Class Note for BIOC 460 at UA
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Date Created: 02/06/15
Bioc 460 Dr Miesfeld Fall 2008 Lecture 30 Carbon Fixation Key Concepts Three stages of the Calvin Cycle Stage 1 Fixation of C02 to form 3phosphoglycerate Stage 2 Reduction of 3phosphoglycerate to from hexose sugars Stage 3 Regeneration of ribulose 15bisphosphate Regulation of the Calvin Cycle The C4 and CAM pathways reduce photorespiration in hot climates Key Concept Question concerning the Calvin Cycle How do sugarcane plants and saguaro cacti avoid the wasteful side effects of photorespiration Biochemical Applications of the Calvin Cvce Photorespiration is a wasteful side reaction catalyzed by the enzyme ribulose15bisphosphate carboxylaseoxygenase rubisco which reduces crop yields by competing for the C3 carbon dioxide fixation pathway The C4 pathway in corn Cel and TISSLE Culture plants leads to reduced rates of photorespiration because 39 x C02 is concentrated in the chloroplast stroma and out 352353322 w competes 02 for rubisco binding In an effort to increase crop J f yields in C3 plants such as rice transgenic plants are being noigg fenednas 39 developed that contain corn genes encoding multiple lem lire enzymes in the C4 pathway Three stages of the Calvin cycle Plants store light energy in the form of carbohydrate primarily starch and sucrose The carbon and oxygen for this process comes from C02 and the energy for carbon fixation is derived from the ATP and NADPH made during photosynthesis The conversion of C02 to carbohydrate is called the Calvin cycle and is named after Melvin Calvin who discovered it This cycle requires the enzyme ribulose 15bisphosphate carboxylase commonly called rubisco As summarized in figure 1 the Calvin cycle generates the triose phosphates 3 Figure 1 phosphoglycerate Starch glyceraldehyde3P GAP Sum storage Cellulose transport cell wall and dlhydroxyacetone phosphate all of which are used to synthesize the Hex se ph 5phates Metabolic RD hexose phosphates fructose Pent 5e intermediates I Protein 16bisphosphate and Triose phosphates Ph 5phates Lipid fructose 6phosphate ADP Hexose phosphates NADP produced by the Calvin cycle ATP are converted to 1 sucrose NADPH for transport to other plant 5 C02 20 tissues 2 starch for energy Lightdependent stores within the cell 3 reactions of cellulose for cell wall synthesis and 4 pentose phosphates for metabolic intermediates 1 of11 pages Bioc 460 Dr Miesfeld Fall 2008 The basic scheme of the Calvin cycle is illustrated in figure 2 where it can be seen that it consists of three stages Fixation Reduction and Regeneration Note that three turns of the cycle results in the fixation of three molecules of C02 into the triose phosphate 3 phosphoglycerate through a reaction catalyzed by the rubisco enzyme W This key reaction in stage 1 combines three molecules of ET ribulose15bisphosphate Stage 33 RuBP a five carbon cs Refemmm 3 5 smge 1 compound with three molecules of o acceptor CH20 Fixation C02 to form six molecules of the C3 5V Ribulose15 compound 3phosphoglycerate bisphosphate Using energy available from ATP EHO l3 foo hydrolysis and NADPH oxidation 1 CHOH fHOH the six molecules of 3 CH20 CH20 phosphoglycerate are converted into GlyceraIdehyczgfphosphate 3quot quot 5P39 6 939V erate six molecules of glyceraldehydes 3P of which one is used to synthesize glucose stage 2 and the other five are recycled to Pi generate RuBP stage 3 The Calvin cycle is sometimes called the Dark Reactions but do not be fooled by this name the Calvin Cycle is 5 the most active during the daylight hours when ATP and NADPH are plentiful Stage 2 Reduction The net reaction of three turns of the Calvin cycle shown in figure 2 can be written as 3 C02 3 RuBP 6 NADPH 9 ATP 6 H20 gt 1 GAP 3 RuBP 