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Chapter 20:The Calvin Cycle and the
Pentose Phosphate Pathway
Copyright © 2007 by W. H. Freeman and Company
Berg • Tymoczko • Stryer
BiochemistrySixth Edition
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Photosynthesis Dark Reactions (The Calvin cycle)
Reductive conversion of CO2 into carbohydrate. This process fixes ~1010 tons of CO2 annually.
Process is powered by ATP and NADPH which are products of the light reactions of photosynthesis.
Dark reactions occur in chloroplast stroma.
Called the Calvin-Benson-Bassham pathway or the reductive pentosephosphate cycle (RPP)
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Net equation for the Calvin cycle
3 CO2 + 9 ATP + 6 NADPH + 5 H2O
9 ADP + 8 Pi + 6 NADP+ + *Triose phosphate
From an energy standpoint this is an expensive process: 3 ATP and 2 NADPH per CO2 incorporated.
*(G3P or DHAP)
These reactions occur in the stroma.
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Calvin Cycle
Fixation,Reduction,Regeneration
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Stage 1: Incorporation of CO2
Catalyzed by the enzyme Rubisco.Ribulose-1,5-bisphosphate carboxylase-oxygenase
Plants fed CO2, yield 3-phosphoglycerate as the first compound detected.
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Rubisco
8 subunits @ 530008 subunits @ 14000~540000 d total
It is the most abundant enzyme in the biosphere
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Rubisco Mechanism
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Mg++ binding
to Rubisco
Bound using Glu, Asp and a Lys carbamate. Thecarbamate formation is catalyzed by Rubisco Activase
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Stage 2
Phosphorylation and
Reduction
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Hexose synthesis
3-phospho glycerate to hexose-P
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Stage 3: Regeneration
These reactions serve to regenerate ribulose-1,5-bisphosphate from glyceraldehyde-3-phosphate.
Two group transfer reactions are common here:
1. a transketolase reaction using TPP and
2. a transaldolase reaction
Then an isomerase, an epimerase and a kinase complete the cycle.
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Example Transketolase
2 C transfer from a ketose
TPP
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Example Transaldolase
3 C transfer from a ketose
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Sugar Interconversions
2 C transfer from ketose
TPP
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Sugar Interconversions
3 C transfer from ketose
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Sugar Interconversions
2 C transfer from ketose
TPP
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Sugar Interconversions
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Calvin Cycle
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Enzymes of the Calvin Cycle
1. Rubisco 2. Phosphoglycerate kinase 3. Glyceraldehyde-3-phosphate dehydrogenase 4. Triosephosphate isomerase 5. Aldolase (transaldolase) 6. Fructose bisphosphatase 7. Transketolase 8. Sedoheptulose-7-phosphatase 9. Phosphopentose isomerase
10. Phosphopentose epimerase 11. Ribulose-5-phosphate
kinase
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Carbon Flow in the Calvin Cycle
3 C5 + 3 C1 ---> 6 C3
2 C3 ---> 1 C6
C6 + C3 ---> C4 + C5
C4 + C3 ---> C7
C7 + C3 ---> 2 C5
-----------------------------------
net 3 C1 ---> 1 C3
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Synthesis of Sucrose
Occurs in the cytosol.
There is a triose-P:Pi antiport in the chloroplast membrane.
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Regulation of the Calvin Cycle
Rubisco has three forms:
1. E binds R-1,5-B in the dark and is inactive. R-1,5-B is an inhibitor in the dark (blocks the carbamylation site). Rubisco activase causes dissociation of R-1,5-B and catalyzes ATP dependent attachment of CO2.
2. EC carbamylated at Lys201 is still inactive.
3. ECM has bound Mg++ and is active.
Rubisco and rubisco activase are light activated.
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Regulation of the Calvin Cycle
Rubisco activation requires: light, CO2, Mg++ & pH 7.4 (optimum is 8.1)
Functioning PSII-Cytbf-PSI cause proton pumping which leaves the stroma basic. The potential developed promotes translocation of Mg++ and Cl-. Mg++ is needed in the stroma for rubisco and both phosphatases. The stroma can reach pH 9.0. The high pH activates rubisco.
Light produces a conformational change in rubisco activase enhancing its activity.
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Regulation of the Calvin Cycle
Functioning PSII-Cytbf-PSI also produces reduced ferredoxin and NADPH.
