Amino acid

This is a good article. Click here for more information.
Source: Wikipedia, the free encyclopedia.
(Redirected from
Amino-acid
)
This article is about the class of chemicals. For the structures and properties of the standard proteinogenic amino acids, see Proteinogenic amino acid.

Structure of a typical L-alpha-amino acid in the "neutral" form

Amino acids are

amino and carboxylic acid functional groups.[1] Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins.[2] Only these 22 appear in the genetic code of life.[3][4]

Amino acids can be classified according to the locations of the core structural functional groups (

aromatic, polar, etc.). In the form of proteins, amino acid residues form the second-largest component (water being the largest) of human muscles and other tissues.[5] Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. It is thought that they played a key role in enabling life on Earth and its emergence
.

Amino acids are formally named by the IUPAC-IUBMB Joint Commission on Biochemical Nomenclature in terms of the fictitious "neutral" structure shown in the illustration. For example, the systematic name of alanine is 2-aminopropanoic acid, based on the formula CH3−CH(NH2)−COOH. The Commission justified this approach as follows:[6]

The systematic names and formulas given refer to hypothetical forms in which amino groups are unprotonated and carboxyl groups are undissociated. This convention is useful to avoid various nomenclatural problems but should not be taken to imply that these structures represent an appreciable fraction of the amino-acid molecules.

History

The first few amino acids were discovered in the early 1800s.

Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound from asparagus that was subsequently named asparagine, the first amino acid to be discovered.[9][10] Cystine was discovered in 1810,[11] although its monomer, cysteine, remained undiscovered until 1884.[12][10][a] Glycine and leucine were discovered in 1820.[13] The last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who also determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.[14][15]

The unity of the chemical category was recognized by

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".[19]

General structure

pKa values and charges carried at physiological pH (7.4)

2-, alpha-, or α-amino acids[20] have the generic formula H2NCHRCOOH in most cases,[b] where R is an organic substituent known as a "side chain".[21]

Of the many hundreds of described amino acids, 22 are proteinogenic ("protein-building").[22][23][24] It is these 22 compounds that combine to give a vast array of peptides and proteins assembled by ribosomes.[25] Non-proteinogenic or modified amino acids may arise from post-translational modification or during nonribosomal peptide synthesis.

Chirality

The

stereoisomers
of the alpha carbon.

A few D-amino acids ("right-handed") have been found in nature, e.g., in

bacterial envelopes, as a neuromodulator (D-serine), and in some antibiotics.[26][27] Rarely, D-amino acid residues are found in proteins, and are converted from the L-amino acid as a post-translational modification.[28][c]

Side chains

Polar charged side chains

Five amino acids possess a charge at neutral pH. Often these side chains appear at the surfaces on proteins to enable their solubility in water, and side chains with opposite charges form important electrostatic contacts called

glutamate and histidine. Under certain conditions, each ion-forming group can be charged, forming double salts.[32]

The two negatively charged amino acids at neutral pH are aspartate (Asp, D) and glutamate (Glu, E). The anionic carboxylate groups behave as Brønsted bases in most circumstances.[31] Enzymes in very low pH environments, like the aspartic protease pepsin in mammalian stomachs, may have catalytic aspartate or glutamate residues that act as Brønsted acids.

Functional groups found in histidine (left), lysine (middle) and arginine (right)

There are three amino acids with side chains that are cations at neutral pH: arginine (Arg, R), lysine (Lys, K) and histidine (His, H). Arginine has a charged guanidino group and lysine a charged alkyl amino group, and are fully protonated at pH 7. Histidine's imidazole group has a pKa of 6.0, and is only around 10 % protonated at neutral pH. Because histidine is easily found in its basic and conjugate acid forms it often participates in catalytic proton transfers in enzyme reactions.[31]

Polar uncharged side chains

The polar, uncharged amino acids serine (Ser, S), threonine (Thr, T), asparagine (Asn, N) and glutamine (Gln, Q) readily form hydrogen bonds with water and other amino acids.[31] They do not ionize in normal conditions, a prominent exception being the catalytic serine in serine proteases. This is an example of severe perturbation, and is not characteristic of serine residues in general. Threonine has two chiral centers, not only the L (2S) chiral center at the α-carbon shared by all amino acids apart from achiral glycine, but also (3R) at the β-carbon. The full stereochemical specification is (2S,3R)-L-threonine.

Hydrophobic side chains

Nonpolar amino acid interactions are the primary driving force behind the processes that fold proteins into their functional three dimensional structures.[31] None of these amino acids' side chains ionize easily, and therefore do not have pKas, with the exception of tyrosine (Tyr, Y). The hydroxyl of tyrosine can deprotonate at high pH forming the negatively charged phenolate. Because of this one could place tyrosine into the polar, uncharged amino acid category, but its very low solubility in water matches the characteristics of hydrophobic amino acids well.