6 NADP 9 ADP 9 Pi If we just look at the fate of the carbons coming from C02 C1 in this reaction we see that one net C3 compound glyceraldehydes3P is formed and three C5 molecules RuBP are regenerated 3C13C5 gt1C33C5 Stage 1 Fixation of CO to form 3phosphoglycerate When Calvin began his experiments in the 1940s it was known that C02 in plant mm chloroplasts was converted to hexose phosphates however it wasn39t clear how this was accomplished To identify the metabolic intermediates in this process 3 Ph 5Ph S39Y e39a e I Calvin and his colleagues used radioactive quot labeling with 14C02 to follow carbon fixation 5 5 60 5 o in photosynthetic algae cells grown in culture As shown in figure 3 they found that within a few seconds of adding 14C02 to the culture the cells accumulated 14Clabeled 3 phosophoglycerate suggesting that this was the first product of the carboxylation reaction V thin 2 of 11 pages Bioc 460 Dr Miesfeld Fall 2008 a minute of adding 14C02 to the culture they found numerous compounds labeled with 14C many of which were later identi ed as Calvin cycle intermediates The work of Calvin and others led to the identification and characterization of rubisco as the key enzyme in the carbon fixation reaction As shown in figure 4 the rubisco reaction can be broken down into four basic steps beginning with the formation of an enediolate intermediate of RuBP which requires the participation of a carbamoylated lysine residue Lys 201 in the enzyme active site Carboxylation then occurs in step 2 by nucleophilic attack on the C02 to form an unstable C5 intermediate 2carboxy3 keto Darabinitol 15bisphosphate that is hydrated in step 3 Aldol cleavage in step 4 leads to the formation of two molecules of 3 phosphoglycerate The rubisco reaction is very exergonic AG 39 351 kJmol with the aldol cleavage step being a major contributor to the favorable change in free energy Figure 4 Cmopoj 7 7 7 L HO Ci I CHOPO c n0PoJ cmomu CHJOPm LOU H39 lt rquot r r m i C O Cl 0 UL HO C 1 K O C 39U HHSPhosphoglycerate n C CH quot c on C 0 quot 90 c oH w l cow H C OH HiciOH H C OH H C OH l H7 0 OH CHOP03quot CH10P05739 CHJOPOJ39 CHOPO739 010003quot Ribulose Enediolale 2 Carboxy 3kelm Hydrated 3Phosphoglycerate 15 bisphosphate intermediate narabinitol intermediate 15bisphosphale Rubisco is a multisubunit enzyme consisting of eight identical catalytic subunits at the core surrounded by eight smaller subunits that function to stabilize the complex and presumably enhance enzyme activity The molecular structure of the spinach plant rubisco enzyme is shown in gure 5 Because of its low catalytic ef ciency a mere three carboxylation reactions per second kca Figure 5 3sec chloroplasts contain large amounts ofthe enzyme in order to meet the demands ofthe cell In fact 50 ofthe total protein in a spinach leaf is rubisco and the concentration of rubisco in chloroplast stroma is a staggering 250 mgml Considering that rubisco plays a central role in all photosynthetic autotrophic organisms on earth of which 85 are photosynthetic plants and microorganisms that inhabit the oceans rubisco is arguably the most abundant enzyme and perhaps the most abundant protein on this planet Stage 2 Reduction of 3phosphoglycerate to from hexose sugars The 3 phosphoglycerate product of the rubisco reaction is converted to glyceraldehyde3 phosphate GAP by two isozymes of phosphoglycerate kinase and glyceraldehydesP dehydrogenase as shown in figure 6 In the Calvin cycle 3 phosphoglycerate kinase uses ATP in a phosphoryl transfer reaction to generate 13 bisphosphoglycerate which is then reduced to GAP by glyceraldehyde3P dehydrogenase This stromal enzyme is similar to the cytosolic version except it uses NADPH instead of NADH as the electron pair donor Remember that for every 3 COZ that are xed