C4 carbon fixation


carbon fixation or the Hatch–Slack pathway is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules. It is one of three known processes for carbon fixation. refers to the four-carbon molecule that is the first product of this type of carbon fixation.
fixation is an elaboration of the more common carbon fixation and is believed to have evolved more recently. overcomes the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in the process of photorespiration. This is achieved by ensuring that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen. is shuttled via malate or aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells is released by decarboxylation of the malate. plants use PEP carboxylase to capture more in the mesophyll cells. PEP binds to to make oxaloacetic acid. OAA then makes malate or aspartate, which is transported into the bundle sheath cells, where it releases the. These additional steps, however, require more energy in the form of ATP. Using this extra energy, plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or. Since the more common pathway does not require this extra energy, it is more efficient in the other conditions.
The naming Hatch–Slack pathway is in honor of Marshall Davidson Hatch and Charles Roger Slack, who elucidated it in Australia in 1966.

Discovery and resolution of the mechanism

The first experiments indicating that some plants do not use carbon fixation but instead produce malate and aspartate in the first step of carbon fixation were done in the 1950s and early 1960s by Hugo Peter Kortschak and Yuri Karpilov. The pathway was elucidated by Marshall Davidson Hatch and Charles Roger Slack, in Australia, in 1966; it is sometimes called the Hatch–Slack pathway.

pathway

In plants, the first step in the light-independent reactions of photosynthesis involves the fixation of by the enzyme RuBisCO into 3-phosphoglycerate. However, due to the dual carboxylase and oxygenase activity of RuBisCo, some part 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, plants have developed a mechanism to efficiently deliver to the RuBisCO enzyme. They use their specific leaf anatomy where chloroplasts exist not only in the mesophyll cells in the outer part of their leaves but in the bundle sheath cells as well. Instead of direct fixation by RuBisCO into the three-carbon compound 3-phosphoglycerate, is incorporated into a four-carbon organic acid. The organic acid is produced in the mesophyll cells and then transported through plasmodesmata into the bundle sheath cells, where it is decarboxylated to regenerate. The chloroplasts of the bundle sheath cells can then use this to produce carbohydrates by the conventional pathway.
There are several variants of the pathways, all of which use a four-carbon organic acid to transport from the mesophyll cells to the bundle sheath cells. They achieve this purpose by using different substrates and enzymes. Three examples are provided in the figures on the right. The NADP-ME type pathway is described in detail below.

NADP-ME type

The first step in the NADP-ME type pathway is the conversion of pyruvate to phosphoenolpyruvate, by the enzyme Pyruvate phosphate dikinase. This reaction requires inorganic phosphate and ATP plus pyruvate, producing PEP, AMP, and inorganic pyrophosphate. The next step is the fixation of into oxaloacetate by the PEP carboxylase enzyme. Both of these steps occur in the mesophyll cells:
PEPC has a lower Km for — and, hence, higher affinity — than RuBisCO. Furthermore, O2 is a very poor substrate for this enzyme. Thus, at relatively low concentrations of, most will be fixed by this pathway.
The product is usually converted to malate, a simple organic compound, which is transported to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated by the NADP-malic enzyme to produce and pyruvate. The now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell.

Other types

There are several variants of this pathway, in which different substrates and enzymes are used to transport from the mesophyll cells to the bundle sheath cells:
  1. The four-carbon acid transported from mesophyll cells may be malate, as above, or aspartate, or both.
  2. The three-carbon acid transported back from bundle-sheath cells may be pyruvate, as above, or alanine.
  3. The enzyme that catalyses decarboxylation in bundle-sheath cells differs:

    Efficiency

Since every molecule has to be fixed twice, first in the mesphyll cells and second in the bundle sheath cells, the pathway uses more energy than the pathway. For instance, the pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the NADP-ME type pathway used by maize and sugarcane requires 30 molecules of ATP. This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in photorespiration as occurs in some tropical plants, making it an adaptive mechanism for minimizing the loss.

