Amino acids are organic compounds containing amine functional groups (-NH 2 ) and carboxyl (-COOH), together with side chains (group R) specific to each amino acid. The key elements of amino acids are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), although other elements are found in certain amino acid side chains. About 500 known natural amino acids (although only 20 appear in the genetic code) and can be classified in many ways. They can be classified according to the location of the core structural functional groups as alpha- (? -) , beta- (? -) , gamma- (? -) or delta- (? -) amino acids; other categories are related to the polarity, pH level, and type of side chain groups (aliphatic, acyclic, aromatic, hydroxyl or sulfuric, etc.). In the form of protein, amino acid residues form the second largest component (water is the largest) of human muscle and other tissues. Beyond their role as a residue in proteins, amino acids participate in a number of processes such as transport neurotransmitters and biosynthesis.
In biochemistry, amino acids having amine and carboxylic acid groups attached to the first carbon atom (alpha) have a particular interest. They are known as 2-, alpha -, or ? - amino acids (generic formula H 2 NCHRCOOH in many cases, where R is an organic substituent known as "side chain"); often the term "amino acid" is used to refer specifically to this. They include 22 proteinogenic ("protein-building") amino acids, which merge into peptide chains ("polypeptides") to form the building blocks of a number of proteins. These are all L -stereoisomers ("left-handed" isomers), although some D -amino acids ("right hand") appear in the bacterial envelope, as neuromodulators ( D -serine), and in some antibiotics.
Twenty proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids. The other two ("non-standard" or "non-canonical") are selenocysteine ​​(present in many prokaryotes as well as most eukaryotes, but not directly encoded by DNA), and pyrrolysine (found only in some archea and one bacteria). Pyrrolysine and selenocysteine ​​â € <â €
Many of the important proteinogen and non-proteinogenic amino acids have biological functions. For example, in the human brain, glutamate (standard glutamic acid) and gamma-amino-butyric acid ("GABA", non-standard gamma-amino acids) are, respectively, major excitatory and inhibitory neurotransmitters. Hydroxyproline, a major component of connective tissue collagen, is synthesized from proline. Glycine is a porphyrin biosynthetic precursor used in red blood cells. Carnitine is used in lipid transport.
Nine proteinogenic amino acids are called "essential" to humans because they can not be produced from other compounds by the human body and therefore must be taken as food. Others may be conditionally important for a certain age or medical condition. Essential amino acids may also differ between species.
Because of its biological significance, amino acids are important in nutrients and are commonly used in nutritional supplements, fertilizers, and food technology. Industrial uses include drug production, biodegradable plastics, and chiral catalysts.
Video Amino acid
History
The first amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated compounds in asparagus which were then named asparagins, the first amino acids found. Cystine was discovered in 1810, although monomer, cysteine, remained undiscovered until 1884. Glycine and leucine were discovered in 1820. The last of the 20 common amino acids found was threonine in 1935 by William Cumming Rose, which also determined the amino acid and set minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he did not give a specific name for it. The use of the term "amino acids" in English is from 1898, while the German term, AminosÃÆ'¤ure , was used earlier. Proteins are found to produce amino acids after enzymatic digestion or acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are made up of many amino acids, in which bonds are formed between amino groups of one amino acid with another carboxyl group, producing a linear structure called Fischer as a "peptide".
Maps Amino acid
General structure
In the structure shown at the top of the page, R represents a special side chain for each amino acid. The carbon atom next to the carboxyl group (which is therefore numbered 2 in the carbon chain starting from the functional group) is called "carbon." Amino acids containing an amino group directly bound to alpha carbon are referred to as alpha amino acids This includes amino acids such as proline containing secondary amines, which were formerly referred to as "imino acids".
