Photosynthesis: Carbon Reactions
Photosynthesis:Carbon ReactionsDr. Obaidur Rahman
Topics:Part ITHE CALVIN CYCLE The Calvin Cycle Has Three Stages: Carboxylation, Reduction, andRegeneration The Carboxylation of Ribulose Bisphosphate Is Catalyzed by theEnzyme Rubisco Triose Phosphates Are Formed in the Reduction Step of the CalvinCycle Operation of the Calvin Cycle Requires the Regeneration of Ribulose1,5-Bisphosphate The Calvin Cycle Regenerates Its Own Biochemical Components Calvin Cycle Stoichiometry Shows That Only One-Sixth of the TriosePhosphate Is Used for Sucrose or StarchPart IIREGULATION OF THE CALVIN CYCLE Light-Dependent Enzyme Activation Regulates the Calvin Cycle Rubisco Activity Increases in the Light Light-Dependent Ion Movements Regulate Calvin Cycle Enzymes Light-Dependent Membrane Transport Regulates the Calvin CyclePart IIITHE C2 OXIDATIVE PHOTOSYNTHETIC CARBON CYCLE Photosynthetic CO2 Fixation and Photorespiratory Oxygenation AreCompeting Reactions
Topics:Part IVCO2-CONCENTRATING MECHANISMS I: ALGAL AND CYANOBACTERIALPUMPSPart VCO2-CONCENTRATING MECHANISMS II: THE C4 CARBON CYCLE Malate and Aspartate Are Carboxylation Products of the C4 Cycle The C4 Cycle Concentrates CO2 in Bundle Sheath Cells The Concentration of CO2 in Bundle Sheath Cells Has an Energy Cost Light Regulates the Activity of Key C4 Enzymes In Hot, Dry Climates, the C4 Cycle Reduces Photorespiration andWater LossPart VICO2-CONCENTRATING MECHANISMS III: CRASSULACEAN ACIDMETABOLISM The Stomata of CAM Plants Open at Night and Close during the Day Phosphorylation Regulates the Activity of PEP Carboxylase in C4 andCAM Plants Some Plants Adjust Their Pattern of CO2 Uptake to EnvironmentalConditionsPart VIISYNTHESIS OF STARCH AND SUCROSE
Light reaction of photosynthesis:The photochemical oxidation of water to molecular oxygen is coupled to thegeneration of ATP and reduced pyridine nucleotide (NADPH) by reactionstaking place in the chloroplastthylakoid membrane.The reactions catalyzing the reduction of CO2 to carbohydrate are coupled tothe consumption of NADPH and ATP by enzymes found in the stroma, thesoluble phase of chloroplasts.These stroma reactions were long thought to be independent of light and, asa consequence, were referred to as the dark reactions.However, because these stroma-localizedreactions depend on the products of thephotochemical processes, and are alsodirectly regulated by light, they are moreproperly referred to as the carbonreactions of photosynthesis.
All photosynthetic eukaryotes, from the most primitive algae to the most advancedangiosperm, reduce CO2 to carbohydrate via the same basic mechanism: Calvin cycle or reductive pentose phosphate [RPP] cycle (C3 species) C4 photosynthetic carbon assimilation cycle (C4 species) the photorespiratory carbon oxidation cycle
The Calvin Cycle Has Three Stages:1.Carboxylation, during which CO2 iscovalently linked to a carbon skeleton2.Reduction, during whichcarbohydrate is formed at the expenseof the photochemically derived ATP andreducing equivalents in the form ofNADPH.3. Regeneration, during which the CO2acceptor ribulose-1,5-bisphosphate reforms.
