Topics 1 Regulation of the Calvin Cycle 2

Topics: 1. Regulation of the Calvin Cycle 2. Photorespiration 3. CO 2 concentrating mechanisms 4. Sucrose and starch synthesis

Regulation of the Calvin cycle a. Ru. BP “activase” b. Light induction of Calvin cycle gene expression c. Enzyme activites regulated by redox state of the chloroplast

c: Redox state of stroma: The Ferredoxin-Thioredoxin System NADPH

Topics: 1. Regulation of the Calvin Cycle 2. Photorespiration 3. CO 2 concentrating mechanisms 4. Sucrose and starch synthesis

“The Problem with Oxygen”: RUBISCO reacts with oxygen as well as CO 2 (oxygenase/carboxylase) RUBISCO has a higher affinity for CO 2 compared to O 2 (lower Km) Rubisco : Km (CO 2)= 15 μM Km (O 2)= 550 μM But concentration of O 2 is much higher: Atmophere: 20% O 2 and only 0. 03% CO 2 In solution: CO 2 = 12 μM, O 2= 265 μM NET RESULT: a lot of O 2 gets “fixed” instead of CO 2 This process is called photorespiration.

Ru. Bis Carboxylase/Oxygenase

Calvin Cycle Rib 15 bis. P (C 5) + O 2 P-Glycolate (C 2) O 2 Glycolate (C 2) H 2 O 2 Glycolate (C 2) Glycine (C 2 -N) 3 x. ATP 2 x NADPH 3 -P-Glycerate (C 3) Chl. Fd ATP Glycerate (C 3) NH 4 ATP Hydroxypyruvate (C 3) Serine (C 3 -N) CO 2 NH 4 Per. Mit

The cost of photorespiration 3 x O 2 needs 2 x ATP and 2 x Ferredoxin AND high temperature increases photorespiration: *Modifies Rubisco’s kinetics: oxygenation more favorable *Decreases the CO 2/O 2 ratio in solution

What do plants do?

CO 2 Concentrating Mechanisms a) CO 2 and HCO 3 - Pumps: aquatic organisms b) CO 2 concentrating mechanisms: higher plants

CO 2 Concentrating Mechanisms Clicker question: Is there only one type of CO 2 concentrating mechanisms in higher plants? A. Yes B. No, there are two. C. N, there are many

CO 2 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. CO 2 concentrating mechanisms evolved many times independently: Convergent evolution

CO 2 Concentrating Mechanisms a) CO 2 and HCO 3 - Pumps: aquatic organisms b) CO 2 concentrating mechanisms: higher plants PEP-Carboxylase CH 2 II C-OPO 32 - + HCO 3 I COOPhosphoenolpyruvate COOI CH 2 + HPO 42 I C=O I COOOxaloacetate

Could plants just use PEP-carboxylase instead of Rubisco?

Principles of CO 2 concentration mechanisms HCO 3 - C 3 + HCO 3 C 3 -”recycling” CO 2 Fixation/carboxylation C 3 C 4 transport C 3 + CO 2 Decarboxylation RUBISCO

HCO 3 - C 3 + HCO 3 C 3 -”recycling” CO 2 Fixation/carboxylation C 3 C 4 transport C 3 + CO 2 Decarboxylation RUBISCO

CO 2 Concentrating Mechanisms a) C 4 Photosynthesis: spacial separation b) Crassulacean Acid Metabolism (CAM): temporal separation

The C 4 carbon cycle: Spatial separation a. Different Cells: Bundle Sheath cells/ Kranz anatomy b. Within one cell

Kranz (=Wreath) Anatomy Bundle sheath cells

Single Cell C 4 Photosynthesis (V Borszczowia

CAM: temporal separation Minimizing water loss CAM C 4 C 3 H 20 loss/CO 2 gained (g) 50 -100 g 250 -300 g 400 -500 g

CAM: Day/Night switch

Topics: 1. Regulation of the Calvin Cycle 2. Photorespiration 3. CO 2 concentrating mechanisms 4. Sucrose and starch synthesis

UDP-Glucose

Saccharide Synthesis: Overview Pi Triose-P Glc-1 -P NTP (ATP/UTP) Glc-Nt. DP PPi S a c c h a ri d e s

Plastids: Starch Synthesis

Starch is a branched polymer Remember: Cellulose = b-D-1, 4 -glucosyl

Regulation of starch and sucrose biosynthesis

Pi Suc-6 -P PPi SPS UTP Sucrose UDP-Glc Glc-1 -P Phosphate is generated in the cytosol during sucrose synthesis Glc-6 -P Fru-6 -P Pi Pi PPi Fru-1, 6 -bis. P Triose-P ATP ADP

Balance: Starch vs Sucrose Synthesis Cytosol Plastid Sucrose Synthesis Triose-P Pi Starch Synthesis

Regulation of Starch and Sucrose Synthesis UDP-Glc + Fru-6 -P Glc-1 -P Suc-6 P SPS Glc-6 -P Pi SPS-P Pi ATP 2 x. Pi PPi ADP-Glc Sucrose-P Synthase (SPS) 3 PGA Ferredoxin Red. ADP-Glc Pyrophosphorylase (AGPase)