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glutathione : ウィキペディア英語版
glutathione

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Glutathione (GSH) is an important antioxidant in plants, animals, fungi, and some bacteria and archaea, preventing damage to important cellular components caused by reactive oxygen species such as free radicals, peroxides, lipid peroxides and heavy metals. It is a tripeptide with a gamma peptide linkage between the carboxyl group of the glutamate side-chain and the amine group of cysteine (which is attached by normal peptide linkage to a glycine).
Thiol groups are reducing agents, existing at a concentration around 5 mM in animal cells. Glutathione reduces disulfide bonds formed within cytoplasmic proteins to cysteines by serving as an electron donor. In the process, glutathione is converted to its oxidized form, glutathione disulfide (GSSG), also called L-(–)-glutathione.
Once oxidized, glutathione can be reduced back by glutathione reductase, using NADPH as an electron donor. The ratio of reduced glutathione to oxidized glutathione within cells is often used as a measure of cellular toxicity.
==Biosynthesis==
The biosynthesis pathway for glutathione is found in some bacteria, such as cyanobacteria and proteobacteria, but is missing in many other bacteria. Most eukaryotes synthesize glutathione, including humans, but some do not, such as Leguminosae, ''Entamoeba'', and ''Giardia''. The only archaea that make glutathione are halobacteria.
Glutathione is not an essential nutrient for humans, since it can be biosynthesized in the body from the amino acids L-cysteine, L-glutamic acid, and glycine. The sulfhydryl group (SH) of cysteine serves as a proton donor and is responsible for its biological activity. Cysteine is the rate-limiting factor in cellular glutathione biosynthesis, since this amino acid is relatively rare in foods.
Cells make glutathione in two adenosine triphosphate-dependent steps:
*First, ''gamma''-glutamylcysteine is synthesized from L-glutamate and cysteine via the enzyme gamma-glutamylcysteine synthetase (glutamate cysteine ligase, GCL). This reaction is the rate-limiting step in glutathione synthesis.
*Second, glycine is added to the C-terminal of ''gamma''-glutamylcysteine via the enzyme glutathione synthetase.
Animal glutamate cysteine ligase (GCL) is a heterodimeric enzyme composed of a catalytic and a modulatory subunit. The catalytic subunit is necessary and sufficient for all GCL enzymatic activity, whereas the modulatory subunit increases the catalytic efficiency of the enzyme. Mice lacking the catalytic subunit (i.e., lacking all ''de novo'' GSH synthesis) die before birth. Mice lacking the modulatory subunit demonstrate no obvious phenotype, but exhibit marked decrease in GSH and increased sensitivity to toxic insults.
While all cells in the human body are capable of synthesizing glutathione, liver glutathione synthesis has been shown to be essential. Mice with genetically induced loss of GCLC (i.e., GSH synthesis) only in the liver die within a month of birth.
The plant glutamate cysteine ligase (GCL) is a redox-sensitive homodimeric enzyme, conserved in the plant kingdom. In an oxidizing environment, intermolecular disulfide bridges are formed and the enzyme switches to the dimeric active state. The midpoint potential of the critical cysteine pair is -318 mV. In addition to the redox-dependent control is the plant GCL enzyme feedback inhibited by GSH. GCL is exclusively located in plastids, and glutathione synthetase is dual-targeted to plastids and cytosol, thus are GSH and gamma-glutamylcysteine exported from the plastids. Both glutathione biosynthesis enzymes are essential in plants; knock-outs of GCL and GS are lethal to embryo and seedling.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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