CarboxylationCO2 enters the Calvin cycle byreacting with ribulose-1,5bisphosphate to yield two moleculesof 3-phosphoglycerate.A reaction catalyzed by theThe two molecules of 3-phosphoglycerate—labeledchloroplast enzyme ribulose―upper‖ and ―lower‖ on the figure—are distinguished bybisphosphatethe fact that the upper molecule contains the newlycarboxylase/oxygenase, referred toincorporated carbon dioxide, designated here as *CO2.as rubiscoTwo properties of the carboxylase reaction are especially important:1. The negative change in free energy associated with the carboxylation of ribulose1,5-bisphosphate islarge; thus the forward reaction is strongly favored.2. The affinity of rubisco for CO2 is sufficiently high to ensure rapid carboxylation atthe low concentrations of CO2 found in photosynthetic cells.Rubisco is very abundant, representing up to 40% of the total soluble protein ofmost leaves.
Triose Phosphates Are Formed in the Reduction Step of the Calvin CycleThe 3-phosphoglycerate formedin the carboxylation stageundergoes two modifications:1. It is first phosphorylated via 3-phosphoglyceratekinase to 1,3-bisphosphoglycerate through use ofthe ATP generated in the light reactions2. Then it is reduced to glyceraldehyde-3-phosphate through use of the NADPH generated by the lightreactions .The chloroplast enzyme NADP:glyceraldehyde-3-phosphate dehydrogenase catalyzes this step.
Operation of the Calvin Cycle Requires the Regeneration of Ribulose-1,5BisphosphateTo prevent depletion of Calvin cycle intermediates, three molecules of ribulose-1,5bisphosphate (15 carbons total) are formed by reactions that reshuffle the carbonsfrom the five molecules of triose phosphate (5 3 15 carbons).
Calvin Cycle Stoichiometry Shows That Only One-Sixth of the TriosePhosphate Is Used for Sucrose or StarchAn input of energy, provided by ATP and NADPH, is required in order to keep thecycle functioning in the fixation of CO2.1 molecule of hexose 6 molecules of CO2 are fixedexpense of 18 ATP and 12 NADPH2 molecules of NADPH and 3 molecules of ATP for every molecule of CO2 fixedinto carbohydrateRed light at 680 nm contains 175 kJ (42 kcal) per quantum mole of photons.The minimum quantum requirement is usually calculated to be 8 photons permolecule of CO2 fixed,although the number obtained experimentally is 9 to 10the minimum light energy needed to reduce 6 moles of CO2 to a mole ofhexose isapproximately 6 8 175 kJ 8400 kJ (2016 kcal)A mole of a hexose such as fructose yields only 2804 kJ (673 kcal) when totallyoxidized.Comparing 8400 and 2804 kJ, we see that the maximum overall thermodynamic
Calvin Cycle Stoichiometry Shows That Only One-Sixth of the TriosePhosphate Is Used for Sucrose or StarchMost of the unused light energy is lost in the generation of ATP and NADPH by thelight reactions rather than during operation of the Calvin cycle.the hydrolysis of ATP and the oxidation of NADPH, which are 29 and 217 kJ (7and 52 kcal) per mole, respectively.Therefore the Calvin cycle consumes (12 217) (18 29) 3126 kJ (750kcal) in the form of NADPH and ATP, resulting in a thermodynamic efficiencyclose to 90%.
REGULATION OF THE CALVIN CYCLEIn general, variation in the concentration or in the specific activity ofenzymes modulates catalytic rates, thereby adjusting the level of metabolitesin the cycle.Two general mechanisms can change the kinetic properties of enzymes:1. The transformation of covalent bonds such as the reduction of disulfides and thecarbamylation ofamino groups, which generate a chemically modified enzyme.2. The modification of noncovalent interactions, such as the binding of metabolitesor changes in the composition of the cellular milieu (e.g., pH).In addition, the binding of the enzymes to the thylakoid membranes enhancesthe efficiency of the Calvin cycle, thereby achieving a higher level of organizationthat favors the channeling and protection of substrates.
