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21-441: (Redirected from Hcy ) HCY may refer to: Homocysteine , an amino acid Hôpital Central de Yaoundé , a Cameroonian hospital Tsuen Wan Public Ho Chuen Yiu Memorial College , a Hong Kong secondary school Helios Airways , a Cypriot airline (1998–2006; ICAO: HCY ) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with

42-456: A general base results in the formation of the external aldimine and removal of the lysine residue. The basic lysine residue is then able to deprotonate the alpha carbon, pushing electron density into the nitrogen of the pyridine ring. Pyridoxal phosphate is necessary to stabilize this carbanionic intermediate; otherwise the proton's pKa would be too high. The beta carbon is then deprotonated, creating an alpha-beta unsaturation and pushing

63-402: A lone pair onto the aldimine nitrogen. To reform the aldimine, this lone pair pushes back down, cleaving the sulfur-gamma carbon bond, resulting in the release of cysteine. A pyridoxamine derivative of vinyl glyoxylate remains after the gamma elimination . The lone pair from the pyridine nitrogen pushes electron density to the gamma carbon, which is protonated by lysine. Lysine then attacks

84-561: A reaction catalyzed by S-adenosyl-methionine synthetase , to give S -adenosyl methionine (SAM-e). SAM-e then transfers the methyl group to an acceptor molecule, (e.g., norepinephrine as an acceptor during epinephrine synthesis, DNA methyltransferase as an intermediate acceptor in the process of DNA methylation ). The adenosine is then hydrolyzed to yield L -homocysteine. L -Homocysteine has two primary fates: conversion via tetrahydrofolate (THF) back into L -methionine or conversion to L -cysteine . Mammals biosynthesize

105-462: A significant role in the appearance of life on the early Earth. Homocysteine levels typically are higher in men than women, and increase with age. Common levels in Western populations are 10 to 12 μmol/L, and levels of 20 μmol/L are found in populations with low B-vitamin intakes or in the elderly (e.g., Rotterdam, Framingham). It is decreased with methyl folate trapping, where it

126-405: Is a prosthetic group of this enzyme. Cystathionine γ-lyase also catalyses the following elimination reactions: In some bacteria and mammals , including humans, this enzyme takes part in generating hydrogen sulfide . Hydrogen sulfide is one of a few gases that was recently discovered to have a role in cell signaling in the body. Cystathionase uses pyridoxal phosphate to facilitate

147-411: Is accompanied by decreased methylmalonic acid, increased folate, and a decrease in formiminoglutamic acid . This is the opposite of MTHFR C677T mutations, which result in an increase in homocysteine. The ranges above are provided as examples only; test results always should be interpreted using the range provided by the laboratory that produced the result. Abnormally high levels of homocysteine in

168-491: Is also a member of the broader aspartate aminotransferase family. Like many other PLP-dependent enzymes, cystathionine γ-lyase is a tetramer with D2 symmetry . Pyridoxal phosphate is bound in the active site by Lys . Cysteine is the rate-limiting substrate in the synthetic pathway for glutathione in the eye. Glutathione is an antioxidant that protects crystallins in the eye from reactive oxygen species; denatured crystallins can lead to cataracts . Cystathionase

189-540: Is also a target for reactive oxygen species. Thus as cystathionase is oxidized, its activity decreases, causing a decrease in cysteine and, in turn, glutathione in the eye, leading to a decrease in antioxidant availability, causing a further decrease in cystathionase activity. Deficiencies in cystathionase activity have also been shown to contribute to glutathione depletion in patients with cancer and AIDS . Mutations and deficiencies in cystathionase are associated with cystathioninuria . The mutations T67I and Q240E weaken

210-529: Is managed with vitamin B 6 , vitamin B 9 , and vitamin B 12 supplementation. However, supplementation with these vitamins does not appear to improve cardiovascular disease outcomes. Cystathionine gamma-lyase The enzyme cystathionine γ-lyase (EC 4.4.1.1, CTH or CSE ; also cystathionase ; systematic name L -cystathionine cysteine-lyase (deaminating; 2-oxobutanoate-forming) ) breaks down cystathionine into cysteine , 2-oxobutanoate ( α-ketobutyrate ), and ammonia : Pyridoxal phosphate

