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C4 plants have evolved a mechanism to deliver CO2 to Rubisco
Rubisco has oxygenase activity as well as carboxylase activity; it sometimes fixes O2 to RuBP instead of CO2. The oxygenase activity occurs at low CO2, high O2 conditions, and becomes pronounced at high temperatures. As a result, organic carbon is oxidized, the opposite of photosynthesis, which reduces inorganic carbon to make organic carbon.
These processes are directly relevant to global climate change studies because C3 and C4 plants respond differently to changes in atmospheric carbon dioxide concentration and changes in temperature and water availability. Humans are currently relying on the type of plant that does not do well under warmer, dryer, and erratic conditions: we are going to have to find some way to adapt, and changing photosynthesis processes may be one way to do that.
Evolution of C4 Phosphoenolpyruvate Carboxylase. …
Phosphoenolpyruvate carboxylase was selected for this evolutionary analysis because the biochemistry and molecular biology of this enzyme have been studied intensively both in C4 and C3 plants (; ). PEPC catalyses the irreversible carboxylation of phosphoenolpyruvate (PEP) to form oxaloacetate. The enzyme needs Mg2+ as an essential cofactor and requires that the inorganic carbon be supplied as bicarbonate. Consequently, mesophyll cells of C4 plants contain high amounts of carbonic anhydrase to fulfil the demands of PEPC when the carbon flux through the C4 cycle is high (). Native PEPC is a tetramer comprising four identical subunits each with a molecular mass of about 100 000 Da (). PEPC activity is regulated by metabolites (), but also post‐translationally by phosphorylation ().
The photosynthetic and non‐photosynthetic PEPCs of the C4 plant F. trinervia are encoded by a small gene family which consists of three distinct classes, named ppcA, ppcB and ppcC (Fig. ). The ppcA gene class contains two members and encodes the C4 isoform of PEPC (; ). The ppcB and ppcC gene classes probably consist of only one gene each and code for non‐photosynthetic PEPC isoforms. The exact physiological roles of the ppcB and ppcC PEPCs have not been determined (). Phylogenetic analysis of cDNA sequences revealed that ppcA and ppcB genes are sister gene classes. This indicates that they were derived from an ancestral ppcB‐like gene by gene duplication (Fig. ; ). There may be a fourth gene class, named ppcD. However, the existence of this remains doubtful since neither genomic nor cDNA sequences have been analysed and no expression has been detected ().
what does the enzyme PEP carboxylase do? | Yahoo …
) proposed a molecular phylogeny of the genus Flaveria by using the H‐protein of the glycine cleavage system as a gene marker. Their findings indicate that the group with five to six phyllaries forms a separate clade, thus confirming Powell’s classification (). Two to three thousand base pairs of ppcA1 promoter sequences were isolated from the C4 plants F. trinervia and F. bidentis, the C4‐like species F. palmeri, F. vaginata and F. brownii, the C4–C3‐like species F. pubescens, F. floridana, F. anomala and F. chloraefolia, and the C3 plants F. pringlei and F. cronquistii. The overall structure of the promoters clearly separates the species with five to six phyllaries, i.e. F. brownii, F. pubescens, F. floridana and F. chloraefolia from the other Flaveria species analysed (U. Gowik and P. Westhoff, unpublished data). The 500 base pairs of proximal promoter sequences which can be aligned to each other without major gaps were used to construct a phylogenetic tree. The phylogram obtained supports the above classification and confirms that the species with five to six phyllaries form a distinct group (Fig. B). Based on these ppcA PEPC data one may propose that the evolution from C3 to C4 photosynthesis was initiated at least twice in this genus. However, to confirm this conclusion additional C4‐related genes should be investigated at the phylogenetic level.
Based on the number of phyllaries, ) divided the genus Flaveria into two major branches (Fig. A). The section with species that possess three to four phyllaries contains C3, C3–C4 and C4 species. Within this group are F. trinervia and F. pringlei, which serve as model C4 and C3 species of this genus, respectively. The three to four phyllaries branch also contains F. bidentis which is the only C4 flaveria that is amenable to genetic engineering by tissue‐culture‐based Agrobacteriumtumefaciens‐mediated transformation techniques (). The group with five to six phyllaries is composed of only C3–C4 intermediate species. Within this group is F. brownii, a C4‐like species but with the expression of C4 photosynthesis dependent on environmental conditions, i.e. light intensity and growth temperature (). The group with five to six phyllaries also contains the C3–C4 intermediate species F. pubescens which is the only transformable C3–C4 intermediate flaveria ().
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role of PEPC during photosynthesis by C4 and C3-C4 ..
These plants show 2 types of photosynthetic cells , mesophyll cells and bundle sheath cells. Chloroplasts are dimorphic i.e., those is the mesophyll cells are granal and in bundle sheath Cells are agranal.
· C4 plants can tolerate high temperature and high light intensity, show greater productivity of biomass, and lack photorespiration.
· Primary CO2 acceptor: Phosphoenol pyruvate (PEP) − a 3-carbon molecule.
· PEP Carboxylase fixes CO2 in the mesophyll cells.
such as Kranz anatomy, elevated PEP carboxylase and low CO 2 ..
There, C4 acid breaks down to form C3 acid and CO2, and carbon dioxide enters the C3 cycle).
· C3 acid, so formed, is again transported to the mesophyll cells and regenerated back into PEP.
· C3 cycle cannot directly occur in the mesophyll cells of C4 plants because of the lack of the enzyme RuBisCO in these cells.
· RuBisCO is found in abundance in the bundle sheath cells of C4 plants.
BRENDA - Information on EC 126.96.36.199 - phosphoenolpyruvate carboxylase
Some modifications to C3 plants are thought possible because comparative studies have shown that C3 plants already have some rudimentary genes that are similar in function to C4 plants. The evolutionary process that created C4 out of C3 plants occurred not once but at least 66 times in the past 35 million years. That evolutionary step achieved high photosynthetic performance and high water- and nitrogen- use efficiencies; C4 plants have twice as high photosynthetic capacity, particularly at higher temperatures as those of C3 plants, and can cope with less water and available nitrogen. For this reason, biochemists have been attempting to move C4 traits to C3 plants as a way to offset environmental changes faced by global warming.
Information on EC 188.8.131.52 - phosphoenolpyruvate carboxylase
Although is responsible for the vast bulk of organic carbon on the surface of the Earth, its oxygenase activity can severely reduce photosynthetic efficiency. Some plants have evolved a way to minimize the oxygenase activity of Rubisco.
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