Special case side chains

Several side chains are not described well by the charged, polar and hydrophobic categories.

disulphide bonds, with other cysteines. These bonds influence the folding and stability of proteins, and are essential in the formation of antibodies. Proline (Pro, P) has an alkyl side chain and could be considered hydrophobic, but because the side chain joins back onto the alpha amino group it becomes particularly inflexible when incorporated into proteins. Similar to glycine this influences protein structure in a way unique among amino acids. Selenocysteine (Sec, U) is a rare amino acid not directly encoded by DNA, but is incorporated into proteins via the ribosome. Selenocysteine has a lower redox potential compared to the similar cysteine, and participates in several unique enzymatic reactions.[33] Pyrrolysine (Pyl, O) is another amino acid not encoded in DNA, but synthesized into protein by ribosomes.[34]
It is found in archaeal species where it participates in the catalytic activity of several methyltransferases.

β- and γ-amino acids

Amino acids with the structure NH+3−CXY−CXY−CO−2, such as

β-alanine, a component of carnosine and a few other peptides, are β-amino acids. Ones with the structure NH+3−CXY−CXY−CXY−CO−2 are γ-amino acids, and so on, where X and Y are two substituents (one of which is normally H).[6]

Zwitterions

Main article: Zwitterion
Ionization and Brønsted character of N-terminal amino, C-terminal carboxylate, and side chains of amino acid residues

The common natural forms of amino acids have a

zwitterionic
structure, with −NH+3 (−NH+2− in the case of proline) and −CO−2 functional groups attached to the same C atom, and are thus α-amino acids, and are the only ones found in proteins during translation in the ribosome. In aqueous solution at pH close to neutrality, amino acids exist as zwitterions, i.e. as dipolar ions with both NH+3 and CO−2 in charged states, so the overall structure is NH+3−CHR−CO−2. At physiological pH the so-called "neutral forms" −NH2−CHR−CO2H are not present to any measurable degree.[35] Although the two charges in the zwitterion structure add up to zero it is misleading to call a species with a net charge of zero "uncharged".

In strongly acidic conditions (pH below 3), the carboxylate group becomes protonated and the structure becomes an ammonio carboxylic acid, NH+3−CHR−CO2H. This is relevant for enzymes like pepsin that are active in acidic environments such as the mammalian stomach and

lysosomes
, but does not significantly apply to intracellular enzymes. In highly basic conditions (pH greater than 10, not normally seen in physiological conditions), the ammonio group is deprotonated to give NH2−CHR−CO−2.

Although various definitions of acids and bases are used in chemistry, the only one that is useful for chemistry in aqueous solution is that of Brønsted:[36][37] an acid is a species that can donate a proton to another species, and a base is one that can accept a proton. This criterion is used to label the groups in the above illustration. The carboxylate side chains of aspartate and glutamate residues are the principal Brønsted bases in proteins. Likewise, lysine, tyrosine and cysteine will typically act as a Brønsted acid. Histidine under these conditions can act both as a Brønsted acid and a base.

Isoelectric point

Composite of titration curves of twenty proteinogenic amino acids grouped by side chain category

For amino acids with uncharged side-chains the zwitterion predominates at pH values between the two pKa values, but coexists in equilibrium with small amounts of net negative and net positive ions. At the midpoint between the two pKa values, the trace amount of net negative and trace of net positive ions balance, so that average net charge of all forms present is zero.[38] This pH is known as the isoelectric point pI, so pI = 1/2(pKa1 + pKa2).

For amino acids with charged side chains, the pKa of the side chain is involved. Thus for aspartate or glutamate with negative side chains, the terminal amino group is essentially entirely in the charged form −NH+3, but this positive charge needs to be balanced by the state with just one C-terminal carboxylate group is negatively charged. This occurs halfway between the two carboxylate pKa values: pI = 1/2(pKa1 + pKa(R)), where pKa(R) is the side chain pKa.[37]

Similar considerations apply to other amino acids with ionizable side-chains, including not only glutamate (similar to aspartate), but also cysteine, histidine, lysine, tyrosine and arginine with positive side chains.

Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isoelectric point, and some amino acids (in particular, with nonpolar side chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point.

Physicochemical properties

The 20 canonical amino acids can be classified according to their properties. Important factors are charge,

hydrophobicity scales of amino acid residues.[40]

Some amino acids have special properties. Cysteine can form covalent

disulfide bonds to other cysteine residues. Proline forms a cycle
to the polypeptide backbone, and glycine is more flexible than other amino acids.

