C4 carbon fixation
carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960s discovery by Marshall Davidson Hatch and Charles Roger Slack.
fixation is an addition to the ancestral and more common carbon fixation. The main carboxylating enzyme in photosynthesis is called RuBisCO, which catalyses two distinct reactions using either or oxygen as a substrate. RuBisCO oxygenation gives rise to phosphoglycolate, which is toxic and requires the expenditure of energy to recycle through photorespiration. photosynthesis reduces photorespiration by concentrating around RuBisCO.
To enable RuBisCO to work in a cellular environment where there is a lot of carbon dioxide and very little oxygen, the leaves of plants generally contain two partially isolated compartments called mesophyll cells and bundle-sheath cells. is initially fixed in the mesophyll cells in a reaction catalysed by the enzyme PEP carboxylase in which the three-carbon phosphoenolpyruvate reacts with to form the four-carbon oxaloacetic acid. OAA can then be reduced to malate or transaminated to aspartate. These intermediates diffuse to the bundle sheath cells, where they are decarboxylated, creating a -rich environment around RuBisCO and thereby suppressing photorespiration. The resulting pyruvate, together with about half of the phosphoglycerate produced by RuBisCO, diffuses back to the mesophyll. PGA is then chemically reduced and diffuses back to the bundle sheath to complete the reductive pentose phosphate cycle. This exchange of metabolites is essential for photosynthesis to work.
Additional biochemical steps require more energy in the form of ATP to regenerate PEP, but concentrating allows high rates of photosynthesis at higher temperatures. Higher CO2 concentration overcomes the reduction of gas solubility with temperature. The concentrating mechanism also maintains high gradients of concentration across the stomatal pores. This means that plants have generally lower stomatal conductance, reduced water losses and have generally higher water-use efficiency. plants are also more efficient in using nitrogen, since PEP carboxylase is cheaper to make than RuBisCO. However, since the pathway does not require extra energy for the regeneration of PEP, it is more efficient in conditions where photorespiration is limited, typically at low temperatures and in the shade.
Discovery
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. While Hatch and Slack originally referred to the pathway as the "C4 dicarboxylic acid pathway", it is sometimes called the Hatch–Slack pathway.Anatomy
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. Mesophyll and bundle sheath cells are connected through numerous cytoplasmic sleeves called plasmodesmata whose permeability at leaf level is called bundle sheath conductance. A layer of suberin is often deposed at the level of the middle lamella in order to reduce the apoplastic diffusion of . 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 are separated from decarboxylase enzymes and RuBisCO in the chloroplasts. A diffusive barrier is 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. Much leakage of from around RuBisCO occurs.
There is also evidence 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.
Biochemistry
In plants, the first step in the light-independent reactions of photosynthesis is the fixation of by the enzyme RuBisCO to form 3-phosphoglycerate. However, RuBisCo has a dual carboxylase and oxygenase activity. Oxygenation results in part of the substrate being oxidized rather than carboxylated, resulting in loss of substrate and consumption of energy, in what is known as photorespiration. Oxygenation and carboxylation are competitive, meaning that the rate of the reactions depends on the relative concentration of oxygen and.In order to reduce the rate of photorespiration, plants increase the concentration of around RuBisCO. Often, to facilitate this, two partially isolated compartments differentiate within leaves: the mesophyll and the bundle sheath. Instead of direct fixation by RuBisCO, is initially incorporated into a four-carbon organic acid in the mesophyll. The organic acids then diffuse through plasmodesmata into the bundle sheath cells. There, they are decarboxylated creating a -rich environment. The chloroplasts of the bundle sheath cells convert this into carbohydrates by the conventional pathway.
There is large variability in the biochemical features of C4 assimilation, and it is generally grouped in three subtypes, differentiated by the main enzyme used for decarboxylation. Since PEPCK is often recruited atop NADP-ME or NAD-ME it was proposed to classify the biochemical variability in two subtypes. For instance, maize and sugarcane use a combination of NADP-ME and PEPCK, millet uses preferentially NAD-ME and Megathyrsus maximus, uses preferentially PEPCK.
NADP-ME
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 carboxylation of PEP by the PEP carboxylase enzyme producing oxaloacetate. Both of these steps occur in the mesophyll cells:PEPC has a low KM for — and, hence, high affinity, and is not confounded by O2 thus it will work even at low concentrations of.
The product is usually converted to malate, which diffuses to the bundle-sheath cells surrounding a nearby vein. Here, it is decarboxylated by the NADP-malic enzyme to produce and pyruvate. The is fixed by RuBisCo to produce phosphoglycerate while the pyruvate is transported back to the mesophyll cell, together with about half of the phosphoglycerate. This PGA is chemically reduced in the mesophyll and diffuses back to the bundle sheath where it enters the conversion phase of the Calvin cycle. For each molecule exported to the bundle sheath the malate shuttle transfers two electrons, and therefore reduces the demand of reducing power in the bundle sheath.