In chemistry, a dehydration reaction is a conversion that involves the loss of water from the reacting molecule or ion. Dehydration reactions are the reverse of a hydration reaction. Common dehydrating agents used in organic synthesis include sulfuric alumina. Dehydration reactions are effected with heating; the classic example of a dehydration reaction is the Fischer esterification, which involves treating a carboxylic acid with an alcohol in the presence of a dehydrating agent: RCO2H + R′OH ⇌ RCO2R′ + H2OTwo monosaccharides, such as glucose and fructose, can be joined together using dehydration synthesis. The new molecule, consisting of two monosaccharides, is called a disaccharide; the process of hydrolysis is the reverse reaction, meaning that the water is recombined with the two hydroxyl groups and the disaccharide reverts to being monosaccharides. In the related condensation reaction water is released from two different reactants. In organic synthesis, there are many examples of dehydration reaction, for example dehydration of alcohols or sugars.
Other examples of dehydration synthesis reactions are the formation of triglycerides from fatty acids and the formation of glycosidic bonds between carbohydrate molecules, such as the formation of maltose from two glucose molecules. Hydration reaction
A carboxylic acid is an organic compound that contains a carboxyl group. The general formula of a carboxylic acid is R–COOH, with R referring to the rest of the molecule. Carboxylic acids occur widely. Important examples include acetic acid. Deprotonation of a carboxyl group gives a carboxylate anion. Important carboxylate salts are soaps. Carboxylic acids are identified by their trivial names, they have the suffix -ic acid. IUPAC-recommended names exist. For example, butyric acid is butanoic acid by IUPAC guidelines. For nomenclature of complex molecules containing a carboxylic acid, the carboxyl can be considered position one of the parent chain if there are other substituents, for example, 3-chloropropanoic acid. Alternately, it can be named as a "carboxy" or "carboxylic acid" substituent on another parent structure, for example, 2-carboxyfuran; the carboxylate anion of a carboxylic acid is named with the suffix -ate, in keeping with the general pattern of -ic acid and -ate for a conjugate acid and its conjugate base, respectively.
For example, the conjugate base of acetic acid is acetate. Carboxylic acids are polar; because they are both hydrogen-bond acceptors and hydrogen-bond donors, they participate in hydrogen bonding. Together the hydroxyl and carbonyl group forms the functional group carboxyl. Carboxylic acids exist as dimers in nonpolar media due to their tendency to "self-associate". Smaller carboxylic acids are soluble in water, whereas higher carboxylic acids have limited solubility due to the increasing hydrophobic nature of the alkyl chain; these longer chain acids tend to be rather soluble in less-polar solvents such as ethers and alcohols. Hydrophobic carboxylic acids react aqueous sodium hydroxide to give water soluble sodium salts. For example, enathic acid has a small solubility in water, but its sodium salt is soluble in water: Carboxylic acids tend to have higher boiling points than water, not only because of their increased surface area, but because of their tendency to form stabilised dimers through hydrogen bonds.
For boiling to occur, either the dimer bonds must be broken or the entire dimer arrangement must be vaporised, both of which increase the enthalpy of vaporization requirements significantly. Carboxylic acids are Brønsted -- Lowry acids, they are the most common type of organic acid. Carboxylic acids are weak acids, meaning that they only dissociate into H3O+ cations and RCOO− anions in neutral aqueous solution. For example, at room temperature, in a 1-molar solution of acetic acid, only 0.4% of the acid are dissociated. Electron-withdrawing substituents, such as -CF3 group, give stronger acids. Electron-donating substituents give weaker acids Deprotonation of carboxylic acids gives carboxylate anions; each of the carbon–oxygen bonds in the carboxylate anion has a partial double-bond character. The carbonyl carbon's partial positive charge is weakened by the -1/2 negative charges on the 2 oxygen atoms. Carboxylic acids have strong sour odors. Esters of carboxylic acids tend to have pleasant odors, many are used in perfume.