Glycine and proline are strongly present within low complexity regions of both eukaryotic and prokaryotic proteins, whereas the opposite is the case with cysteine, phenylalanine, tryptophan, methionine, valine, leucine, isoleucine, which are highly reactive, or complex, or hydrophobic.[39][41][42]

Many proteins undergo a range of

posttranslational modifications, whereby additional chemical groups are attached to the amino acid residue side chains sometimes producing lipoproteins (that are hydrophobic),[43] or glycoproteins (that are hydrophilic)[44] allowing the protein to attach temporarily to a membrane. For example, a signaling protein can attach and then detach from a cell membrane, because it contains cysteine residues that can have the fatty acid palmitic acid added to them and subsequently removed.[45]

Table of standard amino acid abbreviations and properties

"Amino acid code" redirects here. For base-pair encoding of amino acids, see Genetic code § Codons.
Main article: Proteinogenic amino acid

Although one-letter symbols are included in the table, IUPAC–IUBMB recommend[6] that "Use of the one-letter symbols should be restricted to the comparison of long sequences".

The one-letter notation was chosen by IUPAC-IUB based on the following rules:[46]

  • Initial letters are used where there is no ambuiguity: C cysteine, H histidine, I isoleucine, M methionine, S serine, V valine,[46]
  • Where arbitrary assignment is needed, the structurally simpler amino acids are given precedence: A Alanine, G glycine, L leucine, P proline, T threonine,[46]
  • F PHenylalanine and R aRginine are assigned by being phonetically suggestive,[46]
  • W tryptophane is assigned based on the double ring being visually suggestive to the bulky letter W,[46]
  • K lysine and Y tyrosine are assigned as alphabetically nearest to their initials L and T (note that U was avoided for its similarity with V, while X was reserved for undetermined or atypical amino acids); for tyrosine the mnemonic tYrosine was also proposed,[47]
  • D aspartate was assigned arbitrarily, with the proposed mnemonic asparDic acid;[48] E glutamate was assigned in alphabetical sequence being larger by merely one methylene –CH2– group,[47]
  • N asparagine was assigned arbitrarily, with the proposed mnemonic asparagiNe;[48] Q glutamine was assigned in alphabetical sequence of those still available (note again that O was avoided due to similarity with D), with the proposed mnemonic Qlutamine.[48]
Amino acid 3- and 1-letter symbols Side chain
Hydropathy
index[49]
Molar absorptivity[50]
 Molecular mass Abundance in
proteins (%)[51] 		Standard genetic coding,
IUPAC notation
3 1 Class Chemical polarity[52] Net charge
at pH 7.4[52] 		Wavelength,
λmax (nm) 		Coefficient ε
(mM−1·cm−1)
Alanine Ala A Aliphatic Nonpolar Neutral 1.8 89.094 8.76 GCN
Arginine Arg R Fixed cation Basic polar Positive −4.5 174.203 5.78 MGR, CGY[53]
Asparagine Asn N Amide Polar Neutral −3.5 132.119 3.93 AAY
Aspartate
 Asp D Anion
Brønsted base
 Negative −3.5 133.104 5.49 GAY
Cysteine Cys C Thiol Brønsted acid Neutral 2.5 250 0.3 121.154 1.38 UGY
Glutamine Gln Q Amide Polar Neutral −3.5 146.146 3.9 CAR
Glutamate
 Glu E Anion Brønsted base Negative −3.5 147.131 6.32 GAR
Glycine Gly G Aliphatic Nonpolar Neutral −0.4 75.067 7.03 GGN
Histidine His H Cationic Brønsted acid and base Positive, 10%
Neutral, 90% 		−3.2 211 5.9 155.156 2.26 CAY
Isoleucine Ile I Aliphatic Nonpolar Neutral 4.5 131.175 5.49 AUH
Leucine Leu L Aliphatic Nonpolar Neutral 3.8 131.175 9.68 YUR, CUY[54]
Lysine Lys K Cation Brønsted acid Positive −3.9 146.189 5.19 AAR
Methionine Met M Thioether Nonpolar Neutral 1.9 149.208 2.32 AUG
Phenylalanine Phe F Aromatic Nonpolar Neutral 2.8 257, 206, 188 0.2, 9.3, 60.0 165.192 3.87 UUY
Proline Pro P Cyclic Nonpolar Neutral −1.6 115.132 5.02 CCN
Serine Ser S Hydroxylic Polar Neutral −0.8 105.093 7.14 UCN, AGY
Threonine Thr T Hydroxylic Polar Neutral −0.7 119.119 5.53 ACN
Tryptophan Trp W Aromatic Nonpolar Neutral −0.9 280, 219 5.6, 47.0 204.228 1.25 UGG
Tyrosine Tyr Y Aromatic Brønsted acid Neutral −1.3 274, 222, 193 1.4, 8.0, 48.0 181.191 2.91 UAY
Valine Val V Aliphatic Nonpolar Neutral 4.2 117.148 6.73 GUN

Two additional amino acids are in some species coded for by

codons that are usually interpreted as stop codons
:

21st and 22nd amino acids 3-letter 1-letter Molecular mass
Selenocysteine Sec U 168.064
Pyrrolysine Pyl O 255.313