Isomerism
Alpha alpha is the most common form in nature, but only when it occurs in L -omer. Alpha carbon is a chiral carbon atom, with the exception of glycine having two distinguishable hydrogen atoms on alpha carbon. Therefore, all alpha but glycine amino acids can exist in one of two enantiomers, called L or D amino acids, which are mirror images of each other (< i> see also Chirality ). While L -amino acids represent all the amino acids found in proteins during translation in the ribosome, D -amino acids are found in some proteins produced by post-translational modification of the enzyme after translation. and translocation to the endoplasmic reticulum, as in exotic marine organisms such as cone snails. They are also an abundant component of the bacterial cell wall of peptidoglycan, and D -serine can act as neurotransmitters in the brain. D -amino acid is used in racemic crystallography to create centrosymmetric crystals, which (depending on the protein) enable the determination of easier and stronger protein structures. The L and D conventions for the amino acid configuration do not refer to the optical activity of the amino acid itself but rather to the optical activity of the glyceraldehyde isomers from which the amino acid can, in theory, synthesized ( D -glyceraldehyde is dextrorotatory; L -glyceraldehyde is levorotatory). In the alternative mode, the designers (S) and (R) are used to indicate absolute stereochemistry. Almost all amino acids in proteins are (S) at? carbon, with cysteine ​​being (R) and non-chiral glycine. Cysteine ​​has its side chains in the same geometric position as other amino acids, but the terminology is reversed because the higher number of sulfur atoms compared to the carboxyl oxygen provides a higher side chain. priority, while atoms in most other side chains give them a lower priority.
Side chain
In amino acids having carbon chains attached to carbon (such as lysine, shown to the right) carbon is labeled agar as?,?,?,?, And so on. In some amino acids, amine groups attached to? or? -carbon, and this is therefore referred to as beta or i gamma amino acids .
Amino acids are usually classified by their side-chain properties into four groups. Side chains can make amino acids into weak acids or weak bases, and hydrophils if the side chains are polar or hydrophobic if nonpolar. The chemical structure of the 22 standard amino acids, along with their chemical properties, is described more fully in an article on these proteinogenic amino acids.
The phrase "branching amino acid" or BCAA refers to an amino acid having an non-linear aliphatic side chain; these are leucine, isoleucine, and valine. Proline is the only proteinogenic amino acid associated with amino-group-side groups, and as such, is also the only proteinogenic amino acid containing secondary amines in this position. In chemical terms, proline is, therefore, an imino acid, since it has no primary amino group, although it is still classified as an amino acid in current biochemical nomenclature, and may also be called "N-alkylation of alpha-amino acids".
Zwitterions
The carboxylic acid groups of the amino acids are weak acids, which means releasing a hydron (like a proton) at a moderate pH value. In other words, carboxylic acid groups (-CO 2 H) can be deprotonated into negative carboxylates (-CO 2 - ). The negatively charged carboxylic ion dominates at a pH value greater than the pKa of the carboxylic acid group (the average for 20 common amino acids is about 2.2, see the amino acid structure table above). In complementary, amine-amine acids are weak bases, meaning that they receive protons with a moderate pH value. In other words, the -amino group (NH 2 -) can be protonated into positive-ammonium groups ( NH 3 -). The positively charged ammonium group predominates at pH values ​​less than pKa of the -amonium group (average for 20 general? -amino acids around 9.4).
Since all amino acids contain amine functional groups and carboxylic acids, they share amphiprotic properties. Below pH 2.2, the dominant form will have neutral carboxylic acid and positive -monium ion (net charge 1), and above pH 9.4, negative and neutral carboxylic groups? -amino (net charge -1). But at pH between 2.2 and 9.4, amino acids usually contain negative carboxylates and positive ammonium groups, as shown in structure (2) on the right, thus having zero zero charge. This molecular state is known as zwitterion, from the German Zwitter which means hermaphrodite or hybrid . The complete neutral shape (structure (1) on the left) is a very small species in aqueous solution across the pH range (less than 1 part in 10 7 ). Amino acids exist as zwitterions also in solid phase, and crystallize with properties like salts unlike typical organic or amine acids.
Isoelectric Point
Variations in the titration curve when amino acids can be grouped by category. With the exception of tyrosine, use titration to distinguish between problematic amino acid hydrophobic.
At the pH value between two pKa values, zwitter dominates, but coexist in dynamic equilibrium with a small amount of negative ions and net net net. At the exact midpoint between two pKa values, the net trace amount is negative and the net positive ion trace is exactly balanced, so that the average net charge of all existing forms is zero. This PH is known as the isoelectric point of pI, so pI = ½ (pKa 1 pKa 2 ). Each amino acid has a slightly different pKa value, so it has a different isoelectric point. For an amino acid with a charged side chain, a side chain pKa is involved. So for Asp, Glu with negative side chains, pI = ½ (pKa 1 pKa R ), where pKa R is the side chain of pKa. Cysteine ​​also has the potential of having negative side chains with pKa R = 8.14, so pI should be calculated as for Asp and Glu, although the side chains are not charged significantly at neutral pH. For His, Lys, and Arg with positive side chains, pI = Ã,½ (pKa R pKa 2 ). Amino acids have zero mobility in electrophoresis at the isoelectric point, although this behavior is more often exploited for peptides and proteins than for single amino acids. Zwitterions have a minimum solubility at the isoelectric point and some amino acids (in particular, with non-polar side chains) can be isolated by deposition of water by adjusting the pH to the required isoelectric point.