Carboxylic acids are identified as such by infrared spectroscopy. They exhibit a sharp band associated with vibration of the C–O vibration bond between 1680 and 1725 cm−1. A characteristic νO–H band appears as a broad peak in the 2500 to 3000 cm−1 region. By 1H NMR spectrometry, the hydroxyl hydrogen appears in the 10–13 ppm region, although it is either broadened or not observed owing to exchange with traces of water. Many carboxylic acids are produced industrially on a large scale, they are pervasive in nature. Esters of fatty acids are the main components of lipids and polyamides of aminocarboxylic acids are the main components of proteins. Carboxylic acids are used in the production of polymers, pharmaceuticals and food additives. Industrially important carboxylic acids include acetic acid and methacrylic acids, adipic acid, citric acid, ethylenediaminetetraacetic acid, fatty acids, maleic acid, propionic acid, terephthalic acid. In general, industrial routes to carboxylic acids differ from those used on smaller scale because they require specialized equipment.
Carbonylation of alcohols as illustrated by the Cativa process for production of acetic acid. Formic acid is prepared by a different carbonylation pathway starting from methanol. Oxidation of aldehydes with air using cobalt and manganese catalysts; the required aldehydes are obtained from alkenes by hydroformylation. Oxidation of hydrocarbons using air. For simple alkanes, this method is inexpensive but not selective enough to be useful. Allylic and benzylic compounds undergo more selective oxidations. Alkyl groups on a benzene ring are oxidized to the carboxylic acid, regardless of its chain length. Benzoic acid from toluene, terephthalic acid from para-xylene, phthalic acid from ortho-xylene are illustrative large-scale conversions. Acrylic acid is generated from propene. Base-cata
Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase. Both DNA and RNA are nucleic acids. During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript. Transcription proceeds in the following general steps: RNA polymerase, together with one or more general transcription factors, binds to promoter DNA. RNA polymerase creates a transcription bubble; this is done by breaking the hydrogen bonds between complementary DNA nucleotides. RNA polymerase adds RNA nucleotides. RNA sugar-phosphate backbone forms with assistance from RNA polymerase to form an RNA strand. Hydrogen bonds of the RNA–DNA helix break, freeing the newly synthesized RNA strand. If the cell has a nucleus, the RNA may be further processed; this may include polyadenylation and splicing. The RNA may exit to the cytoplasm through the nuclear pore complex; the stretch of DNA transcribed into an RNA molecule is called a transcription unit and encodes at least one gene.
If the gene encodes a protein, the transcription produces messenger RNA. Alternatively, the transcribed gene may encode for non-coding RNA such as microRNA, ribosomal RNA, transfer RNA, or enzymatic RNA molecules called ribozymes. Overall, RNA helps synthesize and process proteins. In virology, the term may be used when referring to mRNA synthesis from an RNA molecule. For instance, the genome of a negative-sense single-stranded RNA virus may be template for a positive-sense single-stranded RNA; this is because the positive-sense strand contains the information needed to translate the viral proteins for viral replication afterwards. This process is catalyzed by a viral RNA replicase. A DNA transcription unit encoding for a protein may contain both a coding sequence, which will be translated into the protein, regulatory sequences, which direct and regulate the synthesis of that protein; the regulatory sequence before the coding sequence is called the five prime untranslated region. As opposed to DNA replication, transcription results in an RNA complement that includes the nucleotide uracil in all instances where thymine would have occurred in a DNA complement.
Only one of the two DNA strands serve as a template for transcription. The antisense strand of DNA is read by RNA polymerase from the 3' end to the 5' end during transcription; the complementary RNA is created in the opposite direction, in the 5' → 3' direction, matching the sequence of the sense strand with the exception of switching uracil for thymine. This directionality is because RNA polymerase can only add nucleotides to the 3' end of the growing mRNA chain; this use of only the 3' → 5' DNA strand eliminates the need for the Okazaki fragments that are seen in DNA replication. This removes the need for an RNA primer to initiate RNA synthesis, as is the case in DNA replication; the non-template strand of DNA is called the coding strand, because its sequence is the same as the newly created RNA transcript. This is the strand, used by convention when presenting a DNA sequence. Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA.
