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    Home > Biochemistry News > Plant Extracts News > The Basics of Photoynsy Lecture Series V: Carbon Assemation

    The Basics of Photoynsy Lecture Series V: Carbon Assemation

    • Last Update: 2021-01-07
    • Source: Internet
    • Author: User
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    process by which plants use NADPH and ATP formed in light reactions to convert CO2 into stable carbon water
    compounds
    , called CO2 assimilation or carbon assimilation. According to the number of carbon atoms contained in the initial product of carbon assulation process and the characteristics of carbon metabolism, the carbon assulation pathway is divided into three categories: C3 pathway, C4 pathway and CAM (Crassulacean acid metabolism) pathway.
    , the C3 route
    carbohydrates such as sugar and starch are products of photochemical use, which were known more than 100 years ago, but the reaction steps and intermediates are difficult to determine using common chemical methods. Because there are many kinds of carbon-containing compounds in plants, it is impossible to identify which were made at the time of photocodying and which were originally created. Moreover, the amount of photosynthing intermediate products is very small, the transformation is very fast, it is difficult to catch. In 1946, M. Calvin and A. Benson of the University of California's Radiochemistry Laboratory adopted two new technologies: (1) 14C
    isotopes
    marking and assays (excluding material interference previously present in cells, where all substances labeled by 14C are processed) ;(2) bidirectional paper lithography (which separates photogenesic products). Single-celled algae, such as cytobacteria, are selected as materials, and algae are not only similar to higher plants in
    bio-chemical
    properties, but are also easy to
    culture
    under average conditions, and can be killed quickly within the time required by the test.
    After more than 10 years of careful research, Calvin et al. have finally discovered a series of reaction steps in photocodescence, from CO2 to sucrose, to derive a cycle pathway for photogassing carbon assification, known as the Calvin cycle or the Calvin Benson cycle. Since the original product of CO2 fixation in this pathway is PGA as a tricarbon compound, it is also called the C3 pathway or C3 photosynthe carbon reduction cycle (C3 photosynthe carbon reduction cycle, C3
    PCR
    cycle) and the C3 plant (C3 plant) for plants with only the C3 pathway. Calvin, the study's host, won the 1961 Nobel Prize in Chemistry.
    the response process of the C3 approach is
    The C3 pathway is the most basic cycle in photochemical carbon metabolism and is the common way to assulate CO2 in all oxygenated photochemical organisms.
    1. The process starts at RubP and ends at the end of RubP regeneration, with a total of 14 steps of reaction, all in the chloroerogen's substation. The whole process is divided into three stages: carcification, reduction and regeneration.
    (1) carboxylation phase refers to the process by which CO2 entering the chlorophyte binds to the receptor RubP and hydrolyzs to produce a PGA. Take fixed 3 molecule CO2 as an example:
    3RuBP plus 3CO2 plus 3H2O PGA plus 6H plus
    nuclegide sugar-1,5-dephosphate pyrase/oxygenation Rubisco has a dual function, which enables RubP to react with CO2, promote the C3 carbon cycle, and cause the C2 oxidation cycle, or light breathing, to cause the oxygenation reaction between RubP and O2 (see section II). light breathing). The pyridine phase is carried out in two steps, i.e. pyridine and hydrolysing:
    under Rubisco, Rubp's C-2 position, the carbide reaction occurs to form 2-carbab-3 ketoarabinitol-1, 5-biphosphate, 3-keto-2CABP), a intermediate product that binds to enzymes that is unstable and hydrolyzed to produce 2 molecules of PGA.
    Rubisco has both activation and passivation, and passivated enzymes can be activated by CO2 and Mg2 plus, which rely on an ε-NH2-based reaction associated with Lys, the enzyme activity center. First, the ε-NH2 of passivated enzymes acts with CO2 (active CO22 is not substrate CO2) to form aminomethyl compounds (E-NH). COO-), which forms an active enzyme (E-NH) in the form of Mg2 plus. COO· Mg2 plus, also known as tricosome ECM), and then the substrate RubP and CO2 are then combined with the active enzyme in turn for a pyration reaction:
    Rubisco can only become an active ECM with CO2 and Mg2 plus, and if it is first combined with RubP (or RubPP similar) it will become an inactive E-RuBP. Rubisco activity is also regulated by an enzyme called Rubisco activatorase. About the role of this active enzyme: in the dark passivated Rubisco and RubP binding to form E-RuBP can not react;
    (2) reduction phase refers to the reaction process of reducing 3-phosphate glyceric acid to glycerol-3-phosphoric acid using assaturation force:
    6PGA-6ATP-6NADPH-6H-→→6GAP-6ADP-6 The PGA produced by the NADP plus 6Pi (4-34)
    -carbapene reaction is an
    organic
    acid that, to reach the energy level of sugar, must use the assaturation force generated in the light reaction, and ATP and NADPH can transform the PGA's carboxyl into a GAB aldehyde. When CO2 is restored to GAP, the photocodesy energy storage process is almost complete.
    (3) regeneration phase refers to the process of re-forming naloxone glycerides-1,-5-dphosphoric acid from glycerides-3-phosphate.
