20.5: CO2 uptake - C4 and CAM Pathways

Search Fundamentals of Biochemistry

Learning Goals (ChatGPT o3-mini)

Below are a series of learning goals tailored for junior and senior biochemistry majors based on the provided information about the C4 pathway and associated topics:

• Distinguish C3 and C4 Photosynthetic Pathways:
  • Explain how photorespiration in C3 plants, driven by Rubisco’s competing oxygenase activity and the glycolate salvage (C2) pathway, reduces photosynthetic efficiency.
  • Describe how the C4 pathway circumvents these inefficiencies by initially capturing CO₂ as bicarbonate (HCO₃⁻) and converting it into a 4-carbon (4C) compound prior to RuBisCo fixation.

• Outline the Initial Steps of the C4 Pathway:
  • Describe the role of phosphoenolpyruvate (PEP) carboxylase in mesophyll cells in converting HCO₃⁻ and PEP into oxaloacetate (OAA), a 4C intermediate.
  • Explain subsequent reactions where OAA is converted either to malate (via malate dehydrogenase) or aspartate (via transamination) and how these intermediates transport CO₂ to bundle sheath cells.

• Examine the Role of Malate Decarboxylation and CO₂ Release:
  • Discuss how, in the bundle sheath cells, malate is decarboxylated by NADP-malic enzyme to release CO₂ for fixation by RuBisCo, thereby locally saturating RuBisCo with CO₂ and suppressing its oxygenase activity.

• Understand the Function and Mechanism of Pyruvate Phosphate Dikinase (PPDK):
  • Describe the reaction catalyzed by PPDK, including its net reaction, formation of high-energy phosphorylated intermediates, and the overall cost of ATP (equivalent to two ATPs per CO₂ fixed in the C4 pathway).
  • Analyze the domain organization of PPDK (nucleotide-binding, central catalytic, and PEP/pyruvate binding domains) and explain how conformational “swiveling” enables sequential phosphotransfer events.

• Compare Energy Costs and Thermodynamic Trade-offs:
  • Contrast the ATP requirements for fixing CO₂ in the C4 pathway (approximately 5 ATPs per CO₂ fixed) with those in the C3 pathway (approximately 3 ATPs per CO₂ fixed).
  • Discuss how temperature influences Rubisco affinity for CO₂ and the relative efficiencies of C3 versus C4 pathways.

• Explore the Role of Carbonic Anhydrases:
  • Describe the function and distribution of the three classes of carbonic anhydrase (α, β, and γ) in plants, emphasizing their role in interconverting CO₂ and HCO₃⁻ in different cellular compartments (chloroplast, cytosol, mitochondria) and in the CO₂-concentrating mechanisms of C4 and CAM plants.

• Introduce Crassulacean Acid Metabolism (CAM):
  • Outline the CAM pathway, highlighting its adaptation to water-limited environments through nocturnal CO₂ capture and daytime decarboxylation of stored malic acid, and compare its metabolic organization to that of C3 and C4 plants.

• Discuss Plastid Diversity and Interconversion:
  • Explain the various types of plastids (chloroplasts, amyloplasts, etioplasts, chromoplasts) and the processes (e.g., “greening” and “de-greening”) by which plastids interconvert in response to environmental and developmental cues.

• Examine Rubisco Aggregation and Phase Separation Mechanisms:
  • Describe how the enzyme Rubisco, despite its slow turnover, is present in high concentrations and can undergo phase separation with intrinsically disordered linker proteins (such as EPYC1/LCI5 in algae or CsoS2 in carboxysomes) to enhance local CO₂ concentrations. • Understand the roles of protein-protein interactions, tandem repeats, and charge distribution in promoting phase separation and the formation of structures like pyrenoids and carboxysomes.

• Evaluate the Evolutionary and Ecological Implications:
  • Discuss the evolutionary history of Rubisco, including its adaptation from a high-CO₂, low-O₂ environment to current atmospheric conditions, and the strategies (e.g., C4 and CAM pathways) that have evolved to mitigate its inefficiencies.
  • Consider how rising atmospheric CO₂ and global warming might impact plant metabolism, photosynthetic efficiency, and ultimately food security through changes in nutrient composition.

