The citric acid cycle is the final common pathway for the oxidation of fuel molecules—amino acids, fatty acids, and carbohydrates. Most fuel molecules enter the cycle as acetyl coenzyme A. Under aerobic conditions, the pyruvate generated from glucose is oxidatively decarboxylated to form acetyl CoA. In eukaryotes, the reactions of the citric acid cycle take place inside mitochondria, in contrast with those of glycolysis, which take place in the cytosol.
The citric acid cycle is the central metabolic hub of the cell as well as an important source of precursors for biosynthesis. It is the gateway to the aerobic metabolism of any molecule that can be transformed into an acetyl group or an intermediate of the citric acid cycle. The cycle provides the building blocks of many other molecules such as amino acids, nucleotide bases, and porphyrin. One of the citric acid cycle intermediates, oxaloacetate, is also an important precursor to glucose.
The citric acid cycle includes a series of oxidation-reduction reactions that result in the oxidation of an acetyl group to two molecules of carbon dioxide. This oxidation generates high-energy electrons that will be used to power the synthesis of ATP. The function of the citric acid cycle is the harvesting of high-energy electrons from carbon fuels.
The citric acid cycle reactions start with a four-carbon compound (oxaloacetate) being condensed with a two-carbon acetyl unit to yield a six-carbon tricarboxylic acid. The six-carbon compound releases two molecules of CO2 in two successive oxidative decarboxylations and yields high-energy electrons and converts to one four-carbon compound. This four-carbon compound is further processed to regenerate oxaloacetate, which can initiate another round of the cycle. Two carbon atoms enter the cycle as an acetyl unit and two carbon atoms leave the cycle in the form of two molecules of carbon dioxide. The citric acid cycle, in conjunction with oxidative phosphorylation, can only provide 90% of energy used by aerobic cells, but cannot reach 100% of the energy requirement of the cell. Moreover, the cell needs to constantly supply glucose and oxygen as reactants for these series of reactions which are spontaneous irreversible.
In the above citric acid cycle reactions, the New Human Line can synthesize glucose from water and the CO2 which is released or leaves from the cycle, continuously providing O2 and glucose for reactants. This biochemical reaction is spontaneous reversible, allowing cellular energy production and energy utilization to reach 100% efficiency. Therefore, these series of reactions are named the Citric Acid Cycle and the Carbon Dioxide Fixation, also called the Tricarboxylic Acid Cycle and the Carbon Dioxide Fixation, or briefly named the Linyuan Cycle. |
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The above figure briefly describes the series of reactions in the Citric Acid Cycle and Carbon Dioxide Fixation. Please note that the CO2 released from amino acids and pyruvate can also combine with H2O to form glucose. This series of reactions can also be named the Amino Acid Cycle and the Carbon Dioxide Fixation or Pyruvate Cycle and Carbon Dioxide Fixation. All these cycles can be collectively referred to as the Linyuan Cycle. The Linyuan Cycle is the citric acid cycle in the New Human Line and the process simultaneously converts CO2 to a reductive organic compound. This brand new type of evolution allows the efficiency of cellular energy supply and metabolism to become 100% spontaneous reversible in the New Human Line（p=0.00）.
References：
Ahern, K.G., van Holde, K.E., & Mathews, C.K. (2000). Biochemistry. (3rd ed.). San Francisco, CA: Addison Wesley Longman.
Nelson, D. L., & Cox, M. M. (2005). Principles of biochemistry. (4th ed.). New York, NY: W.H. Freeman and company.
Stryer, L., Tymoczko, J.L., & Berg, J.M. (2012). Biochemistry. (7th ed.). New York, NY: W.H. Freeman and company. |
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