Glycine (symbolGly orG;[6]/ˈɡlaɪsiːn/ⓘ[7]) is an organic compound with the formula C2H5NO2, and is the simplest stableamino acid, distinguished by having a single hydrogen atom as itsside chain. As one of the 20proteinogenic amino acids, glycine is a fundamental building block ofproteins in all life and is encoded by allcodons starting with GG (GGU, GGC, GGA, and GGG).[8][9] Because of its minimal side chain, it is the only common amino acid that is notchiral, meaning it is superimposable on its mirror image.[10][11]
In the body, glycine plays several crucial roles. Its small and flexible structure is vital for the formation of certain protein structures, most notably incollagen, where glycine makes up about 35% of the amino acid content and enables the tight coiling of the collagen triple helix.[8][12] Glycine disrupts the formation ofalpha-helices insecondary protein structure, in favor instead ofrandom coils.[13] Beyond its structural role, glycine functions as an inhibitory neurotransmitter in thecentral nervous system,[14] particularly in thespinal cord andbrainstem, where it helps regulate motor and sensory signals. Disruption of glycine signaling can lead to severeneurological disorders andmotor dysfunction;[15] for example, thetetanus toxin causes spastic paralysis by blocking glycine release.[16] It also serves as a key precursor for the synthesis of other important biomolecules, including theporphyrins that formheme in blood and thepurines used to buildDNA andRNA.[8]
Glycine is a white, sweet-tasting crystalline solid, leading to its name from Greek word glykys (Greek:γλυκύς) or "sweet".[17][7] Whilethe body can synthesize it, it is also obtained from the diet and produced industrially by chemical synthesis for use as afood additive, anutritional supplement, and an intermediate in the manufacture of products such as the herbicideglyphosate.[18] In aqueous solutions, glycine exists predominantly as azwitterion (H3N+CH2COO-), apolar molecule with both a positive and negative charge, making it highly soluble in water.[19] It can also fit intohydrophobic environments due to its minimal side chain.[20]
Its acid–base properties are most important. In aqueous solution, glycine isamphoteric: below pH = 2.4, it converts to the ammonium cation called glycinium. Above about pH 9.6, it converts to glycinate.
Glycine functions as abidentate ligand for many metal ions, formingamino acid complexes.[37] A typical complex is Cu(glycinate)2, i.e. Cu(H2NCH2CO2)2, which exists both in cis and trans isomers.[38][39]
As a bifunctional molecule, glycine reacts with many reagents. These can be classified into N-centered and carboxylate-center reactions.
Glycine is not strictlyessential to the human diet, as it is biosynthesized in the body. However, it is considered semi-essential in that the amount that can be biosynthesized is insufficient for all metabolic uses.[44]
In addition to being synthesized from serine, glycine can also be derived fromthreonine,choline or hydroxyproline via inter-organ metabolism of the liver and kidneys.[47]
Glycine is degraded via three pathways. The predominant pathway in animals and plants is the reverse of the glycine synthase pathway mentioned above. In this context, the enzyme system involved is usually called theglycine cleavage system:[45]
In the second pathway, glycine is degraded in two steps. The first step is the reverse of glycine biosynthesis from serine with serine hydroxymethyl transferase. Serine is then converted topyruvate byserine dehydratase.[45]
The half-life of glycine and its elimination from the body varies significantly based on dose.[48] In one study, the half-life varied between 0.5 and 4.0 hours.[48]
The principal function of glycine is it acts as aprecursor to proteins. Most proteins incorporate only small quantities of glycine, a notable exception beingcollagen, which contains about 35% glycine due to its periodically repeated role in the formation of collagen's helix structure in conjunction withhydroxyproline.[45][49] In thegenetic code, glycine is coded by allcodons starting with GG, namely GGU, GGC, GGA and GGG.[9]
Glycineconjugation pathway has not been fully investigated.[52] Glycine is thought to be a hepatic detoxifier of a number endogenous and xenobiotic organic acids.[53]Bile acids are normally conjugated to glycine in order to increase their solubility in water.[54]
In the US, glycine is typically sold in two grades:United States Pharmacopeia ("USP"), and technical grade. USP grade sales account for approximately 80 to 85 percent of the U.S. market for glycine. If purity greater than the USP standard is needed, for example forintravenous injections, a more expensive pharmaceutical grade glycine can be used. Technical grade glycine, which may or may not meet USP grade standards, is sold at a lower price for use in industrial applications, e.g., as an agent in metal complexing and finishing.[58]
Glycine is not widely used in foods for its nutritional value, except in infusions. Instead, glycine's role in food chemistry is as a flavorant. It is mildly sweet, and it counters the aftertaste ofsaccharine. It also has preservative properties, perhaps owing to its complexation to metal ions. Metal glycinate complexes, e.g.copper(II) glycinate are used as supplements for animal feeds.[35]
Glycine has been researched for its potential toextend life.[62][63] The proposed mechanisms of this effect are its ability to clearmethionine from the body, and activatingautophagy.[62]
Glycine is a significant component of some solutions used in theSDS-PAGE method of protein analysis. It serves as a buffering agent, maintaining pH and preventing sample damage during electrophoresis.[66] Glycine is also used to remove protein-labeling antibodies fromWestern blot membranes to enable the probing of numerous proteins of interest from SDS-PAGE gel. This allows more data to be drawn from the same specimen, increasing the reliability of the data, reducing the amount of sample processing, and number of samples required.[67] This process is known as stripping.
The presence of glycine outside the Earth was confirmed in 2009, based on the analysis of samples that had been taken in 2004 by theNASA spacecraftStardust from cometWild 2 and subsequently returned to Earth. Glycine had previously been identified in theMurchison meteorite in 1970.[68] The discovery of glycine in outer space bolstered the hypothesis of so-calledsoft-panspermia, which claims that the "building blocks" of life are widespread throughout the universe.[69] In 2016, detection of glycine within Comet67P/Churyumov–Gerasimenko by theRosetta spacecraft was announced.[70]
Glycine is proposed to be defined by early genetic codes.[72][73][74][75] For example,low complexity regions (in proteins), that may resemble the proto-peptides of the earlygenetic code are highly enriched in glycine.[75]
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