• Relate Plant Photosynthetic Efficiency to Thermodynamic Models:
  • Compare the thermodynamic efficiencies of C3/C4 photosynthesis with energy conversion in internal combustion engines (e.g., the Otto cycle), highlighting how plants use concentration and storage mechanisms to optimize carbon fixation under varying environmental conditions.

These learning goals are designed to help students integrate structural, mechanistic, and regulatory aspects of the C4 pathway and related carbon fixation processes, as well as appreciate their significance in plant physiology and global carbon cycling.

The source for the organization and some of the text derives from Sindayigaya and Longhini. https://www.peoi.org/Courses/Courses...chem/biochem18 CC - https://creativecommons.org/licenses...sa/3.0/deed.en

Everything in the chapter is related to climate change.  For more general information on biochemistry and climate change, visit Chapter 32 Biochemistry and Climate Change. For more details on carbon fixation or capture, visit Chapter 32.16: Part 4 - Fixing Carbon Fixation.

 

The C4 Pathway

Photorespiration, caused by the oxygenase activity of RuBisCo/Oxygenase and the ensuing glycolate salvage (C2) pathway, significantly diminishes the efficiency of photosynthesis in C3 plants. Rest assured, some plants have found a way around the problem by adding a few steps before RuBisCo. The altered pathway is known as the C4 pathway, and plants that utilize it are referred to as C4 plants. The initial steps involve the temporary capture of CO₂ in the form of HCO₃^-, into a 4C, not 3C sugar. Plants that utilize the C4 pathway include maize, sorghum, sugarcane, and many tropical plants. An overview of the pathways emphasizing the steps that precede RuBisCo is shown in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Carbon assimilation in C4 plant

This extra step (compared to C3 plants) concentrates CO₂ and effectively decrease the competing substrate O₂ to increase the efficiency of Rubisco.

Experimental evidence shows that radiolabeled ¹⁴CO₂ is captured into the 4C molecule, oxaloacetate (OAA), a citric acid cycle and gluconeogenic intermediate, through the enzyme phosphoenolpyruvate carboxylase, which uses HCO₃^- as a substrate. The reaction takes place in mesophyll cells. OAA can be reduced by NADPH using malate dehydrogenase or converted to aspartate through a transamination (not shown in the figure). Malate moves into the bundle sheath cell.  It is then decarboxylated by the NADP-malic enzyme to pyruvate. Pyruvate can move back into the mesophyll cell and be converted to phosphoenolpyruvate (PEP) and then back to OAA by the pyruvate phosphate dikinase and PEP carboxylase. CO₂ from the decarboxylation of malate is delivered as a substrate to RuBisCo.

Pyruvate dikinase is used in bacteria, protozoa, C4 plants, and another type, Crassulacean, discussed below. Its non-plant function is to produce ATP, similar to pyruvate kinase. The net reaction is:

ATP + phosphate + pyruvate = AMP + PP_i + H⁺ + phosphoenolpyruvate

Figure \(\PageIndex{2}\) shows a simplified mechanism for the reaction

Figure \(\PageIndex{2}\): Mechanism of pyruvate phosphate dikinase

The net reaction shows that in C4 plants, two molecules are phosphorylated by ATP. One is pyruvate, and the other is inorganic phosphate (Pi). Hence the name dikinase. The reverse reaction of ATP synthesis occurs in bacteria and protozoans. There are two phosphorylated intermediates, an Enz-His-P and an Enz-His-PP_i. These are "activated" phosphate carriers in the phosphotransfer reactions to pyruvate and P_i, respectively. If PP_i is not hydrolyzed to 2P_i, as illustrated in the top left of Figure 2 above, the reaction is fully reversible.

In C4 plants, there are three reactions

1. Pyr + E-His-P ↔ PEP + E-His
2. E-His + ATP ↔ E-His-PP ^.AMP (^. indicates a noncovalent interaction)
3. E-His-PP ^.AMP + P_i ↔ E-His-P + AMP + PP_i

When PP_i is hydrolyzed, the net input of ATP to phosphorylated pyruvate is two ATP equivalents.

In the next step, PEP is carboxylated in a carbon capture reaction by PEP carboxylase, which, as mentioned above, uses HCO₃^- as a substrate, not CO₂ per se. PEP carboxylase also lacks competing oxidase activity. The product is malate, which releases locally high "saturating" concentrations of CO₂ in the bundle sheath cells, significantly suppressing the oxygenase activity of RuBisCo.