In addition to the specific amino acid codes, placeholders are used in cases where chemical or crystallographic analysis of a peptide or protein cannot conclusively determine the identity of a residue. They are also used to summarize conserved protein sequence motifs. The use of single letters to indicate sets of similar residues is similar to the use of abbreviation codes for degenerate bases.[55][56]

Ambiguous amino acids 3-letter 1-letter Amino acids included Codons included
Any / unknown Xaa X All NNN
aspartate
 Asx B D, N RAY
glutamate
 Glx Z E, Q SAR
Leucine or isoleucine Xle J I, L YTR, ATH, CTY[57]
Hydrophobic
 Φ V, I, L, F, W, Y, M NTN, TAY, TGG
Aromatic
 Ω F, W, Y, H YWY, TTY, TGG[58]
Aliphatic
(non-aromatic) 		Ψ V, I, L, M VTN, TTR[59]
Small π P, G, A, S BCN, RGY, GGR
Hydrophilic
 ζ S, T, H, N, Q, E, D, K, R VAN, WCN, CGN, AGY[60]
Positively-charged
 + K, R, H ARR, CRY, CGR
Negatively-charged
 − D, E GAN

Unk is sometimes used instead of Xaa, but is less standard.

Ter or * (from termination) is used in notation for mutations in proteins when a stop codon occurs. It corresponds to no amino acid at all.[61]

In addition, many

photomethionine (pMet).[62]

Occurrence and functions in biochemistry

polypeptide is an unbranched chain of amino acids.
Diagrammatic comparison of the structures of β-alanine and α-alanine
β-Alanine and its α-alanine isomer
A diagram showing the structure of selenocysteine
The amino acid selenocysteine

Proteinogenic amino acids

Main article:
Posttranslational modification

Amino acids are the precursors to proteins.[25] They join by condensation reactions to form short polymer chains called peptides or longer chains called either polypeptides or proteins. These chains are linear and unbranched, with each amino acid residue within the chain attached to two neighboring amino acids. In nature, the process of making proteins encoded by DNA/RNA genetic material is called translation and involves the step-by-step addition of amino acids to a growing protein chain by a ribozyme that is called a ribosome.[63] The order in which the amino acids are added is read through the genetic code from an mRNA template, which is an RNA copy of one of the organism's genes.

Twenty-two amino acids are naturally incorporated into polypeptides and are called

proteinogenic or natural amino acids.[27] Of these, 20 are encoded by the universal genetic code. The remaining 2, selenocysteine and pyrrolysine, are incorporated into proteins by unique synthetic mechanisms. Selenocysteine is incorporated when the mRNA being translated includes a SECIS element, which causes the UGA codon to encode selenocysteine instead of a stop codon.[64] Pyrrolysine is used by some methanogenic archaea in enzymes that they use to produce methane. It is coded for with the codon UAG, which is normally a stop codon in other organisms.[65] This UAG codon is followed by a PYLIS downstream sequence.[66]

Several independent evolutionary studies have suggested that Gly, Ala, Asp, Val, Ser, Pro, Glu, Leu, Thr may belong to a group of amino acids that constituted the early genetic code, whereas Cys, Met, Tyr, Trp, His, Phe may belong to a group of amino acids that constituted later additions of the genetic code.[67][68][69]

Standard vs nonstandard amino acids

The 20 amino acids that are encoded directly by the codons of the universal genetic code are called standard or canonical amino acids. A modified form of methionine (N-formylmethionine) is often incorporated in place of methionine as the initial amino acid of proteins in bacteria, mitochondria and chloroplasts. Other amino acids are called nonstandard or non-canonical. Most of the nonstandard amino acids are also non-proteinogenic (i.e. they cannot be incorporated into proteins during translation), but two of them are proteinogenic, as they can be incorporated translationally into proteins by exploiting information not encoded in the universal genetic code.

The two nonstandard proteinogenic amino acids are selenocysteine (present in many non-eukaryotes as well as most eukaryotes, but not coded directly by DNA) and

bacterium). The incorporation of these nonstandard amino acids is rare. For example, 25 human proteins include selenocysteine in their primary structure,[70] and the structurally characterized enzymes (selenoenzymes) employ selenocysteine as the catalytic moiety in their active sites.[71] Pyrrolysine and selenocysteine are encoded via variant codons. For example, selenocysteine is encoded by stop codon and SECIS element.[72][73][74]

non-proteinogenic amino acids.[75][76][77]

Non-proteinogenic amino acids

Main article: Non-proteinogenic amino acids

Aside from the 22

GABA, levothyroxine) or are not produced directly and in isolation by standard cellular machinery. For example, hydroxyproline , is synthesised from proline. Another example is selenomethionine
).

