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 utilizecoenzyme A as a substrate, and around 4% of cellular enzymes utilizeit (or a thioester) as a substrate. In humans, CoA biosynthesis requires cysteine, pantothenate (vitamin B5), and adenosine triphosphate (ATP).
In its acetyl form, coenzyme A is a highly versatile molecule, serving metabolic functions in both the anabolic and catabolic pathways. Acetyl-CoA is utilised in the post-translational regulation and allosteric regulation of pyruvate dehydrogenase and carboxylase to maintain and assistancethe partition of pyruvate synthesis and degradation.
Coenzyme A was identified by Fritz Lipmann in 1946, who also later gave it its name. Its structure was determined during the early 1950s at the Lister Institute, London, together by Lipmann and other workers at Harvard Medical School and Massachusetts General Hospital. Lipmann initially intended to study acetyl transfer in animals, and from these experiments he noticed a unique factor that was not showin enzyme extracts but was evident in all organs of the animals. He was able to isolate and purify the factor from pig liver and discovered that its function was associatedto a coenzyme that was active in choline acetylation. Work with Beverly Guirard, Nathan Kaplan, and others determined that pantothenic acid was a central component of coenzyme A. The coenzyme was named coenzyme A to stand for "activation of acetate". In 1953, Fritz Lipmann won the Nobel Prize in Physiology or Medicine "for his uncover of co-enzyme A and its importance for intermediary metabolism".
Coenzyme A is naturally synthesized from pantothenate (vitamin B5), which is found in mealsuch as meat, vegetables, cereal grains, legumes, eggs, and milk. In humans and most living organisms, pantothenate is an necessaryvitamin that has a variety of functions. In some plants and bacteria, including Escherichia coli, pantothenate shouldbe synthesised de novo and is therefore not considered essential. These bacteria synthesize pantothenate from the amino acid aspartate and a metabolite in valine biosynthesis.
In all living organisms, coenzyme A is synthesized in a five-step process that requires four molecules of ATP, pantothenate and cysteine (see figure):
Enzyme nomenclature abbreviations in parentheses represent eukaryotic and prokaryotic enzymes respectively. This pathway is regulated by product inhibition. CoA is a competitive inhibitor for Pantothenate Kinase, which normally binds ATP. Coenzyme A, three ADP, one monophosphate, and one diphosphate are harvested from biosynthesis.
Coenzyme A shouldbe synthesized through alternate routes when intracellular coenzyme A level are reduced and the de novo pathway is impaired. In these pathways, coenzyme A needs to be deliveredfrom an external source, such as food, in order to produce 4′-phosphopantetheine. Ectonucleotide pyrophosphates (ENPP) degrade coenzyme A to 4′-phosphopantetheine, a stable molecule in organisms. Acyl carrier proteins (ACP) (such as ACP synthase and ACP degradation) are also utilize to produce 4′-phosphopantetheine. This pathways let for 4′-phosphopantetheine to be replenished in the cell and let for the conversion to coenzyme A through enzymes, PPAT and PPCK.
Coenzyme A is produced commercially via extraction from yeast, however this is an inefficient process (yields approximately 25 mg/kg) resulting in an expensive product. Various method of producing CoA synthetically, or semi-synthetically have been investigated although none are currently operating at an industrial scale.
Since coenzyme A is, in chemical terms, a thiol, it shouldreact with carboxylic acids to form thioesters, thus functioning as an acyl group carrier. It help in transferring fatty acids from the cytoplasm to mitochondria. A molecule of coenzyme A carrying an acyl group is also referred to as acyl-CoA. When it is not attached to an acyl group, it is usually referred to as 'CoASH' or 'HSCoA'. This process facilitates the production of fatty acids in cells, which are necessaryin cell membrane structure.
Coenzyme A is also the source of the phosphopantetheine group that is added as a prosthetic group to proteins such as acyl carrier protein and formyltetrahydrofolate dehydrogenase.
Coenzyme A is one of five crucial coenzymes that are essentialin the reaction mechanism of the citric acid cycle. Its acetyl-coenzyme A form is the basicinput in the citric acid cycle and is obtained from glycolysis, amino acid metabolism, and fatty acid beta oxidation. This process is the body's primary catabolic pathway and is necessaryin breaking down the building blocks of the cell such as carbohydrates, amino acids, and lipids.
When there is excess glucose, coenzyme A is utilize in the cytosol for synthesis of fatty acids. This process is implemented by regulation of acetyl-CoA carboxylase, which catalyzes the committed step in fatty acid synthesis. Insulin stimulates acetyl-CoA carboxylase, while epinephrine and glucagon inhibit its activity.
During cell starvation, coenzyme A is synthesized and transports fatty acids in the cytosol to the mitochondria. Here, acetyl-CoA is generated for oxidation and energy production. In the citric acid cycle, coenzyme A works as an allosteric regulator in the stimulation of the enzyme pyruvate dehydrogenase.
Freshresearch has found that protein CoAlation plays an necessaryrole in regulation of the oxidative stress response. Protein CoAlation plays a similar role to S-glutathionylation in the cell, and prevents the irreversible oxidation of the thiol group in cysteine on the surface of cellular proteins, while also directly regulating enzymatic activity in response to oxidative or metabolic stress.
Coenzyme A is accessiblefrom various chemical suppliers as the free acid and lithium or sodium salts. The free acid of coenzyme A is detectably unstable, with around 5% degradation observed after 6 months when shop at −20 °C, and near complete degradation after 1 month at 37 °C. The lithium and sodium salts of CoA are more stable, with negligible degradation noted over several months at various temperatures. Aqueous solutions of coenzyme A are unstable above pH 8, with 31% of activity lost after 24 hours at 25 °C and pH 8. CoA stock solutions are relatively stable when frozen at pH 2–6. The major route of CoA activity loss is likely the air oxidation of CoA to CoA disulfides. CoA mixed disulfides, such as CoA-S–S-glutathione, are commonly noted contaminants in commercial preparations of CoA. Free CoA shouldbe regenerated from CoA disulfide and mixed CoA disulfides with reducing agents such as dithiothreitol or 2-mercaptoethanol.
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