Pyruvate phosphate dikinase undergoes a very large conformational change in domain organization during the catalytic cycle. The two kinase activities are located at different sites in the enzyme. The phosphorylation of Pi occurs in the N-terminal domain, while the phosphorylation of pyruvate occurs in the C-terminal domain. The central domain, which links to the N- and C-terminal domains via associated "tethers," is the site of the catalytic histidine involved in phosphotransfer. A swiveling of domains occurs to allow sequential phosphotransfers.

Figure \(\PageIndex{3}\) shows the Pfam domain structure for the protein.

Figure \(\PageIndex{3}\): The green represents the AMP/ATP nucleotide-binding domain (NBD), the blue the PEP/pyruvate binding domain (PBD), and the red the central domain which catalyzes the phosphotransfer from ATP to pyruvate and from PEP to AMP through phosphorylated histidine intermediates. The NBD has three subdomains.

Figure \(\PageIndex{4}\) shows an interactive iCn3D model of an AlphaFold-predicted model of chloroplast pyruvate, phosphate dikinase from Flaveria brownii (Brown's yellowtops) (Q39734)

Figure \(\PageIndex{4}\): AlphaFold-predicted model of chloroplast pyruvate, phosphate dikinase from Flaveria brownii (Brown's yellowtops) (Q39734). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...VB6Aywuc2bLD38

The green represents the N-terminal NBD, the red is the central domain with the catalytic His (side chain shown as CPK-colored spheres and labeled), and the blue is the PEP/Pyr PBD.

Figure \(\PageIndex{5}\) illustrates the conformation changes that occur in the central domain (yellow in this figure) and the N-terminal nucleotide-binding domain (NBD, green) in various ligand-bound states.

Figure \(\PageIndex{5}\): Movement of the PPDK domains

Panel a–c show movement of the central domain (yellow). In (a), it swivels to face the PBD domain (PDB 5JVL/C), and in (c), it faces the NBD domain (TbPPDK (PDB 2X0S)). In (b) it is in an intermediate position.

Panels d-f show the movement of the NBD (three greens depict the three subdomains). Pane (d) is the state without bound nucleotide (PDB 5JVJ/A), while panel (f) shows the fully closed, nucleotide-bound state (PDB 5JVL/A). Panel (e) shows a semi-closed, nucleotide-bound state (PDB 5JVL/C). Minges, A., Ciupka, D., Winkler, C. et al. Structural intermediates and directionality of the swiveling motion of Pyruvate Phosphate Dikinase. Sci Rep 7, 45389 (2017). https://doi.org/10.1038/srep45389. Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/

Figure \(\PageIndex{6}\) shows the interactions of bound substrates in different conformation states of PPDK.

Figure \(\PageIndex{6}\): Substrate binding sites of FtPPDK.

Panel (a) shows the semi-closed state of the PEP binding site (PDB 5JVL/A) with the catalytic H456 (yellow) pointing away from PEP.

Panel (b) shows the closed state of the PEP binding site (PDB 5JVL/C), showing interactions between PEP and surrounding residues, including the catalytic H456 (yellow).

Panel (c) shows the closed state of the nucleotide-binding site of 5JVL/D occupied with 2'-Br-dAppNHp, a nonhydrolyzable ATP analog. Interacting residues are highlighted. Minges, A et al. ibid.

After CO₂ is delivered from malate in the bundle sheath cell using RuBisCo, the remaining reactions are the same as in the C3 pathways.

Once CO₂ is fixed into 3-phosphoglycerate in the bundle-sheath cells, the Calvin cycle's other reactions occur exactly as described earlier. Overall, the C4 pathways require more ATP. A molecule of PEP is required for each CO₂ fixed in the C4 pathway, which takes the equivalent of two ATPs. So, each CO₂ fixed in the C4 pathway takes five ATPs compared to three ATPs in the C3 pathway. As mentioned above, the affinity (estimated from the KM) of CO2 for RuBisCo decreases with increasing temperature, thereby decreasing the energetic efficiency of carbon capture. At higher temperatures (28-30 °C), the extra energy cost for the C4 pathway balances out the extra energy cost for the C3 pathway at higher temperatures.

