Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Photosynthetic Carbon Fixation
Competition between Photorespiration and Photosynthesis

Photosynthesis is invariably accompanied by Photorespiration, a light-dependent process that consumes O₂ and partially converts ribulose 1,5-bisphosphate into СО₂.

The enzyme rubisco (ribulose-1,5-bisphosphate carboxylase) catalyzes two competing reactions (Figure 190):

✵ in one of them (СО₂ fixation), СО₂ is added to ribulose-1,5-bisphosphate to yield two molecules of 3-phosphoglycerate,

✵ in the other (photorespiration), O₂ is added to ribulose-1,5-bisphosphate to yield one molecule of 3-phosphoglycerate and one two-carbon molecule of phosphoglycolate.

Photorespiration is costly for The Cell, as it consumes ATP and O₂ while synthesizing СО₂. Notably, however, all known rubisco Enzymes catalyze photorespiration.

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Figure 190 - Competition between carbon fixation (1) and photorespiration (2)

Apparently, the specific geometric and amino acid Features of the Active Site required for the СО₂ fixation reaction prevented the enzyme from evolving in a way that avoids catalyzing energy-costly photorespiratory reactions.

In hot, dry weather, plants close their gas-exchange pores (Stomata) on leaves to conserve moisture. This causes the internal СО₂ concentration in leaves to drop below the threshold required for rubisco activation; consequently, The rate of photosynthetic СО₂ fixation plummets, favoring photorespiration.

To avoid such metabolic "switching," plants adapted to hot and arid climates (such as corn, sugarcane, and African millet) have evolved a two-stage СО₂ fixation process in which СО₂ accumulation precedes The Calvin Cycle.

This metabolic pathway is known as the C₄-pathway, because its initial products are four-carbon compounds such as oxaloacetate and malate (rather than the three-carbon molecules that initiate the Calvin cycle). The C₄-pathway operates in two distinct types of plant Cells (Figure 191):

1) mesophyll cells, which are located near the leaf surface and can acquire СО₂ directly from the atmosphere,

2) bundle sheath cells, which surround the vascular tissue (xylem and phloem, through which biological fluids circulate within the plant).

Figure 191 - Leaf anatomy of C₄ plants

In the mesophyll cells of C₄ plants, phosphoenolpyruvate—a tricarboxylic molecule produced by Pyruvate phosphorylation—reacts with СО₂ to form oxaloacetate, a four-carbon compound (Figure 192). The enzyme phosphoenolpyruvate carboxylase, which catalyzes this reaction, is found almost exclusively in C₄ plants and, unlike rubisco, is insensitive to СО₂ concentration.

The overall reaction converting pyruvate to oxaloacetate involves the Hydrolysis of one phosphoanhydride bond in ATP and is accompanied by a decrease in Free energy (∆G < 0). As a result, СО₂ fixation proceeds efficiently even under very low СО₂ concentrations.

Oxaloacetate produced in mesophyll cells is reduced to malate, which is then transported by specialized carriers into the bundle sheath cells of C₄ plants, where СО₂ is released via decarboxylation and enters the Calvin cycle (Figure 192).

Figure 192 - Scheme of the C₄ carbon fixation pathway

Due to the supply of СО₂ from mesophyll cells, the intracellular СО₂ concentration in the bundle sheath cells of C₄ plants is much higher than in the surrounding atmosphere. Bundle sheath cells also differ in lacking Photosystem II (PSII) and relying exclusively on cyclic electron flow driven by Photosystem I (PSI), which does not require molecular oxygen (O₂).

The elevated СО₂ and low O₂ concentrations in the bundle sheath cells of C₄ plants promote СО₂ fixation by rubisco into 3-phosphoglycerate while inhibiting The Use of ribulose-1,5-bisphosphate in photorespiration.

In contrast, within the mesophyll cells of C₃ plants, high atmospheric O₂ concentrations favor photorespiration (pathway 2 in Figure 190). As a result, up to 50% of the carbon fixed by rubisco can be re-oxidized back into СО₂ in C₃ plants.

C₄ plants outperform C₃ plants in utilizing available СО₂ because the phosphoenolpyruvate carboxylase enzyme in C₄ plants has a higher affinity for СО₂ than rubisco does in the Calvin cycle of C₃ plants. However, the C₄ cycle consumes one phosphodiester bond of ATP (during The formation of phosphoenolpyruvate from pyruvate); consequently, the net yield of sugar photosynthesis utilizing NADPH and ATP is lower in C₄ plants than in C₃ plants, which rely solely on the Calvin cycle for СО₂ fixation.

Nevertheless, the average rate of photosynthesis in C₄ plants such as corn or sugarcane can be 2 to 3 times higher than in comparable C₃ plants such as wheat, rice, or oats, because C₄ plants lack the losses associated with photorespiration.

Self-check questions

1. What process is known as the Calvin cycle?

2. What are C₃ plants? How do they differ from C₄ plants?

3. What reaction does the rubisco enzyme catalyze?

4. Which enzyme of the Calvin cycle is considered the most abundant enzyme on Earth?

5. How many molecules of glyceraldehyde-3-phosphate are exported from the chloroplast stroma into the cell Cytosol as a result of fixing six CO₂ molecules in the Calvin cycle?

6. What two processes facilitate the increased transport of glyceraldehyde-3-phosphate from the chloroplast to the cytosol?

7. What mechanisms regulate The activity of Calvin cycle enzymes?

8. What process is referred to as photorespiration?

9. Why is photorespiration disadvantageous for the cell?

10. Which enzyme catalyzes both CO₂ fixation and photorespiration?

11. Why has rubisco not evolved to avoid catalyzing energy-wasting photorespiratory reactions?

12. What is the physiological Significance of the two-stage CO₂ fixation process in C₄ plants?

13. In which cell types does the C₄ pathway take place?

14. What are the fundamental differences between the C₄ and C₃ pathways?