How can the Rubisco enzyme be found
RuBisCo - the enzyme responsible for fixing the CO2 during photosynthesis
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Ribulose biphosphate carboxylase (RuBisCo) is the enzyme responsible for fixing CO2 during photosynthesis.
Land plants around the world fix an estimated 120 gigatons of carbon from CO2 every year. This is about one sixth of the total atmospheric CO2 and corresponds to around 17 to 20 times the amount of CO2 released into the atmosphere annually by anthropogenic activities. Of this, around 1–2 gigatons of carbon are currently stored annually in the terrestrial ecosystems through the accumulation of biomass and organic matter in the soil. The rest is released back into the atmosphere through autotrophic and heterotrophic respiration. For fixation, 0.2% of the total protein occurring on earth is required (10 kg RuBisCO2 are distributed evenly to each person on earth).
RuBisCO transfers absorbed CO2 to a sugar molecule (ribulose-1,5-bisphosphate = RuBP). Energy from the light reaction of photosynthesis helps here. At the end of several reaction paths there is the production of starch, from which herbivores get their energy. Because RuBisCO fulfills this important task, plant leaves can consist of up to 50 percent.
This makes it the most common protein on earth. Incidentally, it is the only molecule that nature has ever found in the course of evolution to fix CO2.
With a turnover rate of 17 / s (in the living cell: 3 / s) and the lossy side reaction of photorespiration, RuBisCO appears absurdly as one of the worst optimized enzymes. There has therefore been no lack of attempts to change its properties using genetic engineering in order to achieve theoretical yield increases of up to 100%. However, these experiments soon showed that every increase in the turnover rate was at the expense of specificity: the enzyme was less able to differentiate between oxygen and carbon dioxide, which promoted photorespiration.
It appears that RuBisCO of a particular species is almost completely optimized for the prevailing environmental conditions (concentration of O2 and CO2, temperature) despite its disadvantages mentioned above. The first product of CO2 fixation is a C3 body, 3-phosphoglycerate (hence C3 plants). The RuBisCO can not only bind CO2, but also oxygen. The resulting product is not reused by the plant cell; instead, energy and CO2 are lost again. This process is called light breathing.
Light breathing increases with higher temperatures, so that C3 plants no longer carry out net photosynthesis above 28-30 ° C. In the course of evolution, plants have emerged that can compensate for this loss of efficiency in photosynthesis. In C4 plants, for example, the carbon dioxide is pre-fixed spatially separately in another cell (malate, a C4 compound is pre-fixed in the mesophyll cells and then processed in the bundle sheath cells by RuBisCo.), CAM plants store CO2 separately at night.
The evolution of the C4 metabolism is a biochemical adaptation to the falling CO2 concentration in the atmosphere (from the Oligocene, 30 million years ago). Due to the C4 metabolism with its active CO2 pump, the plant enjoys several ecological advantages over C3 plants, since the carbon dioxide concentration around RuBisCO is greatly increased when energy is consumed. On the one hand, this considerably reduces photorespiratory losses.
While C3 plants lose at least 30% of the photosynthetically obtained carbon dioxide, C4 plants can avoid photorespiration even under rising temperatures. This ecophysiological advantage comes into play particularly at temperatures above 25 ° C, so that C4 plants are widespread in hot climates. When the temperature rises, oxygen dissolves better than CO2, so that in C3 plants there are greater losses through photorespiration due to the oxygenase activity of RuBisCO, which in C4 plants can be reduced or completely suppressed. It is in these areas that the negative effects of photorespiration on C3 plants begin to have the greatest effect.
C4 plants are superior to most C3 plants in that they can use water more economically due to their carbon dioxide enrichment (WUE, water use efficiency): The optimal growth temperature is between 30 and 40 ° C, for C3 plants on the other hand at 20 to 30 ° C. C4 plants can largely, but not completely, close their stomata over a longer period of time in order to reduce water loss without endangering the carbon balance. While C4 plants need 230 to 250 ml of water to produce 1 g of dry matter, the need for C3 plants is two to three times as high.
C4 plants can be used to produce biomass for energy. Chinese reed achieves yields of 15 to 25 tons of dry matter per hectare and year. In the USA corn is used as the basis for biofuel, in Brazil it is mainly sugar cane. Alternatively, cold-tolerant C4 grasses such as switchgrass are being discussed for the production of cellulosic ethanol. Under artificially optimized conditions, for example through adequate irrigation, the productivity rates of C4 plants can generally be increased. Corn or sugar cane plantations, if sufficiently fertilized and irrigated, are among the most productive agricultural ecosystems.
One problem of the growing world population (overpopulation) is the scarcity of food supplies, especially since less and less land will be available for agricultural use. One way to increase crop yields could be through C4 photosynthesis. It is particularly advantageous in warmer regions of the world, since there C3 plants, such as rice, are always inferior to C4 plants in terms of their photosythesis productivity. One approach for this is to cultivate C4 plants that already exist in nature but cannot be used for agriculture, such as chicken millet, into rice-like grains.
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