by carboxylation of 3 RuBP molecules six moles of 3 phosphoglycerate are 3 of 11 pages Bioc 460 Dr Miesfeld Fall 2008 generated by ado cleavage Therefore 6 ATP and 6 NADPH are required for every 3 COZ that are converted to one net glyceradehyde 3P An additional 3 ATP are used in stage 3 to regenerate these 3 RuBP molecules One of the six GAP molecules is isomerized to dihydroxyacetone phosphate DHAP by another glycolytic enzyme triose phosphate isomerase and used for the net synthesis of Figure 6 hexose sugars 7r use r Zl 39quot39i39 C fosol I 777 3 phosphoglycrat phosphates GAP 3Phosphoglycerate mm KQix coo39 and DHAP are lamquot sfroma used to generate l CHID V fructose1 6 c m ADP biSphOSphate a J I l 13Bisphosphoglylterate Eo l i Condensation ll ll l NADPHH CHEM I 39V l l glyceraldehyde 3phosphate lCH l 39 reaction catalyzed l quotfOH dehydrogenase I l by the enzyme l I H f OH Fructose 6phosphate ADV 39 cgt I transaldolase 3 20 lcnzo Glyceraldehyde 3phosphate Elmquot l which leads to 0 hunose transaldolase triose phosphate 10 39 starch synthesIs In quot C H 1 6bisphosphatase isomerase CHM the chloroplast and um 39 i0 6phosphate H c 0H SUCFOSE synthesIs D39hydroxyacewne CHzO LT quot 39 x lo 9 Fructose In the cytosol 15bisphosphate through Fructose 5 V 7 lt 7 39 16bisphosphate P39 i 7 739quot r InterconverSIon Of Prtriose the hexose Dihydroxyacetone PhQSPhate phosphates phosphate anhporter fructose 6 phosphate glucose phosphate K 6phosphate and Glyceraldehyde glucose 1 3phosphate phosphate Since glycolysk the chloroplast membrane is not 1 permeable to triose phosphates GAP and DHAP must be exported to the cytosol through a Pitriose phosphate antiporter which also serves to import Pi into the stroma Stage 3 Regeneration of ribulose 15bisphosphate In this final stage of the Calvin cycle a series of enzyme reactions convert five C3 molecules GAP or DHAP into three C5 molecules RuBP to replenish supplies of this C02 acceptor molecule which is required in the rubisco reaction Two of the primary enzymes in this carbon shuffle are transketolase and transaldolase which are involved in InterconvertIng C3 C4 C6 and C7 molecules as shown In C TImlskmhm C C figure 7 The transketolase reaction transfers a C2 group 6 3 4 5 from a ketose to an aldose and requires thiamine 1lolasc V C4 3 A7 pyrophosphate as a coenzyme in the reaction In one case transketolase transfers the C2 unit COCHZOH C C Transk olw c C from a C6 molecule fructose6P to a C3 molecule GAP 7 3 5 5 forming C4 erythrose 4P and C5 xyulose5P molecules In a second transketolase reaction a 4 of 11 pages Bioc 460 Dr Miesfeld Fall 2008 32 unit is transferred from a 37 molecule sedoheptulose7P to a 33 molecule GAP forming two differenth molecules ribose5P and xylulose5P As we have already seen in stage 2 figure 7 transaldolase catalyzes the formation of a 36 moecue fructose16 BP from two 33 moecues GAP and DHAP and here in stage 3 the same enzyme generates a 37 moecue sedoheptulose17BP from 33 DHAP and 34 moecues erythrose4P Figure 8 summarizes these carbon shufer reactions showing how five 33 molecules of GAP are converted to three 35 moecues consisting of two xylulose5P and one ribose5P The two xylulose5P and one ribose 5P molecules are first converted to three ribulose5P molecules by the enzymes ribulose 5P epimerase and ribose5P isomerase respectively and then the enzyme ribulose5P kinase catalyzes a phosphoryl transfer involving three ATP to generate the final three molecules of RuBP The regeneration ofthe three RuBP moecues used to fix 3 302 requires an additional 3 ATP Figure 8 Glyceraldehyde 3 phosphate Dihydroxyacetone phosphate m V V m D transaldolase Fructose 16bisphosphate O O O O H fructose16bisphosphalase Pi Fructose 6 phosphate Glyceraldehyde 3 phosphate u o o m quot