Biochem 2: L19
Photosynthesis and Carbohydrate Synthesis in Plants
20.3 Evolution of a Universal Mechanism of ATP Synthesis
A proton gradient couples electron flow and phosphorylation.
Photoinduced electron flow leads to a net movement of protons across the membrane from the stromal side to the thylakoid lumen.
The Approximate Stoichiometry of Photophosphorylation Has Been Established
In illuminated chloroplasts, energy stored in the proton gradient per mole of protons is:
Experimental measurements show that 8 photons drive the production of one O2, yielding approximately 3 ATP molecules.
The Overall Reaction for Linear Photophosphorylation
The overall reaction is:
(20-6)
Orientation of ATP Synthase is Fixed Relative to the Proton Gradient
Diagram description: The orientation shows the relationship between mitochondrion, chloroplast, bacteria (E. coli), their respective thylakoid, cytosol, and proton sides.
ATP synthesis relies on the specific orientation of ATP synthase in relation to ATP being produced on the proton side.
The Appearance of Oxygenic Photosynthesis
Cyanobacteria appeared on Earth approximately 2.5 billion years ago.
They acquired two photosystems that operate in tandem: one of Type II and one of Type I.
They had water-splitting activity that released oxygen into the atmosphere.
Photosynthetic Microorganisms Can Acquire Electrons From Other Donors
Many photosynthetic microorganisms obtain electrons from donors such as H2S or lactate.
This process forms oxidized products like elemental sulfur or pyruvate.
Modern Cyanobacteria
Can synthesize ATP by either oxidative phosphorylation or photophosphorylation.
Lack mitochondria or chloroplasts.
Dual Roles of Cytochrome b6f and Cytochrome c6
Functional homology between:
Cyanobacterial cytochrome b6f complex and the mitochondrial cytochrome bc1 complex.
Cyanobacterial cytochrome c6 and plant plastocyanin.
This connection provides evidence of a common evolutionary origin for these processes.
20.4 CO2-Assimilation Reactions
Photosynthetic organisms utilize ATP and NADPH to reduce atmospheric CO2 to trioses.
Flowchart of Products of Photosynthesis:
Input: Sunlight, CO₂
yields Energy for biosynthesis (e.g., Glucose) and Carbon skeletons for amino acids, nucleotides, and other metabolites (such as sucrose and starch for transport/storage).
CO2 Assimilation and CO2 Fixation
CO2 Assimilation: The process of converting CO2 into simple (reduced) organic compounds via the Calvin cycle (reductive pentose phosphate pathway).
CO2 Fixation: The process of incorporating (fixing) CO2 into the triose phosphate 3-phosphoglycerate (3-PGA).
Carbohydrate Metabolism is More Complex in Plant Cells
Plants have the following metabolic pathways:
Glycolysis pathway.
Gluconeogenesis pathway.
Pentose phosphate pathway.
Pathways for the reduction of CO2 to triose phosphates and associated reductive pentose phosphate pathway.
Carbon Dioxide Assimilation Occurs in Three Stages
Occurs in the stroma of chloroplasts:
Stage 1: Fixation
CO2 is fixed with ribulose 1,5-bisphosphate to form 3-phosphoglycerate.
Stage 2: Reduction
3-phosphoglycerate is reduced to triose phosphates.
Stage 3: Regeneration
Ribulose 1,5-bisphosphate is regenerated from triose phosphates.
Stage 1 of Carbon Dioxide Assimilation
CO2 condenses with a five-carbon acceptor, ribulose 1,5-bisphosphate (RuBP), to form two molecules of 3-phosphoglycerate.
A total of six molecules of 3-phosphoglycerate are formed.
Stage 2 of Carbon Dioxide Assimilation
3-phosphoglycerate is reduced to triose phosphates.
A total of six molecules of triose phosphates are formed.
Stage 3 of Carbon Dioxide Assimilation
Five of the six triose phosphate molecules regenerate three ribulose 1,5-bisphosphate molecules.
The sixth triose phosphate serves as the net product of photosynthesis.
Stage 1: Fixation of CO2 into 3-Phosphoglycerate
C3 plants: Plants where the three-carbon compound 3-phosphoglycerate (3-PGA) is the first intermediate in photosynthesis (e.g., trees, wheat, oats, rice, beans, peas, spinach).
Rubisco: Ribulose 1,5-bisphosphate carboxylase/oxygenase, the enzyme that catalyzes the incorporation of CO2 into an organic form.