Genesis and function in biochemistry
Amino acid proteininat
Amino acids are structural units (monomers) that make up proteins. They join together to form short polymer chains called peptides or long chains called polypeptides or proteins. These polymers are linear and unbranched, with each of the amino acids in the chain attached to two adjacent amino acids. The process of making proteins encoded by DNA/RNA genetic material is called translational and involves adding amino acids to the chain of proteins grown by ribozymes called ribosomes. The order in which the amino acid is added is read through the genetic code of the mRNA template, which is a copy of RNA from one of the genes of the organism.
Twenty-two amino acids are naturally incorporated into polypeptides and are called proteogenic or natural amino acids. Of these, 20 are encoded by universal genetic code. The remaining 2, selenocysteine ​​and pyrrolysine, are incorporated into proteins by a unique synthetic mechanism. Selenocysteine ​​is included when the translated mRNA includes the SECIS element, which causes the UGA codon to encode selenocysteine ​​â € <â €
Apart from 22 proteinogenic amino acids, many known non-proteinogenic amino acids are known. They are either not found in proteins (eg carnitine, GABA, levothyroxine) or are not produced directly and isolated by standard cellular machinery (eg, hydroxyproline and selenometionin).
The non-proteinogenic amino acids found in proteins are formed by post-translational modification, which is a modification after translation during protein synthesis. This modification is often essential for protein function or setting. For example, carboxylation of glutamate enables better binding of calcium cations, and collagen contains hydroxyproline, produced by proline hydroxylation. Another example is the formation of hypusine in the initiation of translation factor EIF5A, through the modification of lysine residues. The modification can also determine the localization of proteins, such as the addition of long hydrophobic groups may cause proteins to bind to phospholipid membranes.
Some non-proteinogenic amino acids are not found in proteins. Examples include 2-aminoisobutyric acid and gamma-aminobutyric neurotransmitter acid. Non-proteinogenic amino acids often occur as intermediates in metabolic pathways for standard amino acids - for example, ornithine and citrulline occur in the urea cycle, part of amino acid catabolism (see below). The rare exception to the dominance of amino acids in biology is the amino acid beta-alanine (3-aminopropanoic acid), which is used in plants and microorganisms in the synthesis of pantothenic acid (vitamin B 5). sub>), coenzyme component A. D - natural abundance of amino acids
D -isomers are rare in living organisms. For example, gramicidin is a polypeptide made from a mixture of D - and L -amino acids. Other compounds containing D acids are tyrocidine and valinomycin. These compounds disrupt bacterial cell walls, especially in Gram-positive bacteria. Only 837 D -amino acids were found in the Swiss-Prot database (187 million amino acids were analyzed).
Non-standard amino acids
20 amino acids encoded directly by codons of universal genetic codes are called standard or canonical amino acids. Modified methionine forms ( N -formylmethionine) are often incorporated as methionine as the initial amino acids of proteins in bacteria, mitochondria, and chloroplasts. Other amino acids are called non-standard or non-canonical . Most non-standard amino acids are also non-proteinogenic (ie they can not be incorporated into proteins during translation), but two of them are proteinogenic, since they can be translated into proteins by exploiting unencoded information in the universal genetic code..