transketolase Xylulose 5phusphate Dihydroxyacetone phosphate Erythrose 4phosphate m m V quot 0 transaldolase bum 5 phosphate eplmerase Sedoheptulose17bisphusphate Ribulose 5phosphate o o o H sedoheptulase j 17bisphosphatase a Pi ADP Ribuluse15 ADP bisphosphate Glyceraldehyde 3phosphate Sedoheptulose 7phosphate a OOOH Ribulose 5phosphate transketnlase o o o o o ribulose 5phosphate Ribose 5 phosphate Xylulose 5phosphate epimerase Hm GHQO ribose 5phnsphate a ADP isomevase Ribulose 5phosphate b l Ribulose15 bisphosphate fl u 052 5phosphate kinase 5 of 11 pages Bioc 460 Dr Miesfeld Fall 2008 Let39s review the stoichiometry of the Calvin cycle reactions by summarizing what is required to synthesize one molecule of glucose from six C02 We can break the net reaction of the Calvin cycle down into two components 1 the synthesis of one glucose molecule from six C02 using 12 ATP and 12 NADPH and 2 regeneration ofsix RuBP using six ATP Glucose synthesis 6 C02 6 RuBP 12 NADPH 12 ATP 10 H20 gt 4 GAP 2 DHAP Fructose6P Glucose 12 NADP 12 ADP 16 Pi Regeneration of RuBP 4 GAP 2 DHAP Fructose6P 6 ATP 2 H20 gt 6 RuBP 6 ADP 2 Pi Net reaction from six turns of the Calvin cycle 6 C02 12 NADPH 18 ATP 12 H20 gt Glucose 12 NADP 18 ADP 18 Pi Regulation of the Calvin Cycle At night plant cells rely on gycoysis and mitochondrial aerobic respiration to generate ATP for cellular processes Since photophosphorylation and NADPH production by the photosynthetic electron transport system is shut down in the dark it is crucial that the Calvin cycle only be active in the light figure 9 Otherwise if gycoysis the pentose phosphate pathway and the Calvin cycle were all active at the same time then simultaneous starch degradation and carbohydrate biosynthesis would quickly deplete the ATP and NADPH pools in the stroma Light stimulates the activity of Calvin cycle enzymes by two mechanisms 1 Calvin cycle enzymes are activated by elevated pH and by increased Mg2 concentrations in the stroma Light activation of the photosynthetic electron transport system causes stromal pH to increase from pH 7 to pH 8 as a result of proton pumping into the g g thylakoid lumen As shown in DARK LIGHT figure 10 this influx of H into the Photon Mmquot lumen causes an efflux of Mg2 to Wowas the stroma to balance the charge 2 the thylakoid membrane unlike the mitochondrial inner membrane C is permeable to ion movement Since the Calvin cycle enzymes rubisco and fructose16 bisphosphatase are both activated by elevated pH and Mg2 light activation of photosynthetic COZ electron transport indirectly Lightactivated Photaphosphorylation 39 Calvin stimulates metabolic flux through Cycle the Calvin cycle w A inactive enzymes 2 Calvin cycle enzymes are activated thioredoxin mediated reduction of disul de bridges 6 of 11 pages Bioc 460 Dr Miesfeld Fall 2008 Thioredoxin is a small protein of 12 kDa that is L re 11 found throughout nature and functions as a redox protein that can interconvert disulfide a mam l I bridges and sulfhydrals in cysteine residues of Calvin quot V 9 15 s Ca39V39 cycle target proteins In the dark fructose16 in ll356d bisphosphatase sedoheptulose17 7 7 bisphosphatese RuBP kinase and glyceradehyde3P dehydrogenase all contain disulfide bridges that inhibit enzyme activity Light activation of the photosynthetic electron transport system leads to reduction of thioredoxin present in the stroma which in turn reduces disulfide bridges in these enzymes resulting in their activation As shown in figure photon absorption by the PSI reaction center leads to increased levels of reduced ferredoxin which passes electrons one at a time to the enzyme ferredoxinthioredoxin reductase when NADP becomes limiting high NADPH levels in the stroma Ferredoxinthioredoxin Stroma reductase is a soluble enzyme that contains