Reciprocal regulation is important when anabolic and corresponding catabolic pathways are occurring in the same cellular location. As an example, consider regulation of PFK. It is activated by several molecules, most importantly fructose-2,6- bisphosphate F2,6BP.
This molecule has an inhibitory effect on the corresponding gluconeogenesis enzyme, fructose-1,6-bisphosphatase F1,6BPase. You might wonder why pyruvate kinase, the last enzyme in the pathway, is regulated. The answer is simple. Pyruvate kinase catalyzes the most energetically rich reaction of glycolysis. In other words, it takes two enzymes, two reactions, and two triphosphates to go from pyruvate back to PEP in gluconeogenesis.
Another interesting control mechanism called feedforward activation involves pyruvate kinase. Pyruvate kinase is activated allosterically by F1,6BP. This molecule is a product of the PFK reaction and a substrate for the aldolase reaction. When this happens, some of the excess F1,6BP activates pyruvate kinase, which jump-starts the conversion of PEP to pyruvate. As noted, pyruvate produced in glycolysis can be oxidized to acetyl-CoA, which is itself oxidized in the citric acid cycle to carbon dioxide.
That is not the only metabolic fate of pyruvate, though. Pyruvate in animals can also be reduced to lactate when oxygen is limiting. Thus, fermentation of pyruvate is necessary to keep glycolysis operating when oxygen is limiting. These reactions actually have several names associated with them. Other names for light-independent reactions include the Calvin cycle, the Calvin-Benson cycle, and dark reactions.
The most outdated name is dark reactions, which can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.
These energy-carrying molecules are made in the stroma where the Calvin cycle takes place. The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration. The Calvin Cycle : The Calvin cycle has three stages.
In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule. In the stroma, in addition to CO 2 ,two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase RuBisCO and three molecules of ribulose bisphosphate RuBP.
RuBP has five atoms of carbon, flanked by two phosphates. This is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that a reduction is the gain of an electron by an atom or molecule. Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized.
At this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant.
But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed. Three more molecules of ATP are used in these regeneration reactions. The Calvin Cycle involves the process of carbon fixation to produce organic compounds necessary for metabolic processes. The Calvin Cycle is characterized as a carbon fixation pathway. The Calvin Cycle is also referred to as the reductive pentose phosphate cycle or the Calvin-Benson-Bassham cycle.
The process of carbon fixation involves the reduction of carbon dioxide to organic compounds by living organisms. The Calvin cycle is most often associated with carbon fixation in autotrophic organisms, such as plants, and is recognized as a dark reaction. In organisms that require carbon fixation, the Calvin cycle is a means to obtain energy and necessary components for growth.
Some examples of microorganisms that utilize the Calvin cycle include cyanobacteria, purple bacteria, and nitrifying bacteria. Specifically, the Calvin cycle involves reducing carbon dioxide to the sugar triose phosphate, most commonly known as glyceraldehyde 3-phosphate GAP.
Throughout the Calvin Cycle, there are numerous intermediate molecules made which are consistently withdrawn and utilized to create cellular material and participate in cellular processes. The Calvin cycle can be divided into three major phases which include: Phase 1: carbon fixation; Phase 2: reduction; and Phase 3: regeneration of ribulose.
The following is a brief overview of the intermediates created during the Calvin cycle. During phase 1 of this cycle, the CO2 molecule is incorporated into one of two 3-phosphoglycerate molecules 3-PGA. Once 3-PGA is formed, one of two molecules formed continues into the reduction phase phase 2.
The additional 3-PGA is utilized in additional metabolic pathways such as glycolysis and gluconeogenesis. The structure of 3-PGA allows it to be combined and rearranged to form sugars which can be transported to additional cells or stored for energy. During phase 2 of this cycle, the newly formed 3-PGA undergoes phosphorylation by the enzyme phosphoglycerate kinase which utilizes ATP. The result of this phosphorylation is the production of 1,3-bisphosphoglycerates and ADP products.
This energy is necessary for cellular growth and metabolic processes. The inorganic phosphate ion is often a result of regulatory metabolic processes. The GAP molecules at this point are the end product of the Calvin cycle, which is responsible for reducing carbon to a sugar form.
