force across the thylakoid membrane drives the synthesis of ATP… sound familiar??
Thylakoid membrane is impermeable to hydrogen ions. Reaction centers, electron carriers and ATP synthases are located in this membraneUncouplers decouple light absorption from ATP synthesis.
In vitro ATP synthesis
• Incubate chloroplasts in pH 4 buffer in the dark. Buffer slowly entered thylakoids lowering the pH to 4.
• Add ADP and Pi and suddenly increase the pH to 8. (How would you do this?) IN THE DARK.
• !!!! ATP produced!!!
Proton and electron circuits in thylakoids.
Flow of Protons: Mitochondria, Chloroplasts, Bacteria
• According to endosymbiotic theory, mitochondria and chloroplasts arose from entrapped bacteria
• Bacterial cytosol became mitochondrial matrix and chloroplast stroma
Comparison of the topology of proton movement and ATP synthase orientation in the membranes of mitochondria, chloroplasts, and the bacterium E. coli.
Dual roles of cytochrome b6f and cytochrome c6 in cyanobacteria reflect evolutionary origins. Cyanobacteria use cytochrome b6f, cytochrome c6, and plastoquinone for both oxidative phosphorylation and photophosphorylation.
CHAPTER 20 Carbohydrate Biosynthesis
in Plants and Bacteria
– CO2 assimilation in photosynthetic organisms
– Photorespiration in C3 plants
– Avoiding photorespiration in C4 plants
Key topics:
Introduction to Anabolic Pathways
• Anabolism: how to build biomolecules
• Plants are extremely versatile in biosynthesis
– Can build organic compounds from CO2
– Can use energy of sunlight to support biosynthesis
– Can adopt to a variety of environmental situations
Plant versatility
• Autotrophic
• Nonmobile/motile
• CHO synthesis occurs in plastids
• Plants synthesize thick cell walls exterior to the cell containing the bulk of the cell’s CHO—how do they do this???
Assimilation of CO2 by Plants
Plants and Photosynthetic Microorganisms Support the Life of Animals and Fungi
• Plants capture the energy from the ultimate energy source and make it available via carbohydrates to animals and fungi
Photosynthetic organisms use the energy of sunlight to manufacture glucose and other organic products, which heterotrophic cells use as energy and carbon sources.
Biological reproduction occurs with near-perfect fidelity (although no 2 zebras have exactly the same stripes!).
Zebras are herivores.
CO2 Assimilation Occurs in Plastids
• Self-reproducing organelles found in plants and algae• Enclosed by a double membrane• Have their own small genome• Most plastid proteins are encoded in the nuclear DNA• The inner membrane is impermeable to ions such as
H+, and to polar and charged molecules
Amyloplasts filled with starch (dark granules) are stained with iodine in this section of Ranunculus (buttercups) root cells.
Amyloplasts are pastids without the internal membrane or pigments.
Origin and Differentiation of Plastids
• Plastids were acquired during evolution by early eukaryotes via endosymbiosis of photosynthetic cyanobacteria
• Plastids reproduce asexually via binary fission• The undifferentiated protoplastids in plants can
differentiate into several types, each with a distinct function– Chloroplasts for photosynthesis– Amyloplasts for starch storage– Chromoplasts for pigment storage – Elaioplasts for lipid storage– Proteinoplasts for protein storage
Proplastids in nonphotosynthetic tissues (such as root) give rise to amyloplasts, which contain large quantities of starch. All plant cells have plastids, and these organelles are the site of other important processes, including the synthesis of essential amino acids, thiamine, pyridoxal phosphate, flavins, and vitamins A, C, E, and K.
internal membranes lost
CO2 Assimilation
• The assimilation of carbon dioxide occurs in the stroma of chloroplasts via a cyclic process known as the Calvin cycle
• The key intermediate, ribulose 1,5-bisphosphate is constantly regenerated using energy of ATP
• The key enzyme, ribulose 1,5-bisphosphate carboxylase / oxygenase (Rubisco), is probably the most abundant protein on Earth
• The net result is the reduction of CO2 with NADPH
that was generated in the light reactions of photosynthesis
Early studies of the Calvin cycle
• Design an experiment to discover the pathway for carbon assimilation
• First intermediate recognized was 3-PGA
• Search for a 2 carbon acceptor---FAILURE
• Actual acceptor….
CO2 Assimilation
• The assimilation of carbon dioxide occurs in the stroma of chloroplasts via a cyclic process known as the Calvin cycle
• The key intermediate, ribulose 1,5-bisphosphate is constantly regenerated using energy of ATP
• The key enzyme, ribulose 1,5-bisphosphate carboxylase / oxygenase (Rubisco), is probably the most abundant protein on Earth
• The net result is the reduction of CO2 with NADPH
that was generated in the light reactions of photosynthesis
The Calvin Cycle
The Structure and Function of Rubisco
• Rubisco is a large Mg++-containing enzyme that makes a new carbon-carbon bond using CO2
as a substrate
Structure of ribulose 1,5-bisphosphate carboxylase (rubisco). Ribbon model of form II rubisco from the bacterium Rhodospirillum rubrum. The subunits are in gray and blue. A Lys residue at the active site that is carboxylated to a carbamate in the active enzyme
is shown in red. The substrate, ribulose 1,5-bisphosphate, is yellow; Mg2+ is green.