Kranz leaf anatomy

The plants often possess a characteristic leaf anatomy called kranz anatomy, from the German word for wreath. Their vascular bundles are surrounded by two rings of cells; the inner ring, called bundle sheath cells, contains starch-rich chloroplasts lacking grana, which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. The primary function of kranz anatomy is to provide a site in which can be concentrated around RuBisCO, thereby avoiding photorespiration. In order to maintain a significantly higher concentration in the bundle sheath compared to the mesophyll, the boundary layer of the kranz has a low conductance to, a property that may be enhanced by the presence of suberin. The carbon concentration mechanism in plants distinguishes their isotopic signature from other photosynthetic organisms.
Although most plants exhibit kranz anatomy, there are, however, a few species that operate a limited cycle without any distinct bundle sheath tissue. Suaeda aralocaspica, Bienertia cycloptera, Bienertia sinuspersici and Bienertia kavirense are terrestrial plants that inhabit dry, salty depressions in the deserts of the Middle East. These plants have been shown to operate single-cell -concentrating mechanisms, which are unique among the known mechanisms. Although the cytology of both genera differs slightly, the basic principle is that fluid-filled vacuoles are employed to divide the cell into two 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 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 cycle to operate, it is relatively inefficient, with the occurrence of much leakage of from around RuBisCO. There is also evidence for the exhibiting of inducible photosynthesis by non-kranz aquatic macrophyte Hydrilla verticillata under warm conditions, although the mechanism by which leakage from around RuBisCO is minimised is currently uncertain.

Evolution and advantages

plants have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or limitation. When grown in the same environment, at 30 °C, grasses lose approximately 833 molecules of water per molecule that is fixed, whereas grasses lose only 277. This increased water use efficiency of grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.
carbon fixation has evolved on up to 61 independent occasions in 19 different families of plants, making it a prime example of convergent evolution. This convergence may have been facilitated by the fact that many potential evolutionary pathways to a phenotype exist, many of which involve initial evolutionary steps not directly related to photosynthesis. plants arose around during the Oligocene and did not become ecologically significant until around, in the Miocene. metabolism in grasses originated when their habitat migrated from the shady forest undercanopy to more open environments, where the high sunlight gave it an advantage over the pathway. Drought was not necessary for its innovation; rather, the increased resistance to water stress was a byproduct of the pathway and allowed plants to more readily colonize arid environments.
Today, plants represent about 5% of Earth's plant biomass and 3% of its known plant species. Despite this scarcity, they account for about 23% of terrestrial carbon fixation. Increasing the proportion of plants on earth could assist biosequestration of and represent an important climate change avoidance strategy. Present-day plants are concentrated in the tropics and subtropics where the high air temperature contributes to higher possible levels of oxygenase activity by RuBisCO, which increases rates of photorespiration in plants.

Plants that use carbon fixation

About 8,100 plant species use carbon fixation, which represents about 3% of all terrestrial species of plants. All these 8,100 species are angiosperms. carbon fixation is more common in monocots compared with dicots, with 40% of monocots using the pathway, compared with only 4.5% of dicots. Despite this, only three families of monocots use carbon fixation compared to 15 dicot families. Of the monocot clades containing plants, the grass species use the photosynthetic pathway most. 46% of grasses are and together account for 61% of species. has arisen independently in the grass family some twenty or more times, in various subfamilies, tribes, and genera, including the Andropogoneae tribe which contains the food crops maize, sugar cane, and sorghum. Various kinds of millet are also. Of the dicot clades containing species, the order Caryophyllales contains the most species. Of the families in the Caryophyllales, the Chenopodiaceae use carbon fixation the most, with 550 out of 1,400 species using it. About 250 of the 1,000 species of the related Amaranthaceae also use.
Members of the sedge family Cyperaceae, and members of numerous families of eudicots – including Asteraceae, Brassicaceae, and Euphorbiaceae – also use.
There are very few trees which use. Only a handful are known: Paulownia, seven Hawaiian Euphorbia species and a few desert shrubs that reach the size and shape of trees with age.

Converting plants to

Given the advantages of, a group of scientists from institutions around the world are working on the Rice Project to produce a strain of rice, naturally a plant, that uses the pathway by studying the plants maize and Brachypodium. As rice is the world's most important human food—it is the staple food for more than half the planet—having rice that is more efficient at converting sunlight into grain could have significant global benefits towards improving food security. The team claim rice could produce up to 50% more grain—and be able to do it with less water and nutrients.
The researchers have already identified genes needed for photosynthesis in rice and are now looking towards developing a prototype rice plant. In 2012, the Government of the United Kingdom along with the Bill & Melinda Gates Foundation provided US$14 million over three years towards the Rice Project at the International Rice Research Institute.