Regulation of the Calvin Cycle

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• Topics
• Regulation of the Calvin Cycle
• Photorespiration
• CO2 concentrating mechanisms
• Sucrose and starch synthesis

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Regulation of the Calvin cycle

1. RuBP activase
2. Light induction of Calvin cycle gene expression
3. Enzyme activites regulated by redox state of the
   chloroplast

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c Redox state of stroma The Ferredoxin-Thioredo
xin System
NADPH
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• Topics
• Regulation of the Calvin Cycle
• Photorespiration
• CO2 concentrating mechanisms
• Sucrose and starch synthesis

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The Problem with Oxygen RUBISCO reacts with
oxygen as well as CO2 (oxygenase/carboxylase)
RUBISCO has a higher affinity for CO2 compared
to O2 (lower Km) Rubisco Km (CO2) 15
µM Km (O2) 550 µM But concentration of
O2 is much higher Atmophere 20 O2 and only
0.03 CO2 In solution CO2 12 µM, O2 265
µM NET RESULT a lot of O2 gets fixed instead
of CO2 This process is called photorespiration.
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RuBis Carboxylase/Oxygenase
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Calvin Cycle
3xATP
Rib15bisP (C5) O2
2x NADPH
3-P-Glycerate (C3)
P-Glycolate (C2)
Chl.
Fd
Glycerate (C3)
ATP
Glycolate (C2)
Glycolate (C2)
Glycerate (C3)
O2
ATP
Hydroxypyruvate (C3)
NH4
Glycolate (C2)
H2O2
Per.
Serine (C3-N)
Glycine (C2-N)
Serine (C3-N)
Glycine (C2-N)
CO2
NH4
Mit
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The cost of photorespiration
3x O2 needs 2x ATP and 2x Ferredoxin
AND high temperature increases photorespiration
Modifies Rubiscos kinetics oxygenation more
favorable Decreases the CO2/O2 ratio in solution
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What do plants do?
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CO2 Concentrating Mechanisms

1. CO2 and HCO3- Pumps aquatic organisms
2. CO2 concentrating mechanisms higher plants

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CO2 Concentrating Mechanisms

• Clicker question Is there only one type of CO2
  concentrating mechanisms in higher plants?
• A. Yes
• B. No, there are two.
• C. N, there are many

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CO2 Concentrating Mechanisms

• Clicker question Did these different mechanisms
  evolved from one common ancestor?
• A. Yes, modifications occurred later.
• B. No, there are two independent origins.
• C. No , there were many independent origins.

CO2 concentrating mechanisms evolved many times
independently Convergent evolution
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CO2 Concentrating Mechanisms

1. CO2 and HCO3- Pumps aquatic organisms
2. CO2 concentrating mechanisms higher plants

PEP-Carboxylase
CH2 II C-OPO32- HCO3- I COO-
COO- I CH2 HPO42- I CO I COO-
Phosphoenolpyruvate
Oxaloacetate
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Could plants just use PEP-carboxylase instead of
Rubisco?
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Principles of CO2 concentration mechanisms
HCO3-
CO2
Fixation/carboxylation
C3 HCO3-
C3-recycling
C3
C4
C4 transport
C3 CO2
Decarboxylation
RUBISCO
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HCO3-
CO2
Fixation/carboxylation
C3 HCO3-
C3-recycling
C3
C4
C4 transport
C3 CO2
Decarboxylation
RUBISCO
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CO2 Concentrating Mechanisms

• C4 Photosynthesis spacial separation
• Crassulacean Acid Metabolism (CAM) temporal
  separation

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The C4 carbon cycle Spatial separation

1. Different Cells Bundle Sheath cells/ Kranz
   anatomy
2. Within one cell

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Kranz (Wreath) Anatomy
Bundle sheath cells
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(V
Single Cell C4 Photosynthesis
Borszczowia
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CAM temporal separation Minimizing water loss
H20 loss/CO2 gained (g) CAM
50-100g C4 250-300g C3 400-500g
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CAM Day/Night switch
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• Topics
• Regulation of the Calvin Cycle
• Photorespiration
• CO2 concentrating mechanisms
• Sucrose and starch synthesis

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UDP-Glucose
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Saccharide Synthesis Overview
Saccharides
Pi
Triose-P
Glc-1-P
Glc-NtDP
NTP (ATP/UTP)
PPi
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Plastids Starch Synthesis
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Starch is a branched polymer
Remember Cellulose b-D-1,4-glucosyl
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Regulation of starch and sucrose biosynthesis
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Pi
Sucrose
Suc-6-P
UDP-Glc
PPi
UTP
SPS
Phosphate is generated in the cytosol during
sucrose synthesis
Glc-1-P
Glc-6-P
Fru-6-P
ATP
Pi
Pi
PPi
ADP
Fru-1,6-bisP
Triose-P
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Balance Starch vs Sucrose Synthesis
Cytosol
Plastid
Sucrose Synthesis
Triose-P
Triose-P
Pi
Pi
Starch Synthesis
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Regulation of Starch and Sucrose Synthesis
Glc-1-P
UDP-Glc Fru-6-P
Suc-6P
SPS
Pi
ATP
Glc-6-P
Pi
3PGA
PPi
2xPi
Ferredoxin Red.
SPS-P
ADP-Glc
Sucrose-P Synthase (SPS)
ADP-Glc Pyrophosphorylase (AGPase)
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