Carbonic Anhydrases

We first encountered carbonic anhydrase when we discussed its mechanism in Chapter x.xx. We'll now discuss its function and activity in the C4 pathway in detail. Given that we are facing a climate crisis due to the increasing levels of CO₂ in the atmosphere arising from the burning of fossil fuels, removing CO2 from the atmosphere —a process called carbon sequestration for climate purposes —becomes even more important. Understanding the role of carbonic anhydrase in CO2 sequestration also becomes crucial.

Plants have three genes for carbonic anhydrase (α, β, and γ). Each can be differentially spliced, resulting in plants having many different isoforms of this protein. They are most abundant in the chloroplast, cytoplasm, and mitochondria, and they have many additional roles outside of fixing CO₂ in C4 (and CAM) plants. In autotrophic (make their own "food") bacteria (such as cyanobacteria, also known as blue-green algae), there are no internal organelles. However, there are carboxysomes, which are protein-bounded vesicles (much like a bacteriophage head), which contain not nucleic acid but RuBisCo and carbonic anhydrase in their internal compartment. The carbonic anhydrase converts HCO₃^- to CO₂ for reaction with RuBisCo. The carboxysome thus concentrates the CO₂-producing and fixing enzymes for photosynthesis.

Figure \(\PageIndex{7}\):

F

Figure \(\PageIndex{7}\): Electron Micrograph of a carboxysome: (A) A thin-section electron micrograph of H. neapolitanus cells with carboxysomes inside. In one of the cells shown, arrows highlight the visible carboxysomes. (B) A negatively stained image of intact carboxysomes isolated from H. neapolitanus. The features visualized arise from the distribution of stains around proteins that form the shell, as well as around the RuBisCO molecules that fill the carboxysome interior. Scale bars indicate 100 nm. BioLibreText

In C3 plants, CO₂ (aq), which is dissolved CO2, is the actual substrate for RuBisCo, so available HCO3—is converted to CO2 by carbonic anhydrase. In C4 and CAM plants, CO2 (aq) is first converted to bicarbonate by carbonic anhydrase. HCO3- (aq) is then used as the actual substrate for the "carbon fixation" step. Hence, carbonic anhydrase has roles in C3, C4, and CAM plants.

All of the carbonic anhydrases have a Zn⁺² at the active site. The most prominent alpha form in plants was discovered in erythrocytes and is typically active as a monomer. It has one large 10-strand beta sheet surrounded by seven alpha helices. Three histidine side chains and water form a tetrahedral coordination around the Zn²⁺ ion. Gamma carbonic anhydrase is a trimer with three active sites at the interface between pairwise monomers with the Zn-coordinating histidine side chains from two different subunits.

Beta carbonic anhydrase in plants is typically an octamer of identical subunits. Two cysteines, one histidine, and water coordinate the Zn ion. The monomer has four beta strands in a beta-sheet surrounded by alpha helices. An additional beta-strand is involved in monomer interactions. As the active site is located at the interface of two subunits, the functional biological unit is the dimer; however, a tetramer and even an octamer are typically formed. The substrate binding groups have a one-to-one correspondence with the functional groups in the alpha-carbonic anhydrase active site, with the corresponding residues being closely superimposable by a mirror plane of symmetry. Therefore, despite differing folds, alpha- and beta-carbonic anhydrases have converged upon a similar active site design and are likely to share a common mechanism.

Figure \(\PageIndex{8}\) shows an interactive iCn3D model of beta-carbonic anhydrase from Pisum sativum (pea) with bound acetate (1EKJ).

Figure \(\PageIndex{8}\): Beta-carbonic anhydrase from Pisum sativum (pea) with bound acetate (1EKJ). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...VZTEJPy1hPxK27

For the sake of simplicity, only two subunits (A - gray and B - magenta) of the octamer are shown. Acetate, a proxy for HCO₃^-2, is shown in spacefill binding between the two subunits in CPK-colored spheres. The side chains involved in Zn²⁺ binding, Cys 160, His 220, and Cys 223 (unlabeled), are on the A chain. The amino acids that bind acetate are distributed between the A chain (Asp 162, Gly 224, Val 184) and the B chain (Gln 151, Phe 169, and Tyr 205) and are labeled. The same groups are involved in substrate binding in alpha carbonic anhydrase, but in a mirror image orientation (with the normal L-amino acids).

Beta carbonic anhydrase is highly expressed in leaves and is found in chloroplasts, mitochondria, and the cytoplasm. Many plants have it in the cytoplasm and chloroplasts.

• β-carbonic anhydrase (βCA) in C3 plants: Most of the βCA in leaves is in chloroplasts in mesophyll cells and may comprise up to 2% of leaf protein. Yet studies have shown that you can delete the gene for it with minimal effect on the maximal rate of photosynthesis. However, by interference, plant development was affected, so the enzyme is probably most needed to produce HCO₃^- for biosynthesis.
• β-carbonic anhydrase (βCA) in C4 plants: Most of the βCA is found in the cytoplasm of mesophyll cells. There, it catalyzes the first reaction of the C4 pathway, CO₂ (aq) to HCO₃^- (aq). Mitochondrial (βCA) and γCA probably function to fix CO₂ arising from oxidative respiration.

Crassulacean acid metabolism (CAM) pathway

Plants that encounter the chronic stress of low water availability have evolved another pathway to adapt to these conditions. Stomata in C3 plants are open during the day to allow carbon capture from CO₂, but they can close when water is limited. This obviously will inhibit plant growth. In the CAM pathway, the stomata remain open at night, allowing for carbon capture when water loss through the stomata is lower. The incoming CO₂ is fixed through carbonic anhydrase and then a series of several enzymes to form malic acid, which is transported for storage and use during the light in vacuoles. This CAM pathway is described in Figure \(\PageIndex{9}\).

Figure \(\PageIndex{9}\): A simplified diagram of the Crassulacean acid metabolism (CAM) pathway under day and night conditions. Heyduk Karolina, et al. Frontiers in Plant Science.2019. https://www.frontiersin.org/article/...pls.2018.02000. DOI=10.3389/fpls.2018.02000. Creative Commons Attribution License (CC BY).

The proteins and intermediates in the CAM pathways are ALMT9, aluminum-activated malate transporter; CA, carbonic anhydrase; MDH, malate dehydrogenase; OAA, oxaloacetate; ME, malic enzyme (NAD or NADP); P, phosphate; PEPC, phosphoenolpyruvate carboxylase; PEPCK, PEP carboxykinase; PPCK, PEPC kinase; PPDK, pyruvate, phosphate dikinase.

During the day, malic acid moves back into the cytoplasm, where it is decarboxylated by malic enzyme, releasing locally high CO₂ concentrations for use by RuBisCo in the C3 cycle. In plants that use CAM, other changes occur, including modifications to leaf structure and the activation of additional regulatory processes that coordinate metabolic gene expression.

Figure \(\PageIndex{10}\)s shows more details of the CAM cycle.

Figure \(\PageIndex{10}\): Details of the CAM cycle

Thermodynamic comparison between the C3/C4 pathways and the Otto cycle

It is interesting to compare the thermodynamic efficiencies of plants to that of the internal combustion engines, which is governed by the thermodynamic Otto cycle. In the cycle, chemical energy in the form of gasoline and O₂ is converted to thermal energy, which is then converted into mechanical energy. Both photosynthesis (the conversion of the energy of photons into chemical energy) and the Otto cyclehave limited efficiencies.

• In internal combustion engines, power is partly limited by air uptake and different efficiencies when running at non-constant speeds (like stop-and-go).
• In the C3 pathway, efficiency is limited by the oxygenase activity of RuBisCo/Oxygenase (given the much higher concentration of atmospheric O₂ compared to CO₂) and the ensuing photorespiration pathway.
• Water availability also plays a role, as the net reaction of photosynthesis and glucose production, in simplified form, is 6CO₂ + 6H₂O → 6C(H₂O₆), so in high heat and low humidity, the process efficiency decreases.

Figure \(\PageIndex{11}\) shows a comparison of three stages, the storage component, the basic cycle, and a concentration mechanism, in photosynthesis and the internal combustion engine (ICE).

Figure \(\PageIndex{11}\): A comparison of plant photosynthesis and car engine functioning illustrates how the core processes interact with the additional components. Hartzell S, Bartlett M, Yin J, Porporato A (2018). Similarities in the evolution of plants and cars. PLoS ONE 13(6): e0198044. https://doi.org/10.1371/journal.pone.0198044. Creative Commons Attribution License,

A concentrating mechanism in C4 plants and turbocharged cars provides concentrated CO₂ and oxygen, respectively, to the core cycle (upper row). A storage mechanism in CAM plants allows carbon dioxide to be stored as malic acid at night and then passed to the Calvin cycle during the day, while a storage mechanism in hybrid electric vehicles (HEVs) allows energy to be stored in the battery during braking and then passed to the motor to power the drivetrain in parallel with the engine (bottom row).

