Lecture 29

Calvin Cycle Overview

  • The Calvin cycle is a metabolic pathway that occurs in plants and some algae for converting carbon dioxide and other compounds into glucose, primarily consisting of three main phases: carbon fixation, reduction phase, and regeneration.

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Stages of Dark Reactions

  1. Carbon Fixation

    • Three molecules of carbon dioxide ($CO_2$) are incorporated in the cycle.

  2. Use of ATP

    • ATP is utilized to phosphorylate three-phosphoglycerate ($3-PGA$) to 1,3-bisphosphoglycerate.

  3. Reduction Phase

    • This phase involves the conversion of $1,3-bisphosphoglycerate$ into glyceraldehyde 3-phosphate ($G3P$).

  4. Regeneration Phase

    • The final phase where ribulose-1,5-bisphosphate ($RuBP$) is regenerated to enable the cycle to continue.

Key Definitions and Concepts

  • Photorespiration: This is a wasteful pathway that occurs when $O2$ is fixed instead of $CO2$, leading to the production of phosphoglycolate, which is toxic.

    • In photorespiration, oxygen competes with $CO2$ for the active site of rubisco, which can lead to a loss of fixed carbon as $CO2$.

  • Role of Organelles in Photorespiration:

    • Involves collaboration among:

      • Chloroplasts

      • Peroxisomes

      • Mitochondria

Mechanism of Photorespiration

Processing of Phosphoglycolate

Phosphoglycolate is dephosphorylated by a phosphatase to reduce its toxicity.

  • Transferred to the peroxisome, where glycolate oxidase converts it to glyoxylate, generating hydrogen peroxide as a byproduct.

  • Dealing with Hydrogen Peroxide

    • Catalase decomposes hydrogen peroxide into water and oxygen.

Amino Acid Formation

Glyoxylate reacts with ammonium to produce two molecules of glycine. Glycine can be converted into serine and ultimately lead to the production of hydroxypyruvate.

  • Recycling

    • The anionic ammonium ion can be assimilated back into the chloroplast to produce glutamine from glutamate.

Consequences of Photorespiration

  • Carbon lost as $CO_2$ in the metabolic pathway.

  • Ammonium produced is salvaged but requires extra resources, such as ATP, for management.

  • Induces metabolic cost associated with mitigating the inefficiency.

Bypasses in Photorespiration

  • Recent studies have identified engineered pathways that help in avoiding wasteful reactions associated with photorespiration, termed photorepiratory bypasses, which improve photosynthetic efficiency.

Structure of Peroxisomes

  • Characterized as single membrane-bound organelles involved in various metabolic processes, including photorespiration.

  • Bounded by rough endoplasmic reticulum (RER) and rich in reactive enzymes.

Detailed Stages of the Calvin Cycle

Stage 1

- RuBisCO's Role: Involves the fixation of carbon directly with RuBP, catalyzed by rubisco.

Produces three-phosphoglycerate which is phosphorylated to generate 1,3-bisphosphoglycerate using ATP.

Stage 2

  • Reductive Nature of Calvin Cycle

    • NADPH is utilized for the reduction of $3-PGA$ to $G3P$ through the enzyme Glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

    • Interconversion via Triose phosphate isomerase (TPI) yields dihydroxyacetone phosphate (DHAP).

    • Aldolase combines $G3P$ and $DHAP$ into fructose-1,6-bisphosphate, which is further processed to fructose-6-phosphate.

    • Enzyme involved in these reactions includes fructose-1,6-bisphosphatase and phosphoglucomutase.

Stage 3

  • Regeneration of RuBP

    • Fatly complex, requires 3 ATP molecules and involves transketolase and aldolase actions.

    • Transketolase transfers two carbon units between ketoses and aldoses.

    • Formation of ribulose-1,5-bisphosphate from a sequence of enzymatic reactions requiring ATP.

Resources Needed for Fixation

  • ATP and NADPH are predominantly consumed during the Calvin cycle (

    • $18$ ATP and $12$ NADPH$ are needed for the production of one glucose molecule as a result of fixing 6 $CO_2$ molecules).

Storage of Fixed Carbon

  • Fixed carbon can be stored in different forms:

    • Starch as long-term storage in chloroplasts using ADP-glucose.

    • Sucrose for carbohydrate transport and temporary storage, synthesized in cytoplasm using UDP-glucose.

Misconception About the Calvin Cycle

  • Often referred to as dark reactions, yet it relies on light conditions for proper functioning due to activation requirements of crucial enzymes by the ferridoxin-thyrotoxin system which operates only when light is present.

Enzyme Activation via Ferredoxin-Thyrotoxin System

  • Activation requires an alkaline environment in the stroma, elevated magnesium levels, and engagement of the ferredoxin-thyrotoxin system, all conditions optimal in light.

  • Key enzymes activated include:

    • Rubisco

    • Fructose-1,6-bisphosphatase

    • Phosphoribulokinase

Conclusion

  • Full understanding of the intricacies of each phase of the Calvin cycle, especially the collaboration between the different metabolic pathways and organelles, alongside the required resources, is vital for the comprehension of plant photosynthesis as a whole.