Central role of Mg2+ in the catalytic mechanism of
rubisco.
Mg2+ is coordinated in a roughly octahedral complex with six
oxygen atoms: one oxygen in the carbamate on Lys201; two in the
carboxyl groups of Glu204 and Asp203; two at C-2 and C-3 of
the substrate, ribulose 1,5-bisphosphate; and one in the
other substrate, CO2.
First stage of CO2 assimilation: rubisco's carboxylase activity.
Another ene-diol intermediate!!!
Catalytic Role of Mg++ in Rubisco’s Carboxylase Activity
• Notice that Mg++ is held by negatively charged side chains of
• glutamate,
• aspartate, and
• carbamoylated lysine
• Mg++ brings together the reactants in a correct orientation, and stabilizes the negative charge that forms upon the nucleophilic attack of enediolate to CO2
Rubisco is Activated via Covalent Modification of the
Active Site Lysine
Synthesis of Glyceraldehyde-3 Phosphate (First Stage)
• Three rounds of the Calvin cycle fix three CO2
molecules and produce one molecule of 3-phosphoglycerate
Fate of Glyceraldehyde 3-phosphate (Second Stage)
• Converted to starch in the chloroplast• Converted to sucrose for export• Recycled to ribulose 1,5-bisphosphate
Interconversion of Triose Phosphates and Pentose
Phosphates
• This is how ribulose 1,5-bisphosphate is regenerated in the third stage of the Calvin cycle
Sugar interconversions
Transketolase Reactions
Transketolase Uses Thiamine Pyrophosphate as the
Cofactor
Stoichiometry and Energy Cost of CO2 Assimilation
• Fixation of three CO2 molecules yields one
glyceraldehyde 3-phosphate
• Nine ATP molecules and six NADPH molecules are consumed
Photosynthesis: From Light and CO2 to Glyceraldehyde 3-
phosphate
• The photosynthesis of one molecule of glyceraldehyde 3-phosphate requires the capture of roughly 24 photons
ATP and NADPH produced by the light reactions are essential substrates for the reduction of CO2
Enzymes in the Calvin Cycle are Regulated by Light
• Target enzymes are – ribulose 5-phosphate kinase, – fructose 1,6-bisphosphatase, – seduloheptose 1,7-bisphosphatase, and– glyceraldehyde 3-phosphate
dehydrogenase
Light activation of several enzymes of the Calvin cycle
Photorespiration• So far, we saw that plants oxidize water to O2
and reduce CO2 to carbohydrates during the
photosynthesis
• Plants also have mitochondria where usual respiration with consumption of O2 occurs in the
dark
• In addition, a wasteful side reaction catalyzed by Rubisco occurs in mitochondria
• This reaction consumes oxygen and is called photorespiration; unlike mitochondrial respiration, this process does not yield energy
Oxygenase Activity of Rubisco
• The reactive nucleophile in the Rubisco reaction is the electron-rich enediol form of ribulose 1,5-bisphosphate
• The active site meant for CO2 also
accommodates O2
• Mg++ also stabilizes the hydroperoxy anion that forms by electron transfer from the enediol to oxygen
Salvage of 2-Phosphoglycerate
• Complex ATP-consuming process for the recovery of C2 fragments from the
photorespiration
• Requires oxidation of glycolate with molecular oxygen in peroxisomes, and formation of H2O2
• Involves a loss of a carbon as CO2 by
mitochondrial decarboxylation of glycine
Glycolate pathway
Rubisco in C3 Plants Cannot
Avoid Oxygen
• Plants that assimilate dissolved CO2 in the
mesophyll of the leaf into three-carbon 3-phosphoglycerate are called the C3 plants
• Our atmosphere contains about 21% of oxygen and 0.038% of carbon dioxide
• The dissolved concentrations in pure water are about 260 M O2 and 11 M CO2 (at the
equilibrium and room temperature)
• The Km of Rubisco for oxygen is about 350 M
Separation of CO2 capture and the
Rubisco Reaction in C4 Plants
• Many tropical plants avoid wasteful photorespiration by a physical separation of CO2 capture and Rubisco
activity
• CO2 is captured into oxaloacetate (C4) in mesophyll
cells
• CO2 is transported to bundle-sheath cells where
Rubisco is located
• The local concentration of CO2 in bundle-sheath cells
is much higher than the concentration of O2
Carbon assimilation in C4 plants
Chapter 20: Summary
• ATP and NADPH from light reactions are needed in order to assimilate
CO2 into carbohydrates
• Assimilations of three CO2 molecules via the Calvin cycle leads to the
formation of one molecule of 3-phosphoglycerate
• 3-Phosphoglycerate is a precursor for the synthesis of larger
carbohydrates such as fructose and starch
• The key enzyme of the Calvin cycle, Rubisco, fixes carbon dioxide into
carbohydrates
• Low selectivity of Rubisco causes a wasteful incorporation of molecular
oxygen in C3 plants; this is avoided in C4 plants by increasing the
concentration of CO2 near Rubisco
In this chapter, we learned that: