C4 carbon fixation

The pathway

The C4 pathway was discovered by M. D. Hatch and C. R. Slack, two Australian researchers, in 1966, so it is sometimes called the Hatch-Slack pathway.

In RuBisCo, an amount of the substrate is oxidized rather than carboxylated resulting in loss of substrate and consumption of energy, in what is known as photorespiration. In order to bypass the photorespiration pathway , C₄ plants have developed a mechanism to efficiently deliver CO₂ to the C3 pathway.

The first step in the pathway is the conversion of pyruvate to PEP by the enzyme pyruvate-phosphate dikinase (pyruvate, orthophosphate dikinase); this reaction requires inorganic phosphate and ATP plus pyruvate, giving phosphoenolpyruvate, AMP, and PPi (inorganic pyrophosphate) as products. The next step is the fixation of CO₂ by the enzyme phosphoenolpyruvate carboxylase. Both of these steps occur in the mesophyll cells:

pyruvate + Pi + ATP → PEP + AMP + PPi

PEP carboxylase + PEP + CO₂ → oxaloacetate

PEP carboxylase has a lower Km for CO₂—and hence higher affinity—than Rubisco. Furthermore, O₂ is a very poor substrate for this enzyme. Thus, at relatively low concentrations of CO₂, most CO₂ will be fixed by this pathway.

The product is usually converted to malate, a simple Calvin cycle. The decarboxylation leaves pyruvate, which is transported back to the mesophyll cell.

Since every CO₂ molecule has to be fixed twice, the C₄ pathway is more energy-consuming than the C₃ pathway. The C₃ pathway requires 18 ATP for the synthesis of one molecule of glucose while the C₄ pathway requires 30 ATP. But since otherwise tropical plants lose more than half of photosynthetic carbon in photorespiration, the C₄ pathway is an adaptive mechanism for minimizing the loss.

There are several variants of this pathway:

1. The 4-carbon acid transported from mesophyll cells may be malate as above, or may be aspartate.
2. The 3-carbon acid transported back from bundle-sheath cells may be pyruvate as above, or alanine.
3. The enzyme which catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzyme, in millet, it is NAD-malic enzyme, and in Panicum maximum it is PEP carboxykinase.

C4 Leaf Anatomy

The C4 plants possess a characteristic chloroplasts lacking grana which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. This peculiar anatomy is called Kranz Anatomy (Kranz-Crown/Halo). The primary function of the Kranz is to provide a site in which carbon dioxide can be concentrated around rubisco, thus reducing photorespiration. In order to facilitate the maintenance of a significantly higher carbon dioxide concentration in the bundle sheath compared to the mesophyll, the boundary layer of the Kranz has a low conductance to carbon dioxide, a property which may be enhanced by the presence of suberin.

Although most C4 plants exhibit Kranz anatomy, there are a number of species which operate a limited c4 cycle without any distinct bundle sheath tissue. Suaeda aralocaspica (formerly known as Borszczowia aralocaspica), Bienertia cycloptera and Bienertia sinuspersici (all chenopods) are terrestrial plants which inhabit dry, salty depressions in the deserts of south-east Asia. These plants have been shown to operate single-cell c4 carbon dioxide concentrating mechanisms which are unique amongst the known c4 mechanisms. Although the cytology of both species differ slightly, the basic principle is that fluid filled vacuoles are employed to divide the cell into to separate areas. Carboxylation enzymes in the cytosol can therefore be kept separate from decarboxylase enzymes and rubisco in the chloroplasts, and a diffusive barrier can be established between the chloroplasts (which contain rubisco) and the cytosol. This enables a bundle-sheath type area and a mesophyll type area to be established within a single cell. Although this does allow a limited c4 cycle to operate, it is relatively inefficient, with much leakage of CO2 from around rubisco occurring. There is also evidence for the non-Kranz aquatic macrophyte Hydrilla verticillata exhibiting inducible c4 photosynthesis under warm conditions, although the mechanism by which CO2 leakage from around rubisco is minimised is currently uncertain.

The Evolution and Advantages of the C4 Pathway

C4 plants have a competitive advantage over plants possessing the more common switchgrass. C4 plants arose during the Cenozoic Era and did not become common until the Miocene Period. Today they represent about 5% of Earth's plant biomass and 1% of its known plant species. These species are concentrated in the tropics where the high air temperature contributes to higher possible levels of oxygenase activity by Rubisco, which increases rates of photorespiration in C3 plants.

See also

• C3 carbon fixation
• CAM Photosynthesis