Both non-standard proteinogenic amino acids are selenocysteine ​​(present in many non-eukaryotes as well as most eukaryotes, but not directly encoded by DNA) and pyrrolysine (found only in some archaea and one bacterium). The incorporation of non-standard amino acids is rare. For example, 25 human proteins including selenocysteine ​​(Sec) in their main structure, and structurally characterized enzymes (selenoenzymes) use Sec as the catalytic part of their active site. Pyrrolysine and selenocysteine ​​â € <â €
When taken into the human body from the diet, 20 standard amino acids are either used to synthesize proteins and other biomolecules or oxidize into urea and carbon dioxide as an energy source. The oxidation path begins with the removal of amino groups by transaminase; the amino group is then fed into the urea cycle. Another product of transamidation is keto acid which enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, via gluconeogenesis. Of the 20 standard amino acids, nine (Nya, Ile, Leu, Lis, Met, Phe, Thr, Trp and Val) are called essential amino acids because the human body can not synthesize them from other compounds at levels required for normal growth, must be obtained from food. In addition, cysteine, taurine, tyrosine, and arginine are considered as semi-essential amino acids in children (although taurine is technically not an amino acid), because the metabolic pathways that synthesize these amino acids are not fully developed. The amount required also depends on the age and health of the individual, making it difficult to make a general statement about dietary requirements for some amino acids. Dietary exposure to non-standard amino acids BMAA has been linked to human neurodegenerative diseases, including ALS.
Non-protein function
In humans, non-protein amino acids also play important roles as metabolic intermediates, as in the biosynthesis of gamma-amino-butyric neurotransmitter (GABA) acid. Many amino acids are used to synthesize other molecules, for example:
- Tryptophan is a neurotransmitter serotonin precursor.
- Tyrosine (and phenylalanine precursors) is a precursor of dopamine catecholamine, epinephrine and norepinephrine neurotransmitters and various trace amines.
- Phenylalanine is a precursor of phenethylamine and tyrosine in humans. In plants, it is a precursor of various phenylpropanoids, which are important in plant metabolism.
- Glycine is a porphyrin precursor like heme.
- Arginine is a precursor of nitric oxide.
- Ornithine and S-adenosylmethionine are precursors of polyamines.
- Aspartate, glycine, and glutamine are precursors of nucleotides. However, not all other abundant non-standard amino acid functions are known.
Some non-standard amino acids are used as a defense against herbivores in plants. For example, canavanine is an arginine analog found in many legumes, and especially in large quantities at Canavalia gladiata (sword bean). These amino acids protect plants from predators such as insects and can cause disease in humans if some types of legumes are eaten unprocessed. Mimosine of non-protein amino acids is found in other legume species, especially Leucaena leucocephala . This compound is a tyrosine analogue and can poison the animals that graze on this plant.
Industrial use
Amino acids are used for various applications in the industry, but their main use is as an additive for animal feed. This is necessary, since many of the bulk components of this feed, such as soy, have low levels or lack of some of the essential amino acids: lysine, methionine, threonine, and tryptophan are most important in the production of this feed. In this industry, amino acids are also used to chelate metal cations in order to increase the absorption of minerals from supplements, which may be necessary to improve the health or production of these animals.
The food industry is also a major consumer of amino acids, especially glutamic acid, which is used as a flavor enhancer, and aspartame (aspartyl-phenylalanine-1-methyl ester) as a low-calorie artificial sweetener. Similar technologies used for animal nutrition are used in the human nutrition industry to reduce the symptoms of mineral deficiency, such as anemia, by increasing mineral absorption and reducing the negative side effects of inorganic mineral supplementation.
The ability of chelating amino acids has been used in fertilizers for agriculture to facilitate the delivery of minerals to plants to correct mineral deficiencies, such as iron chlorosis. This fertilizer is also used to prevent deficiencies from occurring and improve overall plant health. The remaining production of amino acids is used in the synthesis of drugs and cosmetics.
Similarly, some amino acid derivatives are used in the pharmaceutical industry. They include 5-HTP (5-hydroxytryptophan) used for experimental treatment of depression, L -DOPA ( L -dihydroxyphenylalanine) for Parkinson's treatment, and eflornithine drugs that inhibit ornithine decarboxylase and is used in the treatment of sleeping sickness.
Extended genetic code
Since 2001, 40 non-natural amino acids have been added to proteins by creating unique codons (re-coding) and RNA-transfer couples: aminoacylÃ,-tRNA-synthetase to encode with a variety of physicochemical and biological properties to be used as a tool for exploring structures and protein function or to create new or improved proteins.
Nullomers
Nullomers are codons that are in theory code for amino acids, but in nature there is selective bias towards the use of these codons in favor of others, eg bacteria prefer to use CGA rather than AGA for code for arginine. This creates some sequences that do not appear in the genome. These characteristics can be exploited and used to create new selective cancer-fighting drugs and to prevent cross-contamination of DNA samples from crime scene investigations.
Chemical building block
Amino acids are important as cheap raw materials. These compounds are used in chiral pool synthesis as pure enantiomeric building blocks.