Light-Dependent Enzyme ActivationRegulates the Calvin CycleFive light-regulated enzymes operate in the Calvincycle:1. RubiscoThe last four enzymes2. NADP:glyceraldehyde-3-phosphate dehydrogenasecontain one or more3. Fructose-1,6-bisphosphatasedisulfide4. Sedoheptulose-1,7-bisphosphatase(—S—S—) groups.5. Ribulose-5-phosphate kinaseLight controls the activity of these four enzymes via theferredoxin–thioredoxin system, a covalent thiolbased oxidation–reduction mechanism.
The activation process starts in the light bya reduction of ferredoxin by photosystem IThe reduced ferredoxin plus twoprotons are used to reduce acatalytically active disulfide (—S—S—) group of the iron–sulfurenzyme ferredoxin:thioredoxinreductase, which in turn reducesthe highly specific disulfide (—S—S—) bond of thesmall regulatory proteinthioredoxinIn the light the —S—S— groupis reduced to the sulfhydryl state(—SH HS—)In the dark these residuesexist in the oxidized state (—S—S—), which renders theenzyme inactive or subactive.
The reduced form (—SH HS—)of thioredoxin then reduces thecritical disulfide bond (converts—S—S— to—SH HS—) of a target enzymeand thereby leads to activationof that enzyme.The light signal is thus converted to a sulfhydryl, or —SH, signal via ferredoxin andthe enzymeferredoxin:thioredoxin reductase.
Rubisco Activity Increases in the LightThe activity of rubisco is also regulated by light, but the enzyme itself does notrespond to thioredoxin.One way in which rubisco is activated involves the formation of acarbamate–Mg2 complex on the ε-amino group of a lysine within theactive site of the enzyme.
Rubisco Activity Increases in the LightTwo protons are released during the formation of the ternary complex rubisco–CO2 –Mg2 , so activation is promoted by an increase in both pH and Mg2 concentration.Thus, light-dependent stromal changes in pH and Mg2 appear to facilitate theobserved activation of rubisco by light.
Rubisco Activity Increases in the LightIn the active state, rubisco binds another molecule of CO2 , which reactswith the 2,3-enediol form of ribulose-1,5-bisphosphate (P—O—CH2—COH——COH—CHOH—CH2O—P) yielding 2-carboxy-3-ketoribitol 1,5-bisphosphateThe extreme instability of the latter intermediate leads to the cleavage ofthe bond that links carbons 2 and 3 of ribulose-1,5-bisphosphate, and asa consequence, rubisco releases two molecules of 3-phosphoglycerate.
Rubisco Activity Increases in the LightThe binding of sugar phosphates, such as ribulose1,5-bisphosphate, to rubisco preventsCarbamylation.sugar phosphates bindingThe sugar phosphates can be removed bythe enzyme rubisco activase, in a reactionthat requires ATP.The primary role of rubisco activaseis to accelerate the release of bound sugarphosphates, thus preparing rubisco forcarbamylation
Rubisco Activity Increases in the LightRubisco is also regulated by a naturalsugar phosphate, carboxyarabinitol-1phosphate, that closely resembles thesix-carbon transition intermediate of thecarboxylation reaction.carboxyarabinitol1-phosphate bindingThis inhibitor is present at lowconcentrations inleaves of many species and at highconcentrations in leaves of legumes suchas soybean and bean.Carboxyarabinitol-1-phosphate binds torubisco at night, and it is removed bythe action of rubisco activase in themorning, when photon flux densityincreases.
Light-Dependent Ion Movements Regulate Calvin CycleEnzymesStromaProton (H )LumenProton (H )Cause proton effluxMg2 uptakestromal concentration ofH (pH 7 8) andincrease that of Mg2 Several Calvin cycle enzymes -1,7-bisphosphatase, andribulose-5-phosphate kinase)are more active at pH 8 than at pH 7 andrequire Mg2 as a cofactor for catalysis.