As a result, transcription has a lower copying fidelity than DNA replication. Transcription is divided into initiation, promoter escape and termination. Transcription begins with the binding of RNA polymerase, together with one or more general transcription factors, to a specific DNA sequence referred to as a "promoter" to form an RNA polymerase-promoter "closed complex". In the "closed complex" the promoter DNA is still double-stranded. RNA polymerase, assisted by one or more general transcription factors unwinds 14 base pairs of DNA to form an RNA polymerase-promoter "open complex". In the "open complex" the promoter DNA is unwound and single-stranded; the exposed, single-stranded DNA is referred to as the "transcription bubble."RNA polymerase, assisted by one or more general transcription factors selects a transcription start site in the transcription bubble, binds to an initiating NTP and an extending NTP complementary to the transcription start site sequence, catalyzes bond formation to yield an initial RNA product.
In bacteria, RNA polymerase holoenzyme consists of five subunits: 2 α subunits, 1 β subunit, 1 β' subunit, 1 ω subunit. In bacteria, there is one general RNA transcription factor: sigma. RNA polymerase core enzyme binds to the bacterial general transcription factor sigma to form RNA polymerase holoenzyme and binds to a promoter. In archaea and eukaryotes, RNA polymerase contains subunits homologous to each of the five RNA polymerase subunits in bacteria and contains additional subunits. In archaea and eukaryotes, the functions of the bacterial general transcription factor sigma are performed by multiple general transcription factors that work together. In archaea, there ar
Prenylation is the addition of hydrophobic molecules to a protein or chemical compound. It is assumed that prenyl groups facilitate attachment to cell membranes, similar to lipid anchors like the GPI anchor, though direct evidence of this has not been observed. Prenyl groups have been shown to be important for protein–protein binding through specialized prenyl-binding domains. Protein prenylation involves the transfer of either a farnesyl or a geranyl-geranyl moiety to C-terminal cysteine of the target protein. There are three enzymes that carry out prenylation in the cell, farnesyl transferase, Caax protease and geranylgeranyl transferase I. Farnesylation is a type of prenylation, a post-translational modification of proteins by which an isoprenyl group is added to a cysteine residue, it is an important process to mediate protein–protein interactions and protein–membrane interactions. Farnesyltransferase and Geranylgeranyltransferase I are similar proteins, they consist of two subunits, the α-subunit, common to both enzymes, the β-subunit, whose sequence identity is just 25%.
These enzymes recognise the CaaX box at the C-terminus of the target protein. C is the cysteine, prenylated, a is any aliphatic amino acid, the identity of X determines which enzyme acts on the protein. Farnesyltransferase recognizes CaaX boxes where X = M, S, Q, A, or C, whereas Geranylgeranyltransferase I recognizes CaaX boxes with X = L or E. Rab geranylgeranyltransferase, or Geranylgeranyl transferase II, transfers two geranylgeranyl groups to the cysteine at the C-terminus of Rab proteins; the C-terminus of Rab proteins is referred to as hypervariable. Thus Rab proteins do not have a consensus sequence, such as the CAAX box, which the Rab geranylgeranyl transferase can recognize; the Rab proteins terminate in a CC or CXC motif. Instead, Rab proteins are bound by the Rab escort protein over a more conserved region of the Rab protein and presented to the Rab geranylgeranyltransferase. Once Rab proteins are prenylated, the lipid anchor ensure. REP, plays an important role in binding and solubilising the geranylgeranyl groups and delivers the Rab protein to the relevant cell membrane.
Both isoprenoid chains, geranylgeranyl pyrophosphate and farnesyl pyrophosphate are products of the HMG-CoA reductase pathway. The product of HMG CoA reductase is mevalonate. By combining precursors with 5 carbons, the pathway subsequently produces geranyl pyrophosphate, farnesyl pyrophosphate and geranylgeranyl pyrophosphate. Two farnesyl pyrophosphate groups can be combined to form squalene, the precursor for cholesterol; this means that statins, which inhibit HMG CoA reductase, inhibit the production of both cholesterol and isoprenoids. Note that, in the HMG-CoA reductase/mevalonate pathway, the precursors contain a pyrophosphate group, isoprenoids are produced with a pyrophosphate group. There is no known enzyme activity that can carry out the prenylation reaction with the isoprenoid alcohol. However, enzymatic activity for isoprenoid kinases capable converting isoprenoid alcohols to isoprenoid pyrophosphates have been shown. In accordance with this and geranylgeraniol have been shown to be able to rescue effects caused by statins or nitrogenous bisphosphonates, further supporting that alcohols can be involved in prenylation via phosphorylation to the corresponding isoprenoid pyrophosphate.