Summary

This chapter examines how certain plants overcome the inefficiencies of photorespiration inherent in the C3 pathway by employing the C4 pathway and related adaptations. In C3 plants, Rubisco’s dual carboxylase/oxygenase activity leads to significant energy losses due to photorespiration. The C4 pathway, utilized by plants such as maize, sorghum, and sugarcane, introduces additional steps before CO₂ fixation to concentrate CO₂ and suppress Rubisco’s oxygenase activity.

Key Elements of the C4 Pathway:

• Initial CO₂ Capture:
  In mesophyll cells, atmospheric CO₂ is first converted to bicarbonate (HCO₃⁻) and then rapidly fixed by phosphoenolpyruvate (PEP) carboxylase to form a 4-carbon compound, oxaloacetate (OAA). OAA may be reduced to malate or converted to aspartate, which then transports the captured carbon to the bundle sheath cells.

• CO₂ Release and Concentration:
  Within bundle sheath cells, malate is decarboxylated by NADP-malic enzyme, releasing CO₂ near Rubisco. This localized, high concentration of CO₂ effectively minimizes Rubisco’s competing oxygenase activity and enhances carboxylation efficiency.

• Role of Pyruvate Phosphate Dikinase (PPDK):
  PPDK regenerates PEP from pyruvate in the mesophyll cells. This enzyme undergoes significant conformational changes during its catalytic cycle, transferring high-energy phosphate groups via phosphorylated histidine intermediates. The process consumes two ATP equivalents per CO₂ fixed, which, while energetically more demanding than the C3 cycle, is offset by improved carbon fixation efficiency under conditions where photorespiration would otherwise dominate.

Associated Enzymes and Plastid Adaptations:

• Carbonic Anhydrases:
  These enzymes (α, β, and γ forms) facilitate the rapid interconversion between CO₂ and HCO₃⁻, ensuring efficient substrate availability for PEP carboxylase in C4 and CAM plants. In C3 plants, carbonic anhydrases help generate CO₂ from dissolved bicarbonate for direct fixation by Rubisco.

• Crassulacean Acid Metabolism (CAM):
  As an adaptation to water-limited conditions, CAM plants open their stomata at night to fix CO₂ into malic acid, which is stored in vacuoles. During the day, malic acid is decarboxylated to release CO₂ for the Calvin cycle, reducing water loss while maintaining photosynthetic activity.

• Plastid Interconversion:
  The dynamic nature of plastids, such as the conversion between chloroplasts and amyloplasts, supports the diverse metabolic needs of the plant. These transitions are regulated by light and developmental signals, optimizing both photosynthesis and storage functions.

Integration with Overall Plant Metabolism and Global Impact:

• Energy and Carbon Efficiency:
  The C4 pathway requires additional ATP (approximately 5 ATP per CO₂ fixed compared to 3 ATP in the C3 cycle) due to the extra steps in carbon fixation. However, under high-temperature and high-light conditions, the enhanced CO₂ concentration at Rubisco’s active site improves overall photosynthetic efficiency, making the C4 pathway advantageous in tropical and arid environments.

• Evolutionary Adaptation:
  The evolution of the C4 pathway represents a critical adaptation to declining atmospheric CO₂ and increasing O₂ levels over geological time, addressing the inherent limitations of Rubisco’s carboxylase/oxygenase activity in C3 plants.

• Implications for Global Climate and Food Security:
  Changes in atmospheric CO₂ levels and global warming can affect the balance between CO₂ fixation and photorespiration. Although increased CO₂ may boost carbon assimilation in some contexts, rising temperatures and altered water availability can reduce nutrient content in crops, impacting food security worldwide.

In summary, this chapter details the biochemical and structural adaptations that enable C4 plants—and to some extent CAM plants—to more efficiently fix CO₂ and overcome the limitations of the C3 pathway. It integrates discussions of enzyme mechanisms (PEP carboxylase, PPDK, carbonic anhydrases), plastid dynamics, and the evolutionary and ecological implications of these pathways, providing a comprehensive view of advanced carbon fixation strategies in plants.