a 4Fe4S cluster and is able to donate two electrons to oxidized thioredoxin causing reduction of its disulfide bridge Reduced a thioredoxin then uses these electrons to reduce Lumen the disulfide bridges in target enzymes As long as reduced thioredoxin is present in the stroma these Calvin cycle enzymes are maintained in the active state however when the sun goes down spontaneous oxidation leads to their inactivation activated Ferrodoxin g thioredoxin reductase quot Ferrodoxin NADP reductase The C4 and CAM pathways reduce photorespiration in hot climates Rubisco also catalyzes a oxygenase reaction that combines RuBP with 02 to generate one molecule of 3phosphoglycerate C3 and one molecule of 2phosphoglycolate C2 as shown in figure 12 It is thought that this quotwastefulquot EQLQ reaction belies the ancient DAVO OcO history of the rubisco 39 quot activity l l CH 09 CH 00 39 enzyme which has been I 2 l 2 HC70H HC OH l 902 around since before 02 i0 Q T 0 CHQOO CHZOC levels in the atmosphere Her OH icioH WWWmm mu whomum were as high as they are WfOH Hflsiori today The 2 04200 CH200 O O O O 39 l Phosphog39yco39ate 398 li li l11 lt1i2 02 r r metabolized by the l l WM HC OH CH20 glycolate pathway which quot33353379 4209 l39lll l l lll WV generates C02 and requires a significant amount of cellular energy to salvage the C2 group for reincorporation into the Calvin cycle As shown in figure 13 the 2 phosphoglycolate is converted to glycolate which is exported to peroxisomes to make glyoxylate and glycine which is then exported to mitochondria where two 7 of11 pages Bioc 460 Dr Miesfeld Fall 2008 molecules of glycine are converted to one molecule M of serine Oxygenation of RuBP and metabolism of 2 phosphoglycolate by the glycolate pathway is collectively called photorespiration because 02 is consumed and 002 is released However unlike mitochondrial respiration photorespiration requires energy input and is therefore considered a wasteful pathway in photosynthetic cells Plants in hot sunny climates are especially susceptible to photorespiration due to high 02002 ratios under these conditions 02 is more soluble at higher temperatures In the 1960s Marshall Hatch and Roger Slack plant biochemists at the Colonial Sugar Refining Company in Brisbane Australia used 14002 labeling experiments to determine what the initial products were in the carbon fixation reactions of sugarcane plants To their surprise they found that malate was more quickly labeled with 14C than was 3phosphoglycerate Follow up work showed that plants such as sugarcane and corn and weeds like crabgrass thrive under high temperature conditions by having very low levels of photorespiration Rather than accumulating mutations in rubisco that increase the efficiency of carbon fixation they found that these plants evolved a way to dramatically increase the concentration of 002 inside the stroma thereby preventing significant 02 binding to the rubisco active site The mechanism involves the carboxylation of phosphoenolpyruvate PEP by the enzyme PEP carboxylase to form oxaloacetate 0AA a four carbon 04 intermediate that serves as a transient 002 carrier molecule Two variations of the quotHatchSlackquot pathway have been described 1 the C4 pathway in tropical plants such as sugarcane that utilize two Figure 14 separate cell types one for CO2 Uptake and the Sugarcane plants use the C4 pathway Saguaro cacti use the CAM pathwz other for rubiscomediated carboxylation and 2 39 the CAM pathway found in desert succulents such as the giant saguaro cactus which captures 002 at night when transpiration rates are low figure 14 Gas exchange in land plants occurs through a special cell structure called a stomate consisting of two guard cells that change shape to open and close the stomate in response to