THE C2 OXIDATIVE PHOTOSYNTHETICCARBON CYCLEAn important property of rubisco is its ability to catalyze both the carboxylation andthe oxygenation of RuBP.Oxygenation is the primary reaction in a process known as photorespiration.Because photosynthesis and photorespiration work in diametrically oppositedirections, photorespiration results in loss of CO2 from cells that aresimultaneously fixing CO2by the Calvin cycle
The main reactions of the photorespiratory cycleOperation of the C2 oxidativephotosynthetic cycle involvesthe cooperative interactionamong threeorganelles:1. Chloroplasts2. Mitochondria3. peroxisomes.
The main reactions of the photorespiratory cycleTwo molecules of glycolate (fourcarbons) transported from thechloroplast into the peroxisomeGlycolate transported from thechloroplast into the peroxisomeare converted to glycineGlycine in turn is exported to themitochondrion and transformed toserine (three carbons) with theconcurrent release of carbon
The main reactions of the photorespiratory cycleGlycerate phosphorylatedto 3-phosphoglycerateand incorporated into theCalvin cycleGlycerate flows to thechloroplastSerine transformed toglycerate.Serine is transported tothe peroxisome
The main reactions of the photorespiratory cycleInorganic nitrogen (ammonia)released by the mitochondrion iscaptured by the chloroplast for theincorporationinto amino acids by usingappropriate skeletons (αketoglutarate).
The main reactions of the photorespiratory cyclethe uptake of oxygen inthe peroxisome supports ashort oxygen cyclecoupled to oxidativereactions
Carboxylation and Oxygenation Are CloselyInterlocked in the Intact LeafPhotosynthetic carbon metabolism in the intact leaf reflects the integrated balancebetween two mutually opposing and interlocking cyclesThe Calvin cycle can operateindependently, but the C2 oxidativephotosynthetic carbon cycle depends onthe Calvin cycle for a supply of ribulose1,5-bisphosphate.The balance between the two cycles isdetermined by three factors:1. the kinetic properties ofrubisco2. the concentrations of thesubstrates CO2 and O23. temperature.
Carboxylation and Oxygenation Are CloselyInterlocked in the Intact LeafAs the temperature increases, the concentration of CO2 in a solution in equilibriumwith air decreases more than the concentration of O2 does.Consequently, the concentration ratio ofCO2to O2 decreases as the temperature rises.As a result of this property,photorespiration (oxygenation) increasesrelative to photosynthesis (carboxylation)as the temperature rises.This effect is enhanced by the kinetic properties of rubisco, which also result ina relative increase in oxygenation at higher temperaturesOverall, then, increasing temperatures progressively tilt the balance awayfrom the Calvin cycle and toward the oxidative photosynthetic carboncycle
Other than C3 plants:Many plants either do not photorespire at all, or they do so to only a limitedextent.These plants have normal rubiscos, and their lack of photorespiration is aconsequence of mechanisms that concentrate CO2 in the rubisco environment andthereby suppress the oxygenation reaction.Three mechanisms for concentrating CO2 at the site of carboxylation:1. C4 photosynthetic carbon fixation (C4)2. Crassulacean acid metabolism (CAM)3. CO2 pumps at the plasma membraneThe first two of these CO2 -concentrating mechanisms are found in someangiosperms and involve ―add-ons‖ to the Calvin cycle.Plants with C4 metabolism are often found in hot environmentsCAM plants are typical of desert environments.