Rubisco's Function
Rubisco catalyzes:
Covalent attachment of CO2 to ribulose 1,5-bisphosphate.
Cleavage of the unstable six-carbon intermediate to form two molecules of 3-phosphoglycerate.
Forms of Rubisco
Form I: Enzyme of vascular plants, algae, and cyanobacteria; present in high concentrations due to its low turnover number.
Form II: Enzyme of photosynthetic bacteria.
Central Role of Mg2+ in the Active Site of Rubisco
A carbamoylated Lys side chain binds a Mg2+ ion.
The Mg2+ ion helps to bring together and orient the reactants at the active site.
Steps of CO2 Assimilation Catalyzed by Rubisco
Creation of an Enediolate Intermediate:
Mg2+ facilitates the interaction of carbamoylated Lys with ribulose 1,5-bisphosphate.
Forms an enediolate intermediate.
Nucleophilic Attack to Create a β-Keto Acid Intermediate:
The Mg2+ ion polarizes CO2, allowing for nucleophilic attack by the enediolate.
Hydroxylation at C-3:
Hydroxylation occurs at the C-3 carbonyl of the β-keto acid intermediate.
Cleavage to Yield the First 3-Phosphoglycerate:
Cleavage of the hydrated intermediate produces one molecule of 3-phosphoglycerate.
Protonation and Release of a Second 3-Phosphoglycerate:
Deprotonation of Lys175 resets the active site, generating a second molecule of 3-phosphoglycerate.
Role of Rubisco Activase
Rubisco is inactive until it is carbamoylated on the ε-amino group of Lys201.
Rubisco Activase: Promotes ATP-dependent release of ribulose 1,5-bisphosphate, exposing Lys201 for non-enzymatic carbamoylation.
Stage 2: Conversion of 3-Phosphoglycerate to Glyceraldehyde 3-Phosphate
Stromal 3-phosphoglycerate kinase: Catalyzes the transfer of a phosphoryl group from ATP to 3-phosphoglycerate, yielding 1,3-bisphosphoglycerate.
Glyceraldehyde 3-phosphate dehydrogenase: Catalyzes a reduction between NADPH and 1,3-bisphosphoglycerate, producing glyceraldehyde 3-phosphate and Pi.
Stage 3: Regeneration of Ribulose 1,5-Bisphosphate from Triose Phosphates
Stromal enzymes rearrange carbon skeletons of triose phosphates to generate intermediates.
The pentose phosphates are converted to ribulose 5-phosphate, phosphorylated to regenerate ribulose 1,5-bisphosphate, completing the Calvin cycle.
Third Stage of CO2 Assimilation
Three exergonic reactions make the whole process irreversible:
Fructose 1,6-bisphosphatase.
Sedoheptulose 1,7-bisphosphatase.
Ribulose 5-phosphate kinase.
Synthesis of Each Triose Phosphate from CO2 Requires Six NADPH and Nine ATP
Fixation of three CO2 yields one glyceraldehyde 3-phosphate for anabolic processes.
ATP and NADPH are provided by the light-dependent reactions of photosynthesis.
A Transport System Exports Triose Phosphates from the Chloroplast and Imports Phosphate
Sucrose is synthesized in the cytosol.
The inner chloroplast membrane is impermeable to phosphorylated compounds.
Role of the Antiporter in the Transport of ATP and Reducing Equivalents
Oxidation of dihydroxyacetone phosphate in the cytosol generates ATP and NADH, moving ATP and reducing equivalents from the chloroplast to the cytosol.
Four Enzymes of the Calvin Cycle Are Indirectly Activated by Light
When chloroplasts are illuminated, the following increases:
Concentration of ATP and NADPH.
Stromal pH.
Stromal [Mg2+].
Some Stromal Enzymes Are More Active in an Alkaline Environment and at High [Mg2+]
Fructose 1,6-bisphosphatase activity increases over 100-fold when pH and [Mg2+] rise.
Light-Driven Reduction of Disulfide Bonds Activates Enzymes
Light-driven reduction affects enzymes including:
Ribulose 5-phosphate kinase.
Fructose 1,6-bisphosphatase.
Sedoheptulose 1,7-bisphosphatase.
Glyceraldehyde 3-phosphate dehydrogenase.
These enzymes remain inactive in the dark to prevent competition with glycolysis.