physiological cues Sugarcane grows in a moist hot climate and the stomata plural for stomate open during the day to capture 002 when photosynthetic electron transport rates are high due to intense sunlight In contrast saguaro cacti live in a dry hot climate and only open their stomata at night therefore they must rely on the captured 002 to drive the Calvin cycle reactions during the day when sunlight is available and the stomata are closed Chloroplast 2phosphoglycerate 3 phosphoglycerate Glycerate Glycerate r 02 Hydroxypyruvate Peroxisome qi Serine Serine Glycine 2 NADH H N Mitochondrion decarboxylase NH3 8 of11 pages Bioc 460 Dr Miesfeld Fall 2008 The 04 pathway in sugarcane is M shown in figure 15 where it can be seen that 5mm E E 02 mesophyll cells are responsible for 002 capture whereas interior bundle sheath cells further away from atmospheric 02 Mes neilulhyll use 002 released from the 04 intermediate malate to carry out the Calvin cycle reactions The 04 and 03 pathways are physically separated in the two cell types with malate functioning as the 002 transporter molecule into the bundle sheath cell and pyruvate serving to recycle the carbons back to the mesophyll cell This quotseparation of laborquot between the two cell types essentially eliminates the oxygenase reaction in rubisco and thereby blocks photorespiration Note that the 04 pathway has a price in that two high energy phosphate bonds AMP PR are required to convert pyruvate to phosphoenolpyruvate in mesophyll cells The additional input of energy required to temporarily store the 002 would seem to put 04 plants at a disadvantage However the metabolic cost is more than compensated for by the increased carboxylation efficiency of rubisco in these 4 HCO 39 7 2Pi PEP PEP 0AA carboxylase S cytosol chloroplast PEP 0 N Malate dehydrogenase NADP AMP Pyruvate H PPl phosphate dikinase ATP V AD P ruvate malate v 39 I NADPAmalic enzyme BundEsiheath cell plants once temperatures reach 2830 00 In fact considering that photorespiration in 03 plants is a significant problem at high ambient temperatures 04 plants have a slight advantage under these conditions because the cost of two additional ATP to store 002 is slightly less than the one ATP and one NADPH needed to recycle 2 phosphoglycerate in the glycolate pathway photorespiration As schematically shown in figure 16 this growth advantage of 04 plants at high temperatures is evident in the heat of summer where crabgrass a 04 plant is able to invade a turf lawn consisting of 03 grasses that are growthinhibited by high rates of photorespiration However at more moderate temperatures in the spring when photorespiration rates are low in 03 plants the higher energy cost of the 04 pathway in the crabgrass is a disadvantage and the turf grass is able to prevail The 0AM pathway was first discovered in succulent plants of the Crassulaceae family and is therefore called Crassulacean Acid Metabolism CAM pathway Just like the 04 pathway in sugarcane the CAM pathway functions to concentrate 002 levels in the Figure 16 Turf C3 plant outgrows crab grass C4 plant quotT 39l Nill W JillWW JV l lg gi ll ll Ell l 5 l ll quot l jl 46 Summertime 30 C l MW l ll ll l on W 111 N l l l 39NlJVNVNlll vCir aib grass C4 plant outgrows Turf C3 plant chloroplast stroma to limit the oxygenase activity of rubisco The main difference is that instead of 9 of11 pages Bioc 460 Dr Miesfeld Fall 2008 using two cell types to physically Fume 17 separate atmospheric 02 from N h 20 C 0 rubisco as we saw in C4 plants 39g t l Day 40 Q CAM plants like the saguaro cactus 0 use a temporal separation As 02 02 2 2 shown in figure 17 during the night 0 II when the stomata are open C02 is H O Mesophy C Mesophy 2 cell H20 cell captured by the mesophyll cells and incorporated into OAA by