Amino acids have been studied as chiral catalyst precursors, for example, for asymmetric hydrogenation reactions, although there is no commercial application.
Biodegradable Plastics
Amino acids are under development as a component of various biodegradable polymers. These materials have applications as an environmentally friendly packaging and in medicine in drug delivery and the construction of prosthetic implants. These polymers include polypeptides, polyamides, polyesters, polysulfides, and polyurethanes with amino acids that form part of their major chains or are bonded as side chains. This modification alters the physical properties and reactivity of the polymer. An interesting example of the material is polyaspartate, a water-soluble biodegradable polymer that may have applications in disposable diapers and agriculture. Because of its solubility and its ability to churn metal ions, polyaspartate is also used as a biodegradable anti-scaling agent and corrosion inhibitor. In addition, aromatic amino acid tyrosine is being developed as a possible substitute for toxic phenols such as bisphenol A in the manufacture of polycarbonate.
Reaction
Since amino acids have primary amine groups and primary carboxyl groups, these chemicals can experience most of the reactions associated with this functional group. These include nucleophilic addition, the formation of amide bonds, and the formation of imines for amine groups, and esterification, formation of amide bonds, and decarboxylates for carboxylic acid groups. This combination of functional groups allows amino acids to be an effective polydentat ligand for amino acid chelates. Some amino acid side chains can also undergo chemical reactions. These types of reactions are determined by groups on this side chain and therefore, differ between different types of amino acids.
Chemical synthesis
There are several methods to synthesize amino acids. One of the oldest methods begins with bromination in? carbon from carboxylic acids. The nucleophilic substitution with ammonia then converts alkyl bromide to amino acid. In alternative fashion, the synthesis of the Strecker amino acid involves the treatment of aldehydes with potassium cyanide and ammonia, this results in? -amino nitrile as an intermediary. Hydrolysis of nitrile in the acid then generates the amino acid. Using ammonia or ammonium salts in this reaction gives unsubstituted amino acids, whereas substitutes of primary and secondary amines will produce substituted amino acids. Similarly, using ketones, not aldehydes, giving ?,? - the amino acid is substituted. Classical synthesis gives a racemic mixture of "amino acids" as a product, but some alternative procedures using asymmetric auxiliaries or asymmetric catalysts have been developed.
At the moment, the most widely adopted method is automatic synthesis of solid support (eg, polystyrene beads), using protective groups (eg, Fmoc and t-Boc) and enabling groups (eg DCC and DIC).
Peptide bond formation
Since both amino acid and carboxylic acid groups can react to form amide bonds, one amino acid molecule can react with the other and become joined through the amide relationship. It is this amino acid polymerization that creates proteins. This condensation reaction produces newly formed peptide bonds and water molecules. In the cell, this reaction does not occur directly; instead, the amino acid is first activated by attachment to the transfer RNA molecule through the ester bond. This aminoacyl-tRNA is produced in an ATP-dependent reaction by aminoacyl tRNA synthetase. The aminoacyl-tRNA then becomes the substrate for the ribosome, which catalyzes the attack of the protein chain amino group extending on the ester bond. As a result of this mechanism, all proteins made by ribosomes are synthesized starting from N-terminus and moving toward C-terminus.
However, not all peptide bonds are formed in this way. In some cases, peptides are synthesized by specific enzymes. For example, tripeptide glutathione is an important part of cell defense against oxidative stress. This peptide is synthesized in two steps of free amino acids. In the first step, gamma-glutamylcysteine ​​synthetase condenses cysteine ​​and glutamic acid through a peptide bond formed between the carboxyl side chain of glutamate (this side chain gamma carbon) and the amino cysteine ​​group. The dipeptide is then condensed with glycine by glutathione synthetase to form glutathione.
In chemistry, peptides are synthesized by various reactions. One of the most used in synthesis of solid phase peptides uses aromatic oxide derivatives of amino acids as active units. These are added sequentially to the growing peptide chain, which is attached to the solid resin support. The ability to easily synthesise large numbers of different peptides by varying the types and sequences of amino acids (using combinatorial chemistry) has made peptide synthesis very important in creating peptide libraries for use in drug discovery through high throughput screening.
