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C4 carbon fixation is one of three biochemical processes, along with and CAM photosynthesis, used to fix carbon. It is named for the 4-carbon molecule present in the first product of carbon fixation in the small subset of plants that use that process, in contrast to the 3-carbon molecule products in plants. fixation is an elaboration of the more common carbon fixation and is believed to have evolved more recently. and CAM overcome the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in the process of photorespiration. This is achieved in a more efficient environment for RubisCo by shuttling via malate or aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells, RuBisCO is isolated from atmospheric oxygen and saturated with the released by decarboxylation of the malate or oxaloacetate. These additional steps, however, require more energy in the form of ATP. Because of this extra energy requirement, plants are able to more efficiently fix carbon in only certain conditions, with the more common pathway being more efficient in other conditions. == pathway== The first experiments indicating that some plants do not use C3 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 P. Kortschak and Yuri Karpilov. The pathway was elucidated by Marshall Davidson Hatch and C. R. Slack, in Australia, in 1966; it is sometimes called the Hatch-Slack 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 utilize 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 to RuBisCO in the Calvin cycle, is incorporated into a 4-carbon organic acid, which has the ability to regenerate in the chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilize this to generate carbohydrates by the conventional pathway. The first step in the pathway is the conversion of pyruvate to phosphoenolpyruvate (PEP), by the enzyme pyruvate orthophosphate dikinase. This reaction requires inorganic phosphate and ATP plus pyruvate, producing phosphoenolpyruvate, AMP, and inorganic pyrophosphate (PPi). The next step is the fixation of into oxaloacetate by the enzyme PEP carboxylase. Both of these steps occur in the mesophyll cells: :pyruvate + Pi + ATP → PEP + AMP + PPi :PEP + CO2 → oxaloacetate PEP carboxylase 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 to produce and pyruvate. The now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell. Since every molecule has to be fixed twice, first by 4-carbon organic acid and second by RuBisCO, the pathway uses more energy than the pathway. The pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the pathway 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. There are several variants of this pathway: #The 4-carbon acid transported from mesophyll cells may be malate, as above, or aspartate #The 3-carbon acid transported back from bundle-sheath cells may be pyruvate, as above, or alanine #The enzyme that 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「C4 carbon fixation」の詳細全文を読む スポンサード リンク
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