Proteins that undergo prenylation include Ras, which plays a central role in the development of cancer. This suggests. In the case of the K- and N-Ras forms of Ras, when cells are treated with FTIs, these forms of Ras can undergo alternate prenylation in the form of geranylgeranylation. Recent work has shown that farnesyltransferase inhibitors inhibit Rab geranylgeranyltransferase and that the success of such inhibitors in clinical trials may be as much due to effects on Rab prenylation as on Ras prenylation. Inhibitors of prenyltransferase enzymes display different specificity for the prenyltransferases, dependent upon the specific compound being utilized. In addition to GTPases, the protein kinase GRK1 known as rhodopsin kinase has been shown to undergo farnesylation and carboxyl methylation directed by the carboxyl terminal CVLS CaaX box sequence of the protein; the functional consequence of these post-translational modifications have been shown to play a role in regulating the light-dependent phosphorylation of rhodopsin, a mechanism involved in light adaptation.
FTIs can be used to inhibit farnesylation in parasites such as trypanosoma brucei and malaria. Parasites seem to be more vulnerable to inhibition of Farnesyl transferase. In some cases, this may be because they lack Geranylgeranyltransferase I. Thus, it may be possible for the development of antiparastic drugs to'piggyback' on the development of FTIs for cancer research. In addition, FTIs have shown some promise in treating a mouse model of progeria, in May 2007 a phase II clinical trial using the FTI Lonafarnib was started for children with progeria. In signal transduction via G protein, palmitoylation of the α subunit, prenylation of the γ subunit, myristoylation is involved in tethering the G protein to the inner surface of the plasma membrane so that the G protein can interact with its receptor. Small molecules can undergo prenylation, such as in the case of prenylflavonoids. Prenylation of a vitamin B2 derivative was described. A 2012 study found that statin treatment increases lifespan and imp
Secretion is the movement of material from one point to another, e.g. secreted chemical substance from a cell or gland. In contrast, excretion, is the removal of certain substances or waste products from a cell or organism; the classical mechanism of cell secretion is via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell. Secretion in bacterial species means the transport or translocation of effector molecules for example: proteins, enzymes or toxins from across the interior of a bacterial cell to its exterior. Secretion is a important mechanism in bacterial functioning and operation in their natural surrounding environment for adaptation and survival. Eukaryotic cells, including human cells, have a evolved process of secretion. Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum.
As they are synthesized, these proteins translocate into the ER lumen, where they are glycosylated and where molecular chaperones aid protein folding. Misfolded proteins are identified here and retrotranslocated by ER-associated degradation to the cytosol, where they are degraded by a proteasome; the vesicles containing the properly folded proteins enter the Golgi apparatus. In the Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational modifications, including cleavage and functionalization, may occur; the proteins are moved into secretory vesicles which travel along the cytoskeleton to the edge of the cell. More modification can occur in the secretory vesicles. There is vesicle fusion with the cell membrane at a structure called the porosome, in a process called exocytosis, dumping its contents out of the cell's environment. Strict biochemical control is maintained over this sequence by usage of a pH gradient: the pH of the cytosol is 7.4, the ER's pH is 7.0, the cis-golgi has a pH of 6.5.
Secretory vesicles have pHs ranging between 5.0 and 6.0. There are many proteins like FGF2, interleukin-1 etc. which do not have a signal sequence. They do not use the classical ER-golgi pathway; these are secreted through various nonclassical pathways. At least four nonclassical protein secretion pathways have been described, they include 1) direct translocation of proteins across the plasma membrane through membrane transporters, 2) blebbing, 3) lysosomal secretion, 4) release via exosomes derived from multivesicular bodies. In addition, proteins can be released from cells by mechanical or physiological wounding and through nonlethal, transient oncotic pores in the plasma membrane induced by washing cells with serum-free media or buffers. Many human cell types have the ability to be secretory cells, they have a well-developed endoplasmic reticulum and Golgi apparatus to fulfill their function. Tissues in humans that produce secretions include the gastrointestinal tract which secretes digestive enzymes and gastric acid, the lung which secretes surfactants, sebaceous glands which secrete sebum to lubricate the skin and hair.