    5GAP-3ATP→→→3RuBP-3ADP-2Pi-3H-
    includes a series of reactions to the formation of
    phosphatization
    3, 4, 5, 6 and 7 carbon sugars. Finally, it is catalyzed by
    -5-phosphoric acid
    (Ru5PK), which consumes 1 molecule ATP and then ruBP.
    reaction of the C3 path can be written as:
    3CO2 plus 5H2O plus 9ATP plus 6NADPH→ GAP plus 9ADP plus 8Pi plus 6NADP plus 3H plus (4-36)
    is visible, each ASA2 needs to consume 3 ATP and 2 NADPH, reduce 3 CO2 can output 1 phosphate propylene sugar (GAP or DHAP), fixed 6 CO2 can form 1 phosphate hexarose (G6P or F6P). The resulting propylene phosphate can be transported out of the carbathing body, synthesized into sucrose in the cytoste or involved in other reactions, and the resulting phosphate hexarose is left in the carbathic body and converted into starch and temporarily stored.
    2. Energy conversion efficiency The assulation of 3 CO2s to form 1 propion phosphate as an example. For every 1mol GAP storage capacity of 1460 kJ, 32 kJ per hydrolyzed 1mol ATP, and 220 kJ per oxidized 1mol NADPH, the energy conversion efficiency of the C3 pathway is 91% (1460/(32×9 plus 220×6), which is a high value. However, in the physiological state, the activity of various compounds is less than 1.0, with the above standard state is different, in addition, to maintain the normal operation of the C3 photoresort reduction cycle, itself also needs to consume energy, so it is generally believed that the energy conversion efficiency in the C3 route is about 80%.
    the adjustment of the C3 approach (ii)
    1. The rate of CO2 assulation of plants by autocatalysis is largely determined by the operating state of the photolytic carbon reduction cycle and the number of photolytic intermediates. Dark blades move to light, the initial fixed CO2 rate is very low, need to go through a "lag period" before they can reach the "steady state" phase of the photonation rate. One reason for this is the low content of photolytic intermediates in the dark leafy green body substate, especially RubP. In the C3 approach, there is a mechanism to automatically adjust the ruBP concentration, that is, when the RubP content is low, the initial assulation of CO2 formed by propylene phosphate does not export the cycle, but is used for the growth of RubP to speed up the co2 fixed rate, when the photocoated carbon reduction cycle reaches a "steady state", the formation of propylene phosphate re-output. This mechanism, which regulates the content of photolytic intermediates such as RubP and makes the assulation CO2 rate in a certain "steady state", is called the self-catalytic action of the C3 pathway.
    2. Light regulates the activity of photolytic enzymes in addition to providing assulation force for CO2 assification through photoreactive reactions. Rubisco, PGAK, GAPDH, FBPase, SBPase, Ru5PK in the C3 cycle are all photomodulation enzymes. These enzymes are increased in light and are reduced or lost in the dark. The regulation of the activity of the enzyme by light can be divided into two kinds of conditions, one is by changing the microenviron environment and the other is regulated by producing effects.
    (1) Microenciential regulation Light-driven electron transfer causes H-plus to transfer to the cystic cavity, while Mg2 plus transfers from the cystic cavity to the substation, causing the pH of the curly-body substation to increase from 7 to 8,Mg2-plus concentration. The higher pH and Mg2 plus concentrations in which Rubisco photolyzases are active.
    (2) effect is regulated by the hypothesis that the light regulatory enzyme can be regulated by the Fd-Td (ferrhexyprotein-sulfur-oxygen-also-protein) system. Enzymes such as FBPase, GAPDH, Ru5PK contain the desulfur bond (-S-S-) and are active when reduced to 2 -SH. Light-driven electron transfer reduces Fd in the substate, which in turn reduces Td (thioxygen, thioredoxin), and the reduced Td turns the second sulfur bond on adjacent cysteine on enzymes such as FBPase and Ru5PK into 2 -base, and the enzyme is active. In the dark, on the contrary, -based oxidation forms a disulfur bond, and the enzyme insulates.
    3. Regulation of the output rate of photochemical products According to the law of mass action, the increase in product concentration slows down the speed of chemical reactions. Propylene phosphate is a photogenic product that can transport chlorogretes, while sucrose is a form of transport in which photolytic products are transported out of cells. Propion phosphate is transported out of the carbapenea through the Pi runter on the leafy body membrane, while the medium cytosphageal Pi is transported into the leafy body. Propylene phosphate is used in cytosin to synthesize sucrose while releasing Pi. If the shipment of sucrose is blocked, or if the use slows down, the synthesis speed decreases, and the release of Pi is reduced, which hinders the shipment of propylene phosphate. In this way, propion phosphate accumulates in the leafy body, thus affecting the normal operation of the C3 photocosm carbon reduction ring. In addition, a decrease in the Pi concentration of chlorogrethrogens inhibits photophosphorylation, which prevents ATP from being synthesized properly, which in turn inhibits Rubisco activated enzyme activity and the reaction that requires the use of ATP.
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