CO2 -CONCENTRATING MECHANISMS I: ALGAL AND CYANOBACTERIALPUMPS At the concentrations of CO2 found in aquatic environments, rubisco operatesfar below its maximal specific activity. Marine and freshwater organisms overcome this drawback by accumulatinginorganic carbon by the use of CO2 and HCO3 – pumps at the plasmamembrane. ATP derived from the light reactions provides the energy necessary for theactive uptake of CO2 and HCO3 – The accumulated HCO3 – is converted to CO2 by the enzyme carbonicanhydrase, and the CO2 enters the Calvin cycle The metabolic consequence of this CO2 enrichment is suppression of theoxygenation of ribulose bisphosphate and hence also suppression ofphotorespiration. The energetic cost of this adaptation is the additional ATP needed forconcentrating the CO2
CO2 -CONCENTRATING MECHANISMS II: THE C4 CARBON CYCLEThere are differences in leaf anatomy between plants that have a C4 carboncycle (called C4 plants) and those that photosynthesize solely via the Calvinphotosynthetic cycle (C3 plants).A cross section of a typical C3 leaf revealsone majorcell type that has chloroplasts, themesophyll.In contrast, a typical C4 leaf has two distinctchloroplast-containing cell types:mesophyll and bundle sheath cellsThere is considerable anatomic variation inthe arrangement of the bundle sheath cellswith respect to the mesophyll and vasculartissue.No mesophyll cell of a C4 plant is more than two or three cells away from thenearest bundle sheath cell
CO2 -CONCENTRATING MECHANISMS II: THE C4 CARBON CYCLEInvolves four stages in two different celltypes:1.Fixation of CO2 into a four-carbonacid in a mesophyll cell;2.Transport of the four-carbon acid fromthemesophyll cell to a bundle sheath cell(3) Decarboxylation of the four-carbonacid, and the generation of a high CO2concentration in the bundlesheath cell. The CO2 released is fixed byrubisco and converted to carbohydrate bythe Calvin cycle.(4) Transport of the residual three-carbonacidback to the mesophyll cell, where theoriginal CO2acceptor, phosphoenolpyruvate, is
The Concentration of CO2 in Bundle Sheath Cells Has an EnergyCostThe calculation shows that theCO2 concentrating processconsumes two ATP equivalents(2 ―high-energy‖ bonds) per CO2molecule transported.Thus the total energyrequirement for fixing CO2 by thecombined C4 and Calvin cyclesis five ATP plus two NADPH perCO2 fixed.
Light Regulates the Activity of Key C4 EnzymesRegulated in response to variations inphoton flux density by two differentNADP:malate dehydrogenase is regulatedprocesses:via the thioredoxin system of the1. reduction–oxidation of thiolchloroplast.groups andPEP carboxylase is activated by a light-dependent phosphorylation–2. phosphorylation–dephosphorylation mechanism yet to be osphate dikinase, is rapidly inactivated by an unusual ADPdependent phosphorylation of the enzyme when the photon flux density drops
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CO2 -CONCENTRATING MECHANISMS III: CRASSULACEAN ACIDMETABOLISMCAM plants are typical of desert environments.Water efficiency is important.CAM is not restricted to the family Crassulaceae (Crassula, Kalanchoe, Sedum);it is found in numerous angiosperm families.Cacti and euphorbias are CAM plants, as well as pineapple, vanilla, and agave.How much water efficiency is CAM plants?For every gram of CO2 gained:C3 plants loses 400-500 g waterC4 plants loses 250-300 g waterCAM plants loses only 50-100 g of water
CO2 -CONCENTRATING MECHANISMS III: CRASSULACEAN ACIDMETABOLISMHow this water efficient process occur in CAM?Both water and CO2 pathway is through stomata.So if CO2 require it open the stomata and atmospheric CO2 uptake forphotosynthetic reaction, but opening of stoma permits loss of H2O.So for CAM stomata has to open cool, desert nights and closing themduring the hot, dry days.Closing the stomata during the day minimizes water loss.
CO2 -CONCENTRATING MECHANISMS III: CRASSULACEAN ACIDMETABOLISMThe elevated internal concentration of CO2effectively suppresses the photorespiratoryoxygenation of ribulose bisphosphate andfavors carboxylation.