The G3P, which is destined to exit the cycle, will be used for carbohydrate synthesis and additional pathways. Outline the three major phases of the Calvin cycle: carbon fixation, reduction, and regeneration of ribulose. The Calvin cycle is a process utilized to ensure carbon dioxide fixation. In this process, carbon dioxide and water are converted into organic compounds that are necessary for metabolic and cellular processes.
There are various organisms that utilize the Calvin cycle for production of organic compounds including cyanobacteria and purple and green bacteria. The Calvin cycle requires various enzymes to ensure proper regulation occurs and can be divided into three major phases: carbon fixation, reduction, and regeneration of ribulose.
Each of these phases are tightly regulated and require unique and specific enzymes. Overview of the Calvin cycle : An overview of the Calvin cycle and the three major phases. During the first phase of the Calvin cycle, carbon fixation occurs. The carbon dioxide is combined with ribulose 1,5-bisphosphate to form two 3-phosphoglycerate molecules 3-PG.
The enzyme that catalyzes this specific reaction is ribulose bisphosphate carboxylase RuBisCO. RuBisCO is identified as the most abundant enzyme on earth, to date. RuBisCO is the first enzyme utilized in the process of carbon fixation and its enzymatic activity is highly regulated. RuBisCO is only active during the day as its substrate, ribulose 1,5-bisphosphate, is not generated in the dark.
During the second phase of the Calvin cycle, reduction occurs. The 3-PG molecules synthesized in phase 1 are reduced to glyceraldehydephosphate G3P. The enzyme aldolase is typically characterized as a glycolytic enzyme with the ability to split fructose 1,6-bisphosphate into DHAP and G3P. However, in this specific phase of the Calvin cycle, it is used in reverse.
Therefore, aldolase is said to regulate a reverse reaction in the Calvin cycle. Additionally, aldolase can be utilized to promote a reverse reaction in gluconeogenesis as well. The fructose-1,6-bisphosphate formed in phase 2 is then converted into fructosephosphate.
This specific phase involves a series of reactions in which there are a variety of enzymes required to ensure proper regulation. This phase is characterized by the conversion of G3P, which was produced in earlier phase, back to ribulose 1,5-bisphosphate. This process requires ATP and specific enzymes.
The enzymes involved in this process include: triose phosphate isomerase, aldolase, fructose-1,6-bisphosphatase, transketolase, sedoheptulase-1,7-bisphosphatase, phosphopentose isomerase, phosphopentose epimerase, and phosphoribulokinase. The following is a brief summary of each enzyme and its role in the regeneration of ribulose 1,5-bisphosphate in the order it appears in this specific phase. After this final enzyme performs this conversion, the Calvin cycle is considered complete.
The regulation of the Calvin cycle requires many key enzymes to ensure proper carbon fixation. List the enzymes and function that are unique to the reverse TCA cycle ATP citrate lyase; 2-oxoglutarate:ferredoxin oxidoreductase; pyruvate:ferredoxin oxidoreductase.
The citric acid cycle TCA or Krebs cycle, is a process utilized by numerous organisms to generate energy via the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide. The cycle plays a critical role in the maintenance of numerous central metabolic processes. However, there are numerous organisms that undergo reverse TCA or reverse Krebs cycles. This process is characterized by the production of carbon compounds from carbon dioxide and water.
The chemical reactions that occur are the reverse of what is seen in the TCA cycle. There are numerous anaerobic organisms that utilize a cyclic reverse TCA cycle and an example includes organisms classified as Thermoproteus.
The following is a brief overview of the reverse TCA cycle. The reverse TCA cycle is a series of chemical reactions by which organisms produce carbon compounds from carbon dioxide and water. The results showed that the shuttle was used extensively with reduced substrate such as ethanol, whereas the more oxidized substrates lactate and pyruvate, did not provoke any activity of the shuttle.
However, the absence of a functional G3P shuttle did not affect the growth rate or growth yield of the cells, not even during growth on ethanol.
Presumably, there must be alternative systems for maintaining a cytoplasmic redox balance, e.
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