The generic structure of an alpha amino acid in its unionized form

Amino acids (,, or ) are biologically important molecules made from amine (- NH < sub > 2 </ sub >) and carboxylic acid (- COOH ) functional groups, along with a side-chain specific to each amino acid.

The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen.

Structurally they can be classified according to the functional groups ' locations as alpha-( α -), beta-( β -), gamma-( γ -) or delta-( δ -) amino acids ; other categories relate to polarity, acid / base / neutral, and side chain group type ( including: aliphatic, acyclic, hydroxyl or sulphur-containing, aromatic ).

It lacks the NH < sub > 2 </ sub > group because of the cyclization of the side-chain and is known as an imino acid ; it falls under the category of special structured amino acids .</ ref > where R is an organic substituent known as a " side-chain "); often the term " amino acid " is used to refer specifically to these.

For example the standard glutamic acid ( glutamate ) and the non-standard gamma-amino acid gamma-amino-butyric acid ( GABA ) are respectively the brain's main excitatory and inhibitory neurotransmitters, hydroxyproline-a major component of the connective tissue collagen-is synthesised from proline, the standard amino acid glycine is used to synthesise porphyrins used in red blood cells, and the non-standard carnitine is used in lipid transport.

In 1806, the French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus that was subsequently named asparagine, the first amino acid to be discovered.

Another amino acid that was discovered in the early 19th century was cystine, in 1810, although its monomer, cysteine, was discovered much later, in 1884.

The side-chain can make an amino acid a weak acid or a weak base, and a hydrophile if the side-chain is polar or a hydrophobe if it is nonpolar.

Proline is the only proteinogenic amino acid whose side-group links to the α-amino group and, thus, is also the only proteinogenic amino acid containing a secondary amine at this position.

In chemical terms, proline is, therefore, an imino acid, since it lacks a primary amino group, although it is still classed as an amino acid in the current biochemical nomenclature, and may also be called an " N-alkylated alpha-amino acid ".

The primary structure of a protein simply consists of its linear sequence of amino acids ; for instance, " alanine-glycine-tryptophan-serine-glutamate-asparagine-glycine-lysine -…".

It consists of a triple helix made of the repetitious amino acid sequence glycine-X-Y, where X and Y are frequently proline or hydroxyproline.

Each codon consists of three nucleotide s, usually representing a single amino acid.

Hemoglobin consists mostly of protein subunits ( the " globin " chains ), and these proteins, in turn, are folded chains of a large number of different amino acids called polypeptides.

Autolyzed yeast ( containing the cell walls ) or autolyzed yeast extract consists of concentrations of yeast cells that are allowed to die and break up, so that the yeasts ’ endogenous digestive enzymes break their proteins down into simpler compounds ( amino acids and peptides ).

Like other nuclear receptors, the androgen receptor protein consists of several functional domains: the transactivation domain ( also called the transcription-regulation domain or the amino / NH2-terminal domain ), the DNA-binding domain, the hinge region, and the steroid-binding domain ( also called the carboxyl-terminal ligand-binding domain ).

ACTH consists of 39 amino acids, the first 13 of which ( counting from the N-terminus ) may be cleaved to form α-melanocyte-stimulating hormone ( α-MSH ).

Its primary structure consists of a chain of about 100 amino acids.

The myophosphorylase structure consists of 842 amino acids.

* Polyhistidine-tag, an amino acid motif in proteins that consists of at least five histidine ( His ) residues

The primary structure of renin precursor consists of 406 amino acids with a pre-and a pro-segment carrying 20 and 46 amino acids, respectively.

The nutrient solution consists of water and electrolytes ; glucose, amino acids, and lipids ; essential vitamins, minerals and trace elements are added or given separately.

It consists of a benzene ring substituted with an amino group and a carboxyl group.

For proteins, a prediction consists of assigning regions of the amino acid sequence as likely alpha helices, beta strands ( often noted as " extended " conformations ), or turns.

In polypeptides, the backbone consists of carbon and nitrogen atoms of the constituent amino acids.