Non-proteinogenic amino acids that are found in proteins are formed by post-translational modification. Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a phospholipid membrane.[78] Examples:

Some non-proteinogenic amino acids are not found in proteins. Examples include

beta alanine (3-aminopropanoic acid), which is used in plants and microorganisms in the synthesis of pantothenic acid (vitamin B5), a component of coenzyme A.[83]

In mammalian nutrition

Diagram showing the relative occurrence of amino acids in blood serum as obtained from diverse diets.
Share of amino acid in various human diets and the resulting mix of amino acids in human blood serum. Glutamate and glutamine are the most frequent in food at over 10%, while alanine, glutamine, and glycine are the most common in blood.
Main article: Essential amino acid
Further information: Protein (nutrient) and Amino acid synthesis

Amino acids are not typical component of food: animals eat proteins. The protein is broken down into amino acids in the process of digestion. They are then used to synthesize new proteins, other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy.[84] The oxidation pathway starts with the removal of the amino group by a transaminase; the amino group is then fed into the urea cycle. The other product of transamidation is a keto acid that enters the citric acid cycle.[85] Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.[86]

Of the 20 standard amino acids, nine (His, Ile, Leu, Lys, Met, Phe, Thr, Trp and Val) are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food.[87][88][89]

Semi-essential and conditionally essential amino acids, and juvenile requirements

In addition, cysteine, tyrosine, and arginine are considered semiessential amino acids, and taurine a semi-essential aminosulfonic acid in children. Some amino acids are conditionally essential for certain ages or medical conditions. Essential amino acids may also vary from species to species.[d] The metabolic pathways that synthesize these monomers are not fully developed.[90][91]

Non-protein functions

Biosynthetic pathways for catecholamines and trace amines in the human brain[92][93][94]
The image above contains clickable links
Catecholamines and trace amines are synthesized from phenylalanine and tyrosine in humans.
Further information: Amino acid neurotransmitter

Many proteinogenic and non-proteinogenic amino acids have biological functions beyond being precursors to proteins and peptides.In humans, amino acids also have important roles in diverse biosynthetic pathways. Defenses against herbivores in plants sometimes employ amino acids.[95] Examples:

Standard amino acids

Roles for nonstandard amino acids

Uses in industry

Animal feed

Amino acids are sometimes added to

animal feed because some of the components of these feeds, such as soybeans, have low levels of some of the essential amino acids, especially of lysine, methionine, threonine, and tryptophan.[107] Likewise amino acids are used to chelate metal cations in order to improve the absorption of minerals from feed supplements.[108]

Food

The

artificial sweetener.[110] Amino acids are sometimes added to food by manufacturers to alleviate symptoms of mineral deficiencies, such as anemia, by improving mineral absorption and reducing negative side effects from inorganic mineral supplementation.[111]

Chemical building blocks

Further information:
Asymmetric synthesis

Amino acids are low-cost

chiral pool synthesis as enantiomerically pure building blocks.[112][113]

Amino acids are used in the synthesis of some cosmetics.[107]

Aspirational uses

Fertilizer

The chelating ability of amino acids is sometimes used in fertilizers to facilitate the delivery of minerals to plants in order to correct mineral deficiencies, such as iron chlorosis. These fertilizers are also used to prevent deficiencies from occurring and to improve the overall health of the plants.[114]

Biodegradable plastics

Further information: Biodegradable plastic and Biopolymer

Amino acids have been considered as components of biodegradable polymers, which have applications as

chelate metal ions, polyaspartate is also being used as a biodegradable antiscaling agent and a corrosion inhibitor.[117][118]

Synthesis

Main article: Amino acid synthesis
For the steps in the reaction, see the text.
The Strecker amino acid synthesis

Chemical synthesis

The commercial production of amino acids usually relies on mutant bacteria that overproduce individual amino acids using glucose as a carbon source. Some amino acids are produced by enzymatic conversions of synthetic intermediates.

fumarate using a lyase.[111]

Biosynthesis

In plants, nitrogen is first assimilated into organic compounds in the form of

glutamate, formed from alpha-ketoglutarate and ammonia in the mitochondrion. For other amino acids, plants use transaminases to move the amino group from glutamate to another alpha-keto acid. For example, aspartate aminotransferase converts glutamate and oxaloacetate to alpha-ketoglutarate and aspartate.[119]
Other organisms use transaminases for amino acid synthesis, too.

Nonstandard amino acids are usually formed through modifications to standard amino acids. For example,

post translational modification of proline.[121]

1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is an intermediate in the production of the plant hormone ethylene.[123]

Primordial synthesis

The formation of amino acids and peptides are assumed to precede and perhaps induce the emergence of life on earth. Amino acids can form from simple precursors under various conditions.[124] Surface-based chemical metabolism of amino acids and very small compounds may have led to the build-up of amino acids, coenzymes and phosphate-based small carbon molecules.[125][additional citation(s) needed] Amino acids and similar building blocks could have been elaborated into proto-peptides, with peptides being considered key players in the origin of life.[126]

In the famous

Strecker synthesis whereby hydrogen cyanide, simple aldehydes, ammonia, and water produce amino acids.[124]

According to a review, amino acids, and even peptides, "turn up fairly regularly in the