Ferredoxin-Thioredoxin Reductase generates thioredoxin-(SH)2
Ferredoxin(red) + thioredoxin-S2(ox) ===== > Ferredoxin(ox) + thioredoxin-(SH)2(red)
Thioredioxin (a small protein) activates a number of enzymes through a disulfide -- > dithiol conversion.
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Regulation of Photosynthesis
Light is needed to generate NADPH, FDred and Mg++ transport
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Thioredoxin
This is a small disulfide containing protein.
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Action ofThioredoxin
Enzymes activated by thioredoxin:
Fructose bisphosphataseSedoheptulose bisphosphataseRibulose-5-P kinaseGlyceraldehyde-3-P dehydrogenase
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Regulation
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Photorespiration (Use of O2)
This competes with photosynthesis at higher temperatures so is typically more active in the summer and in the tropics.
Conc. in air KM
O2 250 μM (20%) 200 μM CO2 11 μM (0.04%) 20 μM
However, the affinity of Rubisco for CO2 decreases with increasing temperature.
The immediate products of photorespiration are phosphoglycolate and 3-phosphoglycerate.
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Photorespiration
The mechanism is analogous to that for carboxylation. Rubisco must be carbamylated.
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Photorespiration
The reaction involved in photorespiration require participation of enzymes from the cytosol, chloroplasts, mitochondria and peroxisomes.
Chloroplast: glycolate phosphatasePeroxisome: glycolate oxidase
transaminasehydroxypyruvate reductase
Mitochondria: Glycine cleavage enzyme serine hydroxymethyltransferase
Cytosol: glycerate kinase
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Photo respiration reactions
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Photo respiration reactions
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Photorespiration
CHO
CO2-
H C OH
CH2 -OPO3=
C = O
CH2 -OPO3=
C = O
H C OH
H C OH
CH2 -OH
CH2 -OH
CO2-
CO2-
H C NH2
CH2 -OPO3=
CO2-
H C NH2
CH2 -OH
CO2-
H C OH
CH2 -OH
C = O
CH2 -OH
CO2-
CO2-
CH2 -OPO3=
CO2-
CH2 -OH
CO2-
CO2-
CH2 -NH2
2-P-glycolate glycolate glyoxylate
glycine
3-P-glycerate3-P-hydroxypyruvate 3-P-serine Serine
glycerate hydroxypyruvate
Ribulose-1,5-bisphosphate
Rubisco
+ O2
1 2 3
6
54
7 8
1. phosphoglycerate DH 5. glycerate dehydrogenase2. transaminase 6. glycerate kinase3. phosphoserine phosphatase 7. phosphoglycolate phosphatase4. transaminase 8. glycolate oxidase
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PhotorespirationRun through the previous reactions twice yield two glycines which then react as shown below.So two glycolates produce 1 CO2 and 1 serine.
CO2-
CH2 -NH2
glycine
+ FH4 + NAD+ ---------- > CH2FH4 + NADH + NH4+
+ CO2
CO2-
CH2 -NH2
glycine
+ CH2FH4 ---------------- > FH4 +
CO2-
H C NH2
CH2 -OH
Serine
glycinecleavage
enzyme
serinehydroxymethyl
transferase
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Tetrahydrofolate
FH is a coenzyme that serves as a one carbon carrier.
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Hatch-Slack Pathway
This pathway uses a CO2 concentrating mechanism to permit photosynthesis to surpass photorespiration.
The initially observed compound is this case is oxaloacetate, so this is referred to as the C4 pathway. Similarly, normal photosynthesis is sometimes called the C3 pathway.
C4 plants include crabgrass, bermuda grass, corn, maize and sugarcane. These have an advantage over C3 plants in hot weather.
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C4 Pathway
C4 plants have mesophyll cells which are outer cells that collect CO2. Bundle sheath cells are inside where the Calvin Cycle occurs.
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C4 Pathway
CO2-
C OPO3= C = O
CO2-
CO2-
H C OH
PEP oxaloacetate malatepyruvate
1 2
3
1. PEP carboxylase 2. malate dehydrogenase3. Malic enzyme4. pyruvate:phosphate dikinase
CH2
C = O
CO2-
CH3
bundle sheath cell
mesophyll cell
CO2-
H C OH
4
CH2-CO2-
CH2-CO2- CH2-CO2
-
C = O
CO2-
CH3
CO2
CO2
to Calvin cycle
from air
Pi
NADPH
NADP+
NADPH
NADP+
ATP+Pi
ADP+2Pi
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Hatch-Slack Pathway
Different plants have different mechanisms for moving CO2 into the bundle sheath cells. The Hatch- Slack uses four enzymes in the C4 pathway.
