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Overview of the citric acid cycle. The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle) [1] [2] —is a series of biochemical reactions to release the energy stored in nutrients through the oxidation of acetyl-CoA derived from carbohydrates, fats, proteins, and alcohol.
The citric acid cycle is a series of enzymatic reactions carried out inside the inner membranes of the cell's mitochondria. The process begins when the two-carbon acetyl CoA enters the cycle and joins the four-carbon oxaloacetate to produce the six-carbon citrate.
Citrate synthase (E.C. 2.3.3.1 (previously 4.1.3.7)) is an enzyme that exists in nearly all living cells. It functions as a pace-making enzyme in the first step of the citric acid cycle (or Krebs cycle). [5] Citrate synthase is located within eukaryotic cells in the mitochondrial matrix, but is encoded by nuclear DNA rather than mitochondrial.
All of the enzymes for the citric acid cycle are in the matrix (e.g. citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, fumarase, and malate dehydrogenase) except for succinate dehydrogenase which is on the inner membrane and is part of protein complex II in the electron transport chain.
Amphibolic properties of the citric acid cycle. An amphibolic pathway is one that can be either catabolic or anabolic based on the availability of or the need for energy. [10]: 570 The currency of energy in a biological cell is adenosine triphosphate (ATP), which stores its energy in the phosphoanhydride bonds. The energy is utilized to conduct ...
Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle.All genomes sequenced to date encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it (or a thioester) as a substrate.
Oxoglutarate dehydrogenase is a key control point in the citric acid cycle. It is inhibited by its products, succinyl CoA and NADH. A high energy charge in the cell will also be inhibitive. ADP and calcium ions are allosteric activators of the enzyme.
The function of fumarase in the citric acid cycle is to facilitate a transition step in the production of energy in the form of NADH. [12] In the cytosol, the enzyme functions to metabolize fumarate, which is a byproduct of the urea cycle as well as amino acid catabolism. Studies have revealed that the active site is composed of amino acid ...