Meibomian glands in the eyelid secrete sebum to protect the eye. Secretion is not unique to eukaryotes alone - it is present in bacteria and archaea as well. ATP binding cassette type transporters are common to all the three domains of life; the Sec system constituting the Sec Y-E-G complex is another conserved secretion system, homologous to the translocon in the eukaryotic endoplasmic reticulum and the Sec 61 translocon complex of yeast. Some secreted proteins are translocated across the cytoplasmic membrane by the Sec translocon, which requires the presence of an N-terminal signal peptide on the secreted protein. Others are translocated across the cytoplasmic membrane by the twin-arginine translocation pathway. Gram-negative bacteria have two membranes. There are at least six specialized secretion systems in gram-negative bacteria. Many secreted proteins are important in bacterial pathogenesis. Type I secretion is a chaperone dependent secretion system employing the Tol gene clusters; the process begins as a leader sequence HlyA binds HlyB on the membrane.
This signal sequence is specific for the ABC transporter. The HlyAB complex stimulates HlyD which begins to uncoil and reaches the outer membrane where TolC recognizes a terminal molecule or signal on HlyD. HlyD recruits TolC to the inner membrane and HlyA is excreted outside of the outer membrane via a long-tunnel protein channel. Type I secretion system transports various molecules, from ions, drugs, to proteins of various sizes; the molecules secreted vary in size from the small Escherichia coli peptide colicin V, to the Pseudomonas fluorescens cell adhesion protein LapA of 520 kDa. The best characterized are the lipases. Type I secretion is involved in export of non-proteinaceous substrates like cyclic β-glucans and polysaccharides. Proteins secreted through the type II system, or main terminal branch of the general secretory pathway, depend on the Sec or Tat system for initial transport into the periplasm. Once there, they pass through the outer membrane via a multimeric complex of pore forming secretin proteins.
In addition to the secretin protein, 10–15 other inner and outer memb
Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen and nitrogen, although other elements are found in the side chains of certain amino acids. About 500 occurring amino acids are known and can be classified in many ways, they can be classified according to the core structural functional groups' locations as alpha-, beta-, gamma- or delta- amino acids. In the form of proteins, amino acid residues form the second-largest component of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. In biochemistry, amino acids having both the amine and the carboxylic acid groups attached to the first carbon atom have particular importance, they are known as α-amino acids. They include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins.
These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, in some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids; the other two are selenocysteine, pyrrolysine. Pyrrolysine and selenocysteine are encoded via variant codons. N-formylmethionine is considered as a form of methionine rather than as a separate proteinogenic amino acid. Codon–tRNA combinations not found in nature can be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids have biological functions. For example, in the human brain and gamma-amino-butyric acid are the main excitatory and inhibitory neurotransmitters. Hydroxyproline, a major component of the connective tissue collagen, is synthesised from proline. Glycine is a biosynthetic precursor to porphyrins used in red blood cells.
Carnitine is used in lipid transport. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food. Others may be conditionally essential for medical conditions. Essential amino acids may differ between species; because of their biological significance, amino acids are important in nutrition and are used in nutritional supplements, fertilizers and food technology. Industrial uses include the production of drugs, biodegradable plastics, chiral catalysts; the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus, subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, remained undiscovered until 1884. Glycine and leucine were discovered in 1820; the last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. Usage of the term "amino acid" in the English language is from 1898, while the German term, Aminosäure, was used earlier. Proteins were found to yield amino acids after enzymatic acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". In the structure shown at the top of the page, R represents a side chain specific to each amino acid; the carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids; these include amino acids such as proline which contain secondary amines, which used to be referred to as "imino acids". The alpha amino acids are the most common form found in nature, but only when occurring in the L-isomer.