1. PEP carboxylase: HOH + PEP + CO2 -- > OAA + Pi2. Malate dehydrogenase (NADP+ dependent)3. Malic enzyme (NADP+ dependent) also called malate dehydrogenase decarboxylating 4. Pyruvate:phosphate dikinase
ATP + Pi -- > ADP + PPiADP + E -- > AMP + E~PE~P + Pyruvate -- > PEP + EPPi -- > 2 Pi
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Hexose MonophosphateShunt (HMS)
Pentose phosphate pathway or Phosphogluconate pathway
This pathway is the major site for production of:1. NADPH for anabolism (reductive)2. Ribose-5-phosphate for nucleotide synth.
Other: Makes erythrose for Phe synthesis.
Completely oxidizes glucose without Krebs.No ATP used or made in this pathway.
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Hexose monophosphate
shunt (HMS)
Phase 1 - oxidative.Phase 2 – isomerization, epimerization and rearrangement.
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HMS
This phase is composed of three reactions, two of which are oxidations.
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HMS
Isomerization and epimerizartion.
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HMS
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HMS – Oxidative Phase
The oxidative phase is one-way as written.Reaction 1 is the control site for the HMS. Reaction 3 is the least reversible step. The mechanism involves the decarboxylation of a -ketoacid, similar to isocitrate dehydrogenase.
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HMS – 1st Oxidative Step
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HMS – Lactone Formation
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HMS – 2nd Oxidative Step
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Sugar Interconversions
2 C transfer from ketose
TPP
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Sugar Interconversions
3 C transfer from ketose
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Sugar Interconversions
2 C transfer from ketose
TPP
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Pentose Phosphate PathwayReactions
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Transketolase Mechanism
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Step 1
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Step 2
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Step 3
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Step 4
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Step 5
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Transaldolase Mechanism
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Step 1
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Step 6
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Transaldolase & TransaldolaseActive components of each mechanism
transketolase
transaldolase
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Making Ribose-5-PEnter HMS from F-6-P and convert all to ribose-5-P
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Making NADPH & Ribose-5-PEnter HMS from G-6-P, make NADPH and convert all ribulose-5-P to ribose-5-P
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Making NADPHEnter HMS from G-6-P, convert all ribulose-5-P to fructose-6-P and recycle to glucose-6-P.
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Making NADPH & ATPEnter HMS from G-6-P, convert all ribulose-5-P to fructose-6-P and use it for energy via glycolysis.
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Active HMS
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Regulation of the HMS
Glucose-6-phosphate dehydrogenase is the control point for the pathway. NADPH(-) competes for the binding site with NADP+. Also, fattyacyl CoA(-). Thus, regulation is tied to the need for anabolism and reductive processes. KM of the enzyme for NAD+ is 1000 times greater that that for NADP+.
Normal levels: NAD+/NADH = 700-1000 and a high level of
NAD favors oxidation reactions.
NADP+/NADPH = 0.01-0.014 and high levels of NADP favor reduction reactions.
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Erythrocytes
The need for NADPH in red cells is critical. Red cells lack mitochondria therefore no energy is available from the Krebs cycle or ET/OP. All energy is derived from glycolysis and the HMS.
NADPH is needed to keep hemoglobin and other proteins in the active dithiol form. The active agent here is glutathione, a ubiquitous reducing agent (found in all cells). Glutathione is -glutamyl-cysteinylglycine (GSH).
Activation of an oxidized enzyme:
2 GSH + ES2 -- > GSSG + E(SH)2
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Glutathione (Found in all cells.)
- carboxyl
thiol
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Reduction of GSSG
Glutathione reductase is an NADPH requiring flavoprotein that catalyzes conversion of GSSG back to 2 GSH. FADH2 does not convert GSSG directly but rather goes through a disulfide/ dithiol conversion on the enzyme.
NADPH + H+ + E-FAD -- > NADP+ + E-FADH2
E-FADH2 + ES2 -- > E-FAD + E(SH)2
E(SH)2 + GSSG -- > ES2 + 2 GSH
The normal GSH/GSSG ratio in red cells is ~500.
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End of Chapter 20
Copyright © 2007 by W. H. Freeman and Company
Berg • Tymoczko • Stryer
BiochemistrySixth Edition