Lecture 17 Oct 10, 2005 Photosynthesis II. Calvin Cycle

Lecture Outline The Calvin Cycle fixes carbon makes reduced carbon compounds Reactions of the Calvin Cycle – anabolic pathway input of NADPH + H+, input of ATP Regulation of the Calvin Cycle The problem with oxygen – Photorespiration Tricks some plants use to limit photorespiration - C4 anatomy, C4 metabolism – division of labor - CAM plants, the difference is night and day

DARK REACTIONS energy utilization The Calvin Cycle

The purpose of the Carbon-fixation (Calvin Cycle) Reactions CO2 + NADPH + H+ + ATP C6H12O6 + NADP+ + ADP + Pi carbohydrate Note: synthesis of carbohydrate from CO2 is favorable only because coupled to very favorable reactions NADPH to NADP+ and ATP to ADP + Pi energy released is greater than it costs to make carbohydrate

The Calvin cycle has three phases Carbon fixation Reduction (energy input, reducing equiv input) Regeneration of the CO2 acceptor (energy input – “priming step”)

+ 2 CO2 3 Carbon Acid 5 Carbon Sugar 5 Carbon Sugar 3 Carbon Aldehyde Overview of Carbon-Fixation Reactions R-C-OH = O 12 24 CO2 + R-C-H Higher Energy Compound = O Reduction 3 Carbon Acid with one phosphate on it 2 5 Carbon Sugar with 2 phosphates on it ATP ADP + Pi ATP ADP + Pi NADPH NADP+ 12 24 20/24 5 Carbon Sugar with one phosphate on it 3 Carbon Aldehyde with one phosphate on it 6 Carbon Sugar - Glucose with NO phosphate on it 4/24 2

Carbon Fixation Carried out by the enzyme “rubisco” (ribulose 1,5 bisphosphate carboxylase oxygenase) Do need to know this enzyme Key regulatory enzyme

Acid -COOH Priming Step Regenerate What started with Carbon Fixation Acid -COOH Input of energy Reduction Rubisco Priming Step Aldehyde -C=O H Regenerate What started with

1st Enzyme in Calvin Cycle Regulation of Rubisco 1st Enzyme in Calvin Cycle Substrate/Product availability Allosterically regulated by NADPH and ATP Very Narrow pH optimum pH 8 / pH7 pH 8 or above, inactive at 7 Enzyme must be in reduced form

Electron transport chain Integration of Light-Dependent and Light-Independent Reactions They generally occur AT THE SAME TIME O2 CO2 H2O Light Light reaction Calvin cycle NADP+ ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast Figure 10.21 4

+ + PhotoRespiration - “ the OXYGEN PROBLEM” Oxygen is a competing substrate for the 1st enzyme in the C3 cycle (Rubisco) CO2 5 Carbon Sugar with 2 phosphates on it 3 Carbon Acid with one phosphate on it + Net increase in material make glucose with “extra” O2 5 Carbon Sugar with 2 phosphates on it 3 Carbon Acid with one phosphate on it + 2 Carbon Acid Energy Wasted With NO synthesis of glucose 5

DIVISION OF LABOR BETWEEN CELLS C4 Plants Some Plants Deal with this problem by a DIVISION OF LABOR BETWEEN CELLS C4 Plants Mesophyll cells perform “usual” noncyclic Light-Dependent Reactions make oxygen, ATP and NADPH Do NOT perform the C3 (Calvin cycle) reactions Bundle Sheath cells perform “UNusual” cyclic Light-Dependent Reactions a lot of ATP but very little NADPH and very little O2 PERFORM the usual C3 (Calvin Cycle) reactions 6

C4 leaf anatomy and the C4 pathway Mesophyll cell Mesophyll cell Photosynthetic cells of C4 plant leaf PEP carboxylase CO2 CO2 Bundle- sheath cell Oxaloacetate (4 C) PEP (3 C) NADPH used ADP Vein (vascular tissue) Malate (4 C) ATP C4 leaf anatomy Malate (4 C) Oxaloacetate (4 C) Pyruate (3 C) NADPH regenerated CO2 Mesophyll Produce NADPH and ATP PEP carboxylase insensitive to O2 Malate brings across reducing equivalents Stomata Bundle- Sheath cell Cyclic e- flow Little O2 CALVIN CYCLE Sugar Vascular tissue Figure 10.19

Mesophyll Cell C4 Metabolism PEP carboxylase CO2 NADPH + H+ NADP+ OH / C=O CH2 C=O + Pi \ Oxaloacetate 4C acid OH / C=O CH2 H-C-O-H \ malate 4C acid CH3 C=O \ O-Pi “PEP” ATP ADP CH3 C=O \ O-H pyruvate “carries” Reducing equivalents Needs ATP and NAPDPH + H+ Non-cyclic electron flow

Bundle Sheath - Cell C4 Metabolism CO2 CH3 C=O \ O-H pyruvate OH / C=O CH2 H-C-O-H \ malate 4C acid OH / C=O CH2 \ Oxaloacetate 4C acid NADPH + H+ NADP+ “carries” Reducing equivalents ATP Calvin Cycle glucose NADPH Needs ATP Only Cyclic electron flow Very low O2

Who cares as long as the sun is shining ? Mesophyll cells provide a means for bundle sheath cells to acquire NADPH + H+ reducing power Mesophyll cells provide carbon dioxide to bundle sheath cells at higher concentration than in air Bundle Sheath cells not making oxygen, so very little competitor with C3 reactions Costs more energy to do business this way… but has the advantage when CO2 is limiting (when stomates are closed - like on hot days) Who cares as long as the sun is shining ? ATP is not limiting 7

CAM Plants Cacti, pineapple Open their stomata only at night, too hot during day – survive very adverse (dry) conditions NIGHT Perform PEP carboxylase reaction at night (CO2 assimilation) accumulate malate to high concentration in central vacuole use sugar oxidation/catabolism to power (NADH and ATP) carbon fixation DAY Perform “light” reactions during the day mostly cyclic e- flow to produce ATP (low O2) decarboxylate malate to yield CO2 and NADPH + H+ perform C3 reactions (Calvin Cycle) to produce sugars and starch 2

Generally Slow growing Sugar Oxidation Sunlight Powers Powers One Both Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO2 C4 CAM CO2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO2 to Calvin cycle Figure 10.20 Generally Slow growing Sugar Oxidation Powers One Phase Sunlight Powers Both phases

Summary Photosynthetic “light” reactions produce ATP and reducing potential NADPH + H+ 2. Dark reactions use ATP and reducing potential to synthesize carbohydrates - powers reduction of 3-carbon acid to 3-carbon aldehyde - powers regeneration of starting material 5-carbon di-phosphate (priming step for CO2 fixation) 3. Rubisco enzyme regulated tightly by allosteric modulators pH, and reducing status of stroma 4. O2 interferes with carbon fixation by Rubisco enzyme 5. Metabolic “tricks” to avoid photorespiration - C4 metabolism - CAM metabolism