Codon–amino acids mappings may be the biological information system at the primordial origin of life on Earth.[128] While amino acids and consequently simple peptides must have formed under different experimentally probed geochemical scenarios, the transition from an abiotic world to the first life forms is to a large extent still unresolved.[129]

Reactions

Amino acids undergo the reactions expected of the constituent functional groups.[130][131]

Peptide bond formation

See also: Peptide synthesis and Peptide bond
Two amino acids are shown next to each other. One loses a hydrogen and oxygen from its carboxyl group (COOH) and the other loses a hydrogen from its amino group (NH2). This reaction produces a molecule of water (H2O) and two amino acids joined by a peptide bond (–CO–NH–). The two joined amino acids are called a dipeptide.
The condensation of two amino acids to form a dipeptide. The two amino acid residues are linked through a peptide bond.

As both the amine and carboxylic acid groups of amino acids can react to form amide bonds, one amino acid molecule can react with another and become joined through an amide linkage. This polymerization of amino acids is what creates proteins. This condensation reaction yields the newly formed peptide bond and a molecule of water. In cells, this reaction does not occur directly; instead, the amino acid is first activated by attachment to a transfer RNA molecule through an ester bond. This aminoacyl-tRNA is produced in an ATP-dependent reaction carried out by an aminoacyl tRNA synthetase.[132] This aminoacyl-tRNA is then a substrate for the ribosome, which catalyzes the attack of the amino group of the elongating protein chain on the ester bond.[133] As a result of this mechanism, all proteins made by ribosomes are synthesized starting at their N-terminus and moving toward their C-terminus.

However, not all peptide bonds are formed in this way. In a few cases, peptides are synthesized by specific enzymes. For example, the tripeptide

glutamate through a peptide bond formed between the side chain carboxyl of the glutamate (the gamma carbon of this side chain) and the amino group of the cysteine. This dipeptide is then condensed with glycine by glutathione synthetase to form glutathione.[135]

In chemistry, peptides are synthesized by a variety of reactions. One of the most-used in solid-phase peptide synthesis uses the aromatic oxime derivatives of amino acids as activated units. These are added in sequence onto the growing peptide chain, which is attached to a solid resin support.[136] Libraries of peptides are used in drug discovery through high-throughput screening.[137]

The combination of functional groups allow amino acids to be effective polydentate ligands for metal–amino acid chelates.[138] The multiple side chains of amino acids can also undergo chemical reactions.

Catabolism

lipid synthesis
.
* Amino acids catabolized into both glucogenic and ketogenic products.

Degradation of an amino acid often involves

glutamate. This process involves transaminases, often the same as those used in amination during synthesis. In many vertebrates, the amino group is then removed through the urea cycle and is excreted in the form of urea. However, amino acid degradation can produce uric acid or ammonia instead. For example, serine dehydratase converts serine to pyruvate and ammonia.[100] After removal of one or more amino groups, the remainder of the molecule can sometimes be used to synthesize new amino acids, or it can be used for energy by entering glycolysis or the citric acid cycle
, as detailed in image at right.

Complexation

Amino acids are bidentate ligands, forming transition metal amino acid complexes.[140]

Chemical analysis

The total nitrogen content of organic matter is mainly formed by the amino groups in proteins. The Total Kjeldahl Nitrogen (

TKN) is a measure of nitrogen widely used in the analysis of (waste) water, soil, food, feed and organic matter in general. As the name suggests, the Kjeldahl method is applied. More sensitive methods are available.[141][142]

See also

Notes

  1. ^ The late discovery is explained by the fact that cysteine becomes oxidized to cystine in air.
  2. ^ Proline and other cyclic amino acids are an exception to this general formula. Cyclization of the α-amino acid creates the corresponding secondary amine. These are occasionally referred to as imino acids.
  3. ^ The L and D convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of glyceraldehyde from which that amino acid can, in theory, be synthesized (D-glyceraldehyde is dextrorotatory; L-glyceraldehyde is levorotatory). An alternative convention is to use the (S) and (R) designators to specify the absolute configuration.[29] Almost all of the amino acids in proteins are (S) at the α carbon, with cysteine being (R) and glycine non-chiral.[30] Cysteine has its side chain in the same geometric location as the other amino acids, but the R/S terminology is reversed because sulfur has higher atomic number compared to the carboxyl oxygen which gives the side chain a higher priority by the Cahn-Ingold-Prelog sequence rules.
  4. microbes in the first two stomach chambers
    .