The alpha carbon is a chiral carbon atom, with the exception of glycine which has two indistinguishable hydrogen atoms on the alpha carbon. Therefore, all alpha amino acids but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amin
Glycine is an amino acid that has a single hydrogen atom as its side chain. It is the simplest amino acid, with the chemical formula NH2‐CH2‐COOH. Glycine is one of the proteinogenic amino acids, it is encoded by all the codons starting with GG. Glycine is known as a "helix breaker", due to its ability to act as a hinge in the secondary structure of proteins. Glycine is a sweet-tasting crystalline solid, it is the only achiral proteinogenic amino acid. It can fit into hydrophilic or hydrophobic environments, due to its minimal side chain of only one hydrogen atom; the acyl radical is glycyl. Glycine was discovered in 1820 by the French chemist Henri Braconnot when he hydrolyzed gelatin by boiling it with sulfuric acid, he called it "sugar of gelatin", but the French chemist Jean-Baptiste Boussingault showed that it contained nitrogen. The American scientist Eben Norton Horsford a student of the German chemist Justus von Liebig, proposed the name "glycocoll"; the name comes from the Greek word γλυκύς "sweet tasting".
In 1858, the French chemist Auguste Cahours determined. Although glycine can be isolated from hydrolyzed protein, this is not used for industrial production, as it can be manufactured more conveniently by chemical synthesis; the two main processes are amination of chloroacetic acid with ammonia, giving glycine and ammonium chloride, the Strecker amino acid synthesis, the main synthetic method in the United States and Japan. About 15 thousand tonnes are produced annually in this way. Glycine is cogenerated as an impurity in the synthesis of EDTA, arising from reactions of the ammonia coproduct. In aqueous solution, glycine itself is amphoteric: at low pH the molecule can be protonated with a pKa of about 2.4 and at high pH it loses a proton with a pKa of about 9.6. Glycine is not essential to the human diet, as it is biosynthesized in the body from the amino acid serine, in turn derived from 3-phosphoglycerate, but the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis.
In most organisms, the enzyme serine hydroxymethyltransferase catalyses this transformation via the cofactor pyridoxal phosphate: serine + tetrahydrofolate → glycine + N5,N10-Methylene tetrahydrofolate + H2OIn the liver of vertebrates, glycine synthesis is catalyzed by glycine synthase. This conversion is reversible: CO2 + NH+4 + N5,N10-Methylene tetrahydrofolate + NADH + H+ ⇌ Glycine + tetrahydrofolate + NAD+ 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 called the glycine cleavage system: Glycine + tetrahydrofolate + NAD+ ⇌ CO2 + NH+4 + N5,N10-Methylene tetrahydrofolate + NADH + H+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 converted to pyruvate by serine dehydratase. In the third pathway of glycine degradation, glycine is converted to glyoxylate by D-amino acid oxidase.
Glyoxylate is oxidized by hepatic lactate dehydrogenase to oxalate in an NAD+-dependent reaction. The half-life of glycine and its elimination from the body varies based on dose. In one study, the half-life varied between 4.0 hours. The principal function of glycine is as a precursor to proteins. Most proteins incorporate only small quantities of glycine, a notable exception being collagen, which contains about 35% glycine due to its periodically repeated role in the formation of collagen's helix structure in conjunction with hydroxyproline. In the genetic code, glycine is coded by all codons starting with GG, namely GGU, GGC, GGA and GGG. In higher eukaryotes, δ-aminolevulinic acid, the key precursor to porphyrins, is biosynthesized from glycine and succinyl-CoA by the enzyme ALA synthase. Glycine provides the central C2N subunit of all purines. Glycine is an inhibitory neurotransmitter in the central nervous system in the spinal cord and retina; when glycine receptors are activated, chloride enters the neuron via ionotropic receptors, causing an Inhibitory postsynaptic potential.
Strychnine is a strong antagonist at ionotropic glycine receptors, whereas bicuculline is a weak one. Glycine is a required co-agonist along with glutamate for NMDA receptors. In contrast to the inhibitory role of glycine in the spinal cord, this behaviour is facilitated at the glutamatergic receptors which are excitatory; the LD50 of glycine is 7930 mg/kg in rats, it causes death by hyperexcitability. In the US, glycine is sold in two grades: United States Pharmacopeia, technical grade. USP grade sales account for 80 to 85 percent of the U. S. market for glycine. If purity greater than the USP standard is needed, for example for intravenous 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. USP glycine has a wide variety of uses, including as an additive in pet food and animal feed, in foods and pharmaceuticals as a sweetener/taste enhancer, or as a component of food supplements and protein drinks.
Two glycine molecules in a dipeptide form are referred to as a diglycinate. Because they use a different s