Light Activation of Several Enzymes of the Calvin Cycle
Ferredoxin:thioredoxin reductase: Catalyzes the transfer of electrons from ferredoxin to thioredoxin, which then donates electrons for the reduction of disulfide bonds in light-activated enzymes.
20.5 Photorespiration and the C4 and CAM Pathways
Distinction between processes that produce CO2:
Mitochondrial respiration involves the oxidation of substrates to CO2 and conversion of O2 to H2O; occurs in darkness.
Photorespiration is a costly side reaction of photosynthesis that consumes O2 and produces CO2, driven by light, due to Rubisco's lack of specificity.
Photorespiration Results from Rubisco’s Oxygenase Activity
Rubisco is not specific for CO2; approximately 1 in every 3 or 4 turns, O2 is utilized instead.
2-Phosphoglycolate: A byproduct that is metabolically unneeded, resulting from using O2 as a substrate; carbons must be salvaged.
Phosphoglycolate Is Salvaged in a Costly Set of Reactions in C3 Plants
Glycolate pathway: Converts two 2-phosphoglycolate to a molecule of serine and CO2; consumes O2 and ATP, resulting in loss of carbon via decarboxylation.
Glycine Decarboxylase Complex
Glycine decarboxylase complex: Catalyzes the oxidative decarboxylation of glycine in the mitochondrial matrix, making it a major component of mitochondrial proteins in photosynthetic plants.
Serine hydroxymethyltransferase: Transfers a one-carbon unit from tetrahydrofolate, yielding serine.
The Process of Photorespiration
Photorespiration represents the combined activities of Rubisco’s oxygenase and the glycolate salvage pathway, leading to:
O2 consumption.
CO2 production.
No energy conservation.
Inhibition of net biomass formation.
In C4 Plants, CO2 Fixation and Rubisco Activity Are Spatially Separated
C4 plants: Utilize a CO2-assimilation process to minimize photorespiratory losses; common in tropical and high-light intensity areas.
C4 Pathway: Refers specifically to the CO2-assimilation process in these plants.
CO2 Assimilation in C4 Plants
Diagrammatic summary illustrating mesophyll and bundle sheath cells with PEP carboxylase, malate dehydrogenase, and other pathways that enhance CO2 fixation efficiency and reduce photorespiration.
Phosphoenolpyruvate (PEP) Carboxylase Fixes CO2
Phosphoenolpyruvate (PEP) carboxylase catalyzes the fixation of CO2 (as HCO3−) into oxaloacetate in mesophyll cells.
Reduction or Transamination of Oxaloacetate
Oxaloacetate can either undergo:
Reduction to malate in a reaction that requires NADPH or
Transamination to aspartate.
Oxidation and Decarboxylation of Malate
Malate transitions between cells, where malic enzyme catalyzes its oxidation and decarboxylation, yielding pyruvate and CO2 while also reducing NADP+ to NADPH.
Rubisco Fixes CO2 as in C3 Plants
Rubisco operates similarly in C4 plants, incorporating CO2 into the C-1 position of 3-phosphoglycerate.
Pyruvate Phosphate Dikinase
Pyruvate returns to mesophyll cells, where pyruvate phosphate dikinase catalyzes the conversion of pyruvate back to phosphoenolpyruvate (PEP).
High Concentrations of CO2 Are Released From Malate
The release of CO2 from malate in the bundle-sheath cells ensures a high local concentration of CO2, allowing Rubisco's function at maximal rates and suppressing its oxygenase activity.
Energy Cost of CO2 Assimilation
C4 plants require five ATP molecules to assimilate one CO2.
C3 plants require only three ATP molecules.
In CAM Plants, CO2 Capture and Rubisco Action Are Temporally Separated
CAM plants: Fix CO2 into malate in the dark and store it until daylight, subsequently using the stored malate for Rubisco activity; common in succulents to reduce water loss through stomata.
Comparison of C3, C4, and CAM Plants
Table 20-1 Comparison: An overview comparing C3, C4, and CAM plants on various factors:
C3 Plants: Include spinach, rice; most efficient in temperatures 15-25 °C; path of CO2 fixation through only C3.
C4 Plants: Include maize; efficient in hot, dry conditions; separate pathways in mesophyll and bundle sheath cells.
CAM Plants: Include cacti; effective adaptations for extremely dry environments; utilize CO2 fixation pathways separated temporally between day and night.