References

  1. ISBN 0-7167-4339-6
    .
  2. PMID 31691799
    .
  3. ^ Richard Cammack, ed. (2009). "Newsletter 2009". Biochemical Nomenclature Committee of IUPAC and NC-IUBMB. Pyrrolysine. Archived from the original on 12 September 2017. Retrieved 16 April 2012.
  4. PMID 20847933
    .
  5. ^ Latham MC (1997). "Chapter 8. Body composition, the functions of food, metabolism and energy". Human nutrition in the developing world. Food and Nutrition Series – No. 29. Rome: Food and Agriculture Organization of the United Nations. Archived from the original on 8 October 2012. Retrieved 9 September 2012.
  6. ^ a b c "Nomenclature and Symbolism for Amino Acids and Peptides". IUPAC-IUB Joint Commission on Biochemical Nomenclature. 1983. Archived from the original on 9 October 2008. Retrieved 17 November 2008.
  7. doi:10.1021/cr60033a001
    .
  8. ^ Hansen S (May 2015). "Die Entdeckung der proteinogenen Aminosäuren von 1805 in Paris bis 1935 in Illinois" (PDF) (in German). Berlin. Archived from the original (PDF) on 1 December 2017.
  9. ^ Vauquelin LN, Robiquet PJ (1806). "The discovery of a new plant principle in Asparagus sativus". Annales de Chimie. 57: 88–93.
  10. ^
    ISBN 978-0-12-034226-6
    .
  11. S2CID 110151163
    .
  12. ^ Baumann E (1884). "Über cystin und cystein". Z Physiol Chem. 8 (4): 299–305. Archived from the original on 14 March 2011. Retrieved 28 March 2011.
  13. ^ Braconnot HM (1820). "Sur la conversion des matières animales en nouvelles substances par le moyen de l'acide sulfurique". Annales de Chimie et de Physique. 2nd Series. 13: 113–125.
  14. PMID 12218068. Archived
    from the original on 10 June 2019. Retrieved 4 July 2015.
  15. doi:10.1016/S0021-9258(18)74986-7
    .
  16. ^ Menten, P. Dictionnaire de chimie: Une approche étymologique et historique. De Boeck, Bruxelles. link Archived 28 December 2019 at the Wayback Machine.
  17. ^ Harper D. "amino-". Online Etymology Dictionary. Archived from the original on 2 December 2017. Retrieved 19 July 2010.
  18. doi:10.1002/cber.189402701205. Archived from the original
    on 25 July 2020.
  19. ISBN 978-0-87169-191-0
    .
  20. ^ "Alpha amino acid". Merriam-Webster Medical. Archived from the original on 3 January 2015. Retrieved 3 January 2015..
  21. ^ Clark, Jim (August 2007). "An introduction to amino acids". chemguide. Archived from the original on 30 April 2015. Retrieved 4 July 2015.
  22. ISBN 9783527621170. Archived
    from the original on 17 May 2016. Retrieved 5 January 2016 – via Google Books.
  23. S2CID 3705304
    .
  24. S2CID 5359077.Closed access icon
  25. ^ a b "Chapter 1: Proteins are the Body's Worker Molecules". The Structures of Life. National Institute of General Medical Sciences. 27 October 2011. Archived from the original on 7 June 2014. Retrieved 20 May 2008.
  26. ISBN 978-0-470-14684-2
    .
  27. ^
    ISBN 978-0-7167-7030-5
    .
  28. S2CID 254088816
    .
  29. doi:10.1002/anie.196603851
    .
  30. ^ Hatem SM (2006). "Gas chromatographic determination of Amino Acid Enantiomers in tobacco and bottled wines". University of Giessen. Archived from the original on 22 January 2009. Retrieved 17 November 2008.
  31. ^
    OCLC 297392560
    .
  32. S2CID 265572280
    .
  33. PMID 17508906
    .
  34. S2CID 35519996
    .
  35. ISBN 978-0126654509
    .
  36. doi:10.1002/recl.19230420815
    .
  37. ^
    OCLC 61448218
    .
  38. ISBN 978-0-8247-9691-4
    .
  39. ^
    PMID 31504783
    .
  40. doi:10.1016/S0009-2614(04)01565-9
    .
  41. PMID 10512723
    .
  42. PMID 20962881
    .
  43. PMID 15780596
    .
  44. PMID 17500024
    .
  45. PMID 15189153
    .
  46. ^
    doi:10.1016/S0021-9258(19)34176-6
    .
  47. ^
    doi:10.1016/S0307-4412(97)00167-2
    .
  48. ^
    doi:10.1016/0307-4412(88)90026-X
    .
  49. PMID 7108955
    .
  50. ISBN 978-0-7167-1315-9.[page needed
    ]
  51. PMID 27789699
    .
  52. ^
    ISBN 978-0-87893-214-6
    .
  53. ^ Codons can also be expressed by: CGN, AGR
  54. ^ codons can also be expressed by: CUN, UUR
  55. PMID 11911894
    .