Biosynthesis
In plants, nitrogen is first assimilated into organic compounds in the form of glutamate, formed from alpha-ketoglutarate and ammonia in the mitochondria. To form other amino acids, plants use transaminases to transfer amino groups to other alpha-keto carboxylic acids. For example, aspartate aminotransferase converts glutamate and oxaloacetate into alpha-ketoglutarate and aspartate. Other organisms use transaminases for the synthesis of amino acids as well.
Non-standard amino acids are usually formed by modification of standard amino acids. For example, homocysteine ​​is formed via a transsulfuration or demethylation methionine pathway through the intermediate metabolite of S-adenosyl methionine, while hydroxyproline is made by posttranslational proline modification.
Microorganisms and plants can synthesize many unusual amino acids. For example, some microbes make 2-aminoisobutyric acid and lanthionine, which is a sulphide derivative of alanine. Both of these amino acids are found in peptidik libiotics such as alamethicin. However, in plants, 1-aminocyclopropane-1-carboxylic acid is a substituted cyclic amino acid which is a key intermediary in the production of ethylene plant hormones.
Catabolism
Amino acids must first move from organelles and cells into the blood circulation through amino acid transporters, since amine and carboxylic groups are usually ionized. Amino acid degradation, occurs in the liver and kidneys, often involving deamination by transferring amino groups to alpha-ketoglutarate, forming glutamate. This process involves transaminase, often the same as that used in amination during synthesis. In many vertebrates, the amino group is then excreted through the urea cycle and excreted in urea form. However, the degradation of amino acids can produce uric acid or ammonia instead. For example, serine dehydratase converts serine into pyruvate and ammonia. After removal of one or more amino groups, the remaining molecules can sometimes be used to synthesize new amino acids, or can be used for energy by incorporating glycolysis or citric acid cycles, as described in the right-hand picture.
Physicochemical properties of amino acids
20 amino acids encoded directly by the genetic code can be divided into several groups based on their properties. Important factors are charge, hydrophobicity or hydrophobic, size, and functional groups. These properties are important for protein structure and protein-protein interactions. Water-soluble proteins tend to have hydrophobic residues (Leu, Ile, Val, Phe, and Trp) buried in the middle of the protein, while the hydrophilic side chains are exposed to aqueous solvents. (Note that in biochemistry, the residue refers to a particular monomer in the polymeric chain of polysaccharides, protein or nucleic acid.) Integral membrane proteins tend to have an outer ring of exposed hydrophobic amino acids that trap them into lipid bilayers. In the case of the beak between these two extremes, some peripheral membrane proteins have hydrophobic amino acid patches on their surface that lock into the membrane. In the same way, proteins that must bind to positively charged molecules have surfaces that are rich with negatively charged amino acids such as glutamate and aspartate, while proteins that bind to negatively charged molecules have surfaces that are rich in positively charged chains such as lysine and arginine. There are different hydrophobic scales of amino acid residues.
Some amino acids have special properties such as cysteine, which can form covalent disulfide bonds with other cysteine ​​residues, a proline that cycles into the backbone of polypeptides, and glycine is more flexible than other amino acids.
Many proteins undergo various posttranslational modifications, when additional chemical groups are attached to amino acids in proteins. Some modifications may produce hydrophobic lipoproteins, or hydrophilic glycoproteins. This type of modification allows reversible protein targeting to the membrane. For example, the addition and removal of fatty acid palmitic acid to cysteine ​​residues in some signaling proteins causes the proteins to stick and then detach from the cell membrane.
Abbreviation table and standard amino acid properties
Two additional amino acids in some species are coded by a codon that is usually interpreted as a stop codon:
In addition to certain amino acid codes, placeholders are used in cases where chemical analysis or crystallography of peptides or proteins can not definitively determine residual identity. They are also used to summarize the sequence of preserved protein sequences. The use of single letters to denote similar residual sets is similar to the use of abbreviated codes for a degenerated base.
Unk is sometimes used instead of Xaa , but it's less standard.
In addition, many non-standard amino acids have special codes. For example, some peptide drugs, such as Bortezomib and MG132, are artificially synthesized and retain their protective group, which has a special code. Bortezomib is Pyz-Phe-boroLeu, and MG132 is Z-Leu-Leu-Leu-al. To assist in the analysis of protein structure, reactive photo-amino acid analogs are available. These include photoleucine ( pLeu ) and photomethionine ( pMet ).
See also
References and notes
Further reading
External links
- Media related to Amino Acids on Wikimedia Commons
Source of the article : Wikipedia
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