PEP carboxylase which is then reduced by the enzyme NADmalate dehydrogenase to form malate The quot W39ase NADrmalate malate is stored inside vacuoles PEP dehydrogenasel overnight until sunlight activates the 3332 Malate Calvin cycle enzymes the next quotquotquota e quotmquot morning at which time malate is oxidized and decarboxylated by NADPmalic enzyme to generate C02 and pyruvate Unlike in C4 plants in which the carbon skeleton Wn of pyruvate is recycled directly to CWe PEP the pyruvate is stored as starch during the day At night the starch granules are degraded to form pyruvate which is converted to PEP thereby allowing PEP carboxylase reaction to capture C02 when the stomata are open Photorespiration is kept to a minimum during the day when the stomata are closed and C02 levels in the chloroplast are high due to the decarboxylation of malate Moreover by keeping the stomates closed water loss by transpiration evaporation is inhibited at times of the day when temperatures can reach 40 C in the Sonoran desert Given the increased efficiency of the C4 pathway compared to the C3 pathway at elevated temperatures crop scientists have begun to develop strategies to try and convert C3 crops into pseudo C4 plants One focus of this research has been rice which is a staple food crop that can be grown in tropical areas where water is plentiful and daytime temperatures can be high Traditional rice breeding experiments using C4 plants such as corn are not feasible and therefore plant researchers have recently turned to gene transfer techniques to try and quotbuildquot the C4 pathway into the rice genome using recombinant DNA methods Introduction of single genes from the C4 pathway such as PEP carboxylase do show biochemical activity in the mesophyll cells of rice however photosynthetic efficiency is not significantly altered Current strategies are now aimed at introducing multiple genes into rice for example PEP carboxylase NADPmalate dehydrogenase and NADPmalic enzyme which together would reconstitute the key reactions in the C4 pathway A major challenge in these type of quotmetabolic engineeringquot approaches is being able to regulate the expression and biochemical activity of multiple foreign genes in ways that mimic the finely tuned control of metabolic pathways that have evolved over time OAA I F 39 ll NADP malic NAD ate enzyme NADP 39 H 602 chloroplast cymsol 10 of 11 pages Bloc 460 Dr Miesfeld Fall 2008 ANSWER TO KEY CONCEPT QUEeron Concerning the CaLVIN CYCLE How do sugarcane plants and saguaro cacti avoid the wasteful side effects of photorespiration Sugarcane plants and saguaro cacti avoid the wasteful side effects of photorespiration by separating the process of gas exchange through the stroma from the carbon dioxide xation reaction mediated by the Calvin cycle enzyme Iibulose15bisphosphate carboxylaseoxygenase rubisco In addition to carbon dioxide xation the rubisco also catalyzes an oxygenase reaction that combines ribulose15bisphosphate RuBP with 02 to generate one molecule of 3 phosphoglycerate and one molecule of 2phosphoglycolate This wasteful side reaction called photorespiration not only diminishes net conversion of 002 to carbohydrate but also requires that the Glycolate pathway consume ATP in order to recover the two carbons from 2 phosophochlycholate Sugarcane plants use the 04 pathway to reduce photorespiration by separating gas exchange and 002 fixation into two different cell types mesophyll and bundle sheath cells In contrast saguaro cacti use the CAM pathway which limits photorespiration by capturing 002 at night and then recovering it from the 04 intermediate in the same cells during the day when it is too hot to open the stomates for gas exchange 11 of11 pages
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