Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Tropical plants utilize the C4 pathway, also known as the Hatch-Slack pathway.

In most tropical plants, as well as in temperate-zone crops of tropical origin (such as corn, sugarcane, or sorghum), CO₂ fixation occurs via a pathway known as the C₄ pathway or the Hatch-Slack pathway. It should be understood from the outset, however, that both C₃ and C₄ plants ultimately utilize the aforementioned C₃ pathway, which is examined in detail in Fig. 23-21. Yet, a fundamental difference exists between these two groups of plants. Specifically, in C₄ plants, the Reactions of the C₃ pathway are preceded by additional steps in which CO₂ is preliminarily fixed into a four-carbon compound before being incorporated into phosphoglycerate (Fig. 23-26). Let us now examine how the C₄ pathway Functions.

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Fig. 23-26. C₄ plants first incorporate CO₂ into a C₄ compound and only after two preliminary steps fix CO₂ via the same pathway as C₃ plants.

Fig. 23-27. A. The pathway of CO₂ fixation via intermediate four-carbon products (the Hatch-Slack pathway). This pathway predominates in plants of tropical origin. B. An electron micrograph showing interconnected mesophyll Cell (bottom) and bundle-sheath cell (top). The bundle-sheath cell contains starch grains.

In the 1960s, two Australian plant biochemists, M. Hatch and C. Slack, discovered that in plants of tropical origin, the initial product in which radioactive CO₂ is fixed is the four-carbon compound oxaloacetate. The reaction leading to CO₂ fixation occurs in the mesophyll Cells of the leaf (Fig. 23-27) and is catalyzed by phosphoenolpyruvate carboxylase.

Phosphoenolpyruvate + CO₂ →

→ Oxaloacetate + P_i.

This enzyme, which is absent in animal Tissues, should not be confused with phosphoenolpyruvate carboxykinase (Section 20.2), which catalyzes a reaction occurring during Gluconeogenesis in animals.

The oxaloacetate formed in the mesophyll cells is reduced at the expense of NADPH to yield malate.

Oxaloacetate + NADPH + H⁺ →

→ Malate + NADP⁺.

Next comes a step that is crucial for the C₄ cycle. Malate, produced in the mesophyll cells and containing the fixed CO₂, is transported into neighboring bundle-sheath cells via specialized connections, or channels, linking these two cell types. Within the bundle-sheath cells, malate is decarboxylated to yield Pyruvate and CO₂ through the action of malate dehydrogenase.

Malate + NADP⁺ → Pyruvate +

+ CO₂ + NADPH + H⁺.

The free CO₂ released in the bundle-sheath cells is the very same CO₂ that was initially fixed in the mesophyll as oxaloacetate.

In the bundle-sheath cells, the CO₂ released during malate decarboxylation is fixed once again—this time by the action of ribulose diphosphate carboxylase—in precisely the same reaction that leads to CO₂ fixation as the carboxyl group of 3-phosphoglycerate in C₃ plants. The pyruvate formed from malate decarboxylation in the bundle-sheath cells is transported back to the mesophyll cells and converted there into phosphoenolpyruvate via an unusual enzymatic reaction catalyzed by pyruvate-phosphate dikinase.

Pyruvate + P_i + ATP →

→ Phosphoenolpyruvate + AMP + PP_i.

This enzyme was named dikinase because it catalyzes a reaction in which two different molecules—pyruvate and phosphate—are simultaneously phosphorylated at the expense of a single ATP molecule; pyruvate is phosphorylated to yield phosphoenolpyruvate, and phosphate is phosphorylated to yield pyrophosphate.

Subsequently, this pyrophosphate is hydrolyzed to phosphate, so that ultimately two high-energy ATP bonds are consumed. This reaction thus ensures the regeneration of phosphoenolpyruvate, which can now be used to fix another CO₂ molecule in the mesophyll cells.

Once CO₂ fixation as 3-phosphoglycerate takes place in the bundle-sheath cells (following its preliminary fixation as malate in the mesophyll cells), all subsequent reactions of the C₃ cycle, or Calvin cycle, proceed exactly as shown in Figures 23-20 and 23-21. Thus, in C₄ plants, CO₂ fixation is carried out in the mesophyll cells via the C₄ pathway, whereas glucose synthesis takes place in the bundle-sheath cells via the C₃ pathway.

A second important aspect of CO₂ fixation in C₄ plants is that they expend more energy on this process than C₃ plants. For every molecule of CO₂ fixed via the C₄ pathway, one molecule of phosphoenolpyruvate must be regenerated. As shown above, this regeneration requires two high-energy ATP phosphate groups. Therefore, the fixation of a single CO₂ molecule requires a total of five ATP molecules in C₄ plants, whereas C₃ plants consume only three ATP molecules for the same process.