  56. PMID 5080161
    .
  57. ^ Codons can also be expressed by: CTN, ATH, TTR; MTY, YTR, ATA; MTY, HTA, YTG
  58. ^ Codons can also be expressed by: TWY, CAY, TGG
  59. ^ Codons can also be expressed by: NTR, VTY
  60. ^ Codons can also be expressed by: VAN, WCN, MGY, CGP
  61. ^ "HGVS: Sequence Variant Nomenclature, Protein Recommendations". Archived from the original on 24 September 2021. Retrieved 23 September 2021.
  62. PMID 15782218
    .
  63. PMID 17157507
    .
  64. PMID 12524431
    .
  65. PMID 16256420
    .
  66. PMID 16164991
    .
  67. PMID 1057181
    .
  68. PMID 11164045
    .
  69. S2CID 9039622
    .
  70. S2CID 10363908. Archived
    from the original on 23 July 2018. Retrieved 12 June 2019.
  71. S2CID 1944741
    .
  72. ^ Tjong H (2008). Modeling Electrostatic Contributions to Protein Folding and Binding (PhD thesis). Florida State University. p. 1 footnote. Archived from the original on 28 January 2020. Retrieved 28 January 2020.
  73. ISSN 1574-0889. Archived
    from the original on 14 April 2021. Retrieved 5 January 2016.
  74. ^ Elzanowski A, Ostell J (7 April 2008). "The Genetic Codes". National Center for Biotechnology Information (NCBI). Archived from the original on 20 August 2016. Retrieved 10 March 2010.
  75. PMID 16260173
    .
  76. PMID 19318213
    .
  77. ISBN 978-0-387-22046-8
    .
  78. PMID 8129952
    .
  79. PMID 2183788
    .
  80. S2CID 7211864
    .
  81. PMID 16452303
    .
  82. S2CID 23877884
    .
  83. PMID 16042590
    .
  84. PMID 14144484
    .
  85. PMID 10736367
    .
  86. PMID 11533293
    .
  87. PMID 8064412
    .
  88. PMID 15173430
    .
  89. PMID 10867060
    .
  90. PMID 9437700
    .
  91. PMID 12514918
    .
  92. PMID 19948186
    .
  93. PMID 15860375
    .
  94. PMID 24374199
    .
  95. doi:10.1021/jf60163a003
    .
  96. PMID 18923670
    .
  97. PMID 20276176. Archived
    from the original on 7 May 2022. Retrieved 3 November 2008.
  98. PMID 18815130
    .
  99. PMID 16709566
    .
  100. ^
    ISBN 978-0-7167-4684-3
    .
  101. S2CID 84891972
    .
  102. S2CID 2563606. Archived
    from the original on 13 April 2020. Retrieved 12 June 2019.
  103. S2CID 33603501
    .
  104. S2CID 26273053
    .
  105. S2CID 3144019
    .
  106. PMID 7665380. Archived
    from the original on 7 May 2022. Retrieved 7 May 2022.
  107. ^
    S2CID 24161808
    .
  108. ^ Ashmead HD (1993). The Role of Amino Acid Chelates in Animal Nutrition. Westwood: Noyes Publications.
  109. PMID 10736350
    .
  110. PMID 3300262
    .
  111. ^
    ISBN 978-3527306732
    .
  112. S2CID 43992655
    .
  113. doi:10.1021/cr00013a009
    .
  114. ^ Ashmead HD (1986). Foliar Feeding of Plants with Amino Acid Chelates. Park Ridge: Noyes Publications.
  115. doi:10.1002/(SICI)1521-3935(19991201)200:12<2651::AID-MACP2651>3.0.CO;2-P
    .
  116. PMID 12161646. Archived
    from the original on 25 July 2020. Retrieved 12 June 2019.
  117. ^ Low KC, Wheeler AP, Koskan LP (1996). Commercial poly(aspartic acid) and Its Uses. Advances in Chemistry Series. Vol. 248. Washington, D.C.: American Chemical Society.
  118. S2CID 94818855
    .
  119. ISBN 978-0-943088-39-6
    .
  120. PMID 16702333
    .
  121. PMID 9559057
    .
  122. S2CID 24638475
    .
  123. PMID 12324528
    .
  124. ^
    S2CID 102659869
    .
  125. PMID 28684991
    .
  126. ^
    S2CID 211536416
    .
  127. PMID 31641431
    .
  128. PMID 35743865
    .
  129. S2CID 227037164
    .
  130. ISBN 978-0-521-46827-5
    .
  131. PMID 16214343
    .
  132. PMID 11375928
    .
  133. PMID 4896351
    .
  134. PMID 14988435
    .
  135. PMID 3053703
    .
  136. doi:10.1021/ja00063a082
    .
  137. PMID 18855697
    .
  138. S2CID 94822047
    .
  139. ^ Stipanuk MH (2006). Biochemical, physiological, & molecular aspects of human nutrition (2nd ed.). Saunders Elsevier.
  140. PMID 29712039
    .
  141. PMID 23959242
    .
  142. doi:10.4141/S01-054
    .

Further reading

External links


Retrieved from "https://en.wikipedia.org/w/index.php?title=Amino_acid&oldid=1219455737"