CO2 -CONCENTRATING MECHANISMS III: CRASSULACEAN ACIDMETABOLISMIn C4 plants the carboxylase is ―switched on,‖ or active, during the day and inCAM plants during the night.In both C4 and CAM plants, PEP carboxylase is inhibited by malate andactivated by glucose-6-phosphateCAM plants PEP carboxylaseregulation:PEP carboxylase-kinaseThe synthesis of this kinase is stimulatedby the efflux of Ca2 from the vacuole tothe cytosol and the resulting activationof a Ca2 /calmodulin protein kinasePhosphorylation of a single serine residueof the CAM enzyme diminishes the malateinhibition and enhances the action ofglucose-6-phosphate so that the enzymebecomes catalytically more active
SYNTHESIS OF STARCH AND SUCROSEIn most species, sucrose is the principal form ofcarbohydrate translocated throughout the plant by thephloem.Starch is an insoluble stable carbohydrate reserve that ispresent in almostall plants.Both starch and sucrose are synthesized from the triosephosphate that is generated by the Calvin cycle
The Syntheses of Sucrose and Starch Are CompetingReactionsThe relative concentrations of orthophosphate and triose phosphate are majorfactors that control whether photosynthetically fixed carbon is partitioned as starchin the chloroplast or as sucrose in the cytosol.The phosphate translocator catalyzesthe movement of orthophosphate andtriose phosphate in opposite directionsbetween chloroplast and cytosol.ChloroplastCytosolStarch synthesisPi highPi lowSucrose synthesisPi lowPi high
The Syntheses of Sucrose and Starch Are CompetingChloroplastCytosolReactionsStarch synthesisPi highPi lowSucrose synthesisPi lowPi highThe chloroplast enzyme ADP-glucosepyrophosphorylase is the key enzyme thatregulates the synthesis of starch fromglucose-1-phosphate.This enzyme is stimulated by 3phosphoglycerate and inhibited byorthophosphate.A high concentration ratio of 3phosphoglycerate to orthophosphate istypically found in illuminatedchloroplasts that are activelysynthesizing starch.Reciprocal conditions prevail in thedark.
The Syntheses of Sucrose and Starch Are CompetingReactionsFructose-2,6-bisphosphateis a key controlmolecule that allows increased synthesisof sucrose in the light and decreasedsynthesis in the dark.
The Syntheses of Sucrose and Starch Are CompetingReactionsIncreased cytosolic fructose-2,6-bisphosphate is associated withdecreased rates of sucrose synthesis because fructose-2,6bisphosphate is a powerful inhibitor of cytosolic fructose-1,6bisphosphatase and an activator of the pryophosphate-dependent(PPi-linked) phospho-fructokinase
The Syntheses of Sucrose and Starch Are CompetingReactionsIncreased cytosolic fructose-2,6-bisphosphate is associated withdecreased rates of sucrose synthesis because fructose-2,6bisphosphate is a powerful inhibitor of cytosolic fructose-1,6bisphosphatase and an activator of the pryophosphate-dependent(PPi-linked) phospho-fructokinase
The Syntheses of Sucrose and Starch Are CompetingReactionsFructose-2,6-bisphosphate is synthesized from fructose-6phosphate by a special fructose-6-phosphate 2-kinase (not to beconfused with the fructose-6-phosphate 1-kinase of glycolysis) andis degraded specifically by fructose-2,6- bisphosphatase
The Syntheses of Sucrose and Starch Are CompetingReactionsThe kinase and phosphataseactivities are controlled byorthophosphate and triosephosphate.Light regulates the concentration of these activators and inhibitors through thereactions associated with photosynthesis and thereby controls theconcentration of fructose-2,6-bisphosphate in the cytosol.
The Syntheses of Sucrose and Starch Are CompetingReactionsThe glycolytic enzymephosphofructokinase alsofunctions in the conversionof fructose-6-phosphate tofructose-1,6-bisphosphate,but in plants it is notappreciably affected byfructose-2,6-bisphosphate.The activity of phosphofructokinase in plants appears to be regulatedby the relative concentrations of ATP, ADP, and AMP.
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THE CALVIN CYCLE The Calvin Cycle Has Three Stages: Carboxylation, Reduction, and Regeneration The Carboxylation of Ribulose Bisphosphate Is Catalyzed by the Enzyme Rubisco Triose Phosphates Are Formed in the Reduction Step of the Calvin Cycle Operation of the Calvin Cycle
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