Each codon consists of three nucleotide s, usually representing a single amino acid.

The catalytic domain of PRMTs consists of a SAM binding domain and substrate binding domain ( about 310 amino acids in total ).

An oligopeptide ( oligo -, " few ") consists of between 2 and 20 amino acids and includes dipeptides, tripeptides, tetrapeptides, pentapeptides, etc.

A polyhistidine-tag is an amino acid motif in proteins that consists of at least five histidine ( His ) residues, often at the N-or C-terminus of the protein.

Interstitial fluid consists of a water solvent containing sugars, salts, fatty acids, amino acids, coenzymes, hormones, neurotransmitters, as well as waste products from the cells.

Antithrombin is a glycoprotein produced by the liver and consists of 432 amino acids.

Each codon consists of three nucleotide s, usually representing a single amino acid.

In a typical scenario, an aaRS consists of a catalytic domain ( where both the above reactions take place ) and an anticodon binding domain ( which interacts mostly with the anticodon region of the tRNA and ensures binding of the correct tRNA to the amino acid ).

Almost all of the amino acids in proteins are ( S ) at the α carbon, with cysteine being ( R ) and glycine non-chiral.

Cysteine is unusual since it has a sulfur atom at the second position in its side-chain, which has a larger atomic mass than the groups attached to the first carbon, which is attached to the α-carbon in the other standard amino acids, thus the ( R ) instead of ( S ).

When taken up into the human body from the diet, the 22 standard amino acids either are used to synthesize proteins and other biomolecules or are oxidized to urea and carbon dioxide as a source of energy.

In this dehydration synthesis, a water molecule is removed and the peptide bond connects the nitrogen of one amino acid's amino group to the carbon of the other's carboxylic acid group.

If the amino group is removed from an amino acid, it leaves behind a carbon skeleton called an α-keto acid.

The carbon dioxide is bound to amino groups of the globin proteins as carbaminohemoglobin, and is thought to account for about 10 % of carbon dioxide transport in mammals.

Two percent of the carbon had formed amino acids that are used to make proteins in living cells, with glycine as the most abundant.

However, Bada noted that in current models of early Earth conditions, carbon dioxide and nitrogen ( N < sub > 2 </ sub >) create nitrites, which destroy amino acids as fast as they form.

This suggests the origin of significant amounts of amino acids may have occurred on Earth even with an atmosphere containing carbon dioxide and nitrogen.

The components used in de novo nucleotide synthesis are derived from biosynthetic precursors of carbohydrate and amino acid metabolism, and from ammonia and carbon dioxide.

Once linked in the protein chain, an individual amino acid is called a residue, and the linked series of carbon, nitrogen, and oxygen atoms are known as the main chain or protein backbone.

The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration.

The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.

The cycle converts two amino groups, one from NH < sub > 4 </ sub >< sup >+</ sup > and one from Asp, and a carbon atom from HCO < sub > 3 </ sub >< sup >−</ sup >, to the relatively nontoxic excretion product urea at the cost of four " high-energy " phosphate bonds ( 3 ATP hydrolyzed to 2 ADP and one AMP ).

These compounds, as well as the amino acids, nucleobases, and many other compounds in meteorites, carry deuterium and isotopes of carbon, nitrogen, and oxygen that are very rare on earth, attesting to their extraterrestrial origin.

Note that two natural products were already known with this imidazole riboside structure: substitution at the 5 ' carbon with OH results in pyrazomycin / pyrazofurin, an antibiotic with antiviral properties but unacceptable toxicity, and replacement with an amino group results in the natural purine synthetic precursor 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside ( AICAR ), which has only modest antiviral properties.

The rest of the amino acid is made up of mostly carbon and hydrogen, and is recycled or oxidized for energy.

Gluconeogenesis ( abbreviated GNG ) is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as pyruvate, lactate, glycerol, and glucogenic amino acids.

Transamination or deamination of amino acids facilitates entering of their carbon skeleton into the cycle directly ( as pyruvate or oxaloacetate ), or indirectly via the citric acid cycle.