231-513: The amino acid cysteine via homocysteine. Cystathionine β-synthase catalyses the condensation of homocysteine and serine to give cystathionine . This reaction uses pyridoxine (vitamin B 6 ) as a cofactor. Cystathionine γ-lyase then converts this double amino acid to cysteine, ammonia, and α-ketobutyrate. Bacteria and plants rely on a different pathway to produce cysteine, relying on O -acetylserine. Homocysteine can be recycled into methionine . This process uses N5-methyl tetrahydrofolate as

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252-488: The amino group of propargylglycine attacks the aldimine to form an external aldimine. The β position of the alkyne is then deprotonated to form the allene , which is then attacked by the phenol of Tyr . The internal aldimine can regenerate, but the newly created vinyl ether sterically hinders the active site, blocking cysteine from attacking pyridoxal phosphate. H 2 S decreases transcription of cystathionase at concentrations between 10 and 80μM. However, transcription

273-457: The blood ( hyperhomocysteinemia ) is regarded as a marker of cardiovascular disease, likely working through atherogenesis , which can result in ischemic injury . Therefore, hyperhomocysteinemia is a possible risk factor for coronary artery disease . Coronary artery disease occurs when an atherosclerotic plaque blocks blood flow to the coronary arteries , which supply the heart with oxygenated blood. Hyperhomocysteinemia has been correlated with

294-431: The cleavage of the sulfur-gamma carbon bond of cystathionine, resulting in the release of cysteine. The lysine residue reforms the internal aldimine by kicking off α-iminobutyric acid. Afterwards the external ketimine is hydrolyzed, causing the formation of α-ketobutyrate. The amino group on cystathionine is deprotonated and undergoes a nucleophilic attack of the internal aldimine. An additional deprotonation by

315-495: The enzyme's affinity for pyridoxal phosphate, the co-factor vital to enzymatic function. Low levels of H 2 S have also been associated with hypertension in mice. Excessive levels of H 2 S, due to increased activity of cystathionase, are associated with endotoxemia , acute pancreatitis , hemorrhagic shock , and diabetes mellitus . Propargylglycine and β-cyanoalanine are two irreversible inhibitors of cystathionase used to treat elevated H 2 S levels. Mechanistically,

336-402: The external aldimine, pushing electron density to the beta carbon, which is protonated by a general acid. The imine is then hydrolyzed to release α-ketobutyrate. Deprotonation of the lysine residue causes ammonia to leave, thus completing the catalytic cycle . Cystathionine gamma lyase also shows gamma-synthase activity depending on the concentrations of reactants present. The mechanisms are

357-561: The methyl donor and cobalamin (vitamin B 12 )-related enzymes. More detail on these enzymes can be found in the article for methionine synthase . Homocysteine can cyclize to give homocysteine thiolactone , a five-membered heterocycle . Because of this "self-looping" reaction, homocysteine-containing peptides tend to cleave themselves by reactions generating oxidative stress . Homocysteine also acts as an allosteric antagonist at Dopamine D 2 receptors. It has been proposed that both homocysteine and its thiolactone may have played

378-471: The occurrence of blood clots, heart attacks, and strokes, although it is unclear whether hyperhomocysteinemia is an independent risk factor for these conditions. Hyperhomocysteinemia has ALSO been associated with early-term spontaneous abortions and with neural tube defects . Homocysteine exists at neutral pH values as a zwitterion . Homocysteine is biosynthesized naturally via a multi-step process. First, methionine receives an adenosine group from ATP ,

399-476: The same until they diverge after formation of the vinyl glyoxylate derivative. In the gamma synthase mechanism, the gamma carbon is attacked by a sulfur nucleophile, resulting in the formation of a new sulfur-gamma carbon bond. Cystathionine γ-lyase is a member of the Cys/Met metabolism PLP-dependent enzymes family. Other members include cystathionine γ synthase, cystathionine β lyase, and methionine γ lyase. It

420-644: The serum, above 15 μmol/L, are a medical condition called hyperhomocysteinemia . This has been claimed to be a significant risk factor for the development of a wide range of diseases, in total more than 100 including thrombosis , neuropsychiatric illness, in particular dementia and fractures. It also is found to be associated with microalbuminuria, which is a strong indicator of the risk of future cardiovascular disease and renal dysfunction. Vitamin B 12 deficiency, even when coupled with high serum folate levels, has been found to increase overall homocysteine concentrations as well. Typically, hyperhomocysteinemia

441-514: The title HCY . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=HCY&oldid=1196758564 " Category : Disambiguation pages Hidden categories: Articles containing French-language text Short description is different from Wikidata All article disambiguation pages All disambiguation pages Homocysteine High levels of homocysteine in

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