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Amino acids are the end product of protein. The general structure of an α-amino acid, with the amino group on the left and the carboxyl group on the right.

Amino acids are the building blocks of proteins. In eukaryotes, there are 20 "standard" amino acids out of which almost all proteins are made.

In biochemistry, an amino acid is any molecule that has both amine (NH₂+R) and carboxyl (C=O) functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula H₂NCHRCOOH, where R is one of many side groups (see diagram).

There are hundreds

Across all forms of life, about 500 amino acids are known.^[1] The most important thing that amino acids do is to be part of proteins, which are long chains of amino acids. Every protein has its own sequence of amino acids, and that sequence makes the protein take different shapes, and have different functions. Amino acids are like the alphabet for proteins; even though you only have a few letters, if you connect them, you can make many different sentences.

Nine of the 20 standard amino acids are "essential" amino acids for humans. They cannot be built (synthesised) from other compounds by the human body. They must be taken in as food. Others may be essential for some ages or medical conditions. Essential amino acids may also differ between species. Herbivores have to get their essential amino acids from their diet, which for some is almost entirely grass. Ruminants such as cows get some amino acids via microbes in the first two stomach chambers.

Structure

An amino acid is an organic chemical. It consists of an α-carbon atom that is covalently bonded to four groups.^[2]

a hydrogen atom
an amino group (-NH₂)
a carboxyl group (-COOH)
a variable R group

Every amino acid has at least one amino group (-NH₂) and one carboxyl group (-COOH), except proline.

Gene expression and biochemistry

These are the proteinogenic amino acids, which are the building blocks for proteins. They are produced by cellular machinery coded for in the genetic code of any organism.^[3]

Amino Acid	Short	Abbrev.	Codon(s)	Occurrence in human proteins (%)	Essential in humans
Alanine	A	Ala	GCU, GCC, GCA, GCG	7.8	No
Cysteine	C	Cys	UGU, UGC	1.9	Conditionally
Aspartic acid	D	Asp	GAU, GAC	5.3	No
Glutamic acid	E	Glu	GAA, GAG	6.3	Conditionally
Phenylalanine	F	Phe	UUU, UUC	3.9	Yes
Glycine	G	Gly	GGU, GGC, GGA, GGG	7.2	Conditionally
Histidine	H	His	CAU, CAC	2.3	Yes
Isoleucine	I	Ile	AUU, AUC, AUA	5.3	Yes
Lysine	K	Lys	AAA, AAG	5.9	Yes
Leucine	L	Leu	UUA, UUG, CUU, CUC, CUA, CUG	9.1	Yes
Methionine	M	Met	AUG	2.3	Yes
Asparagine	N	Asn	AAU, AAC	4.3	No
Pyrrolysine	O	Pyl	UAG*	0	No
Proline	P	Pro	CCU, CCC, CCA, CCG	5.2	No
Glutamine	Q	Gln	CAA, CAG	4.2	No
Arginine	R	Arg	CGU, CGC, CGA, CGG, AGA, AGG	5.1	Conditionally
Serine	S	Ser	UCU, UCC, UCA, UCG, AGU, AGC	6.8	No
Threonine	T	Thr	ACU, ACC, ACA, ACG	5.9	Yes
Selenocysteine	U	Sec	UGA**	>0	No
Valine	V	Val	GUU, GUC, GUA, GUG	6.6	Yes
Tryptophan	W	Trp	UGG	1.4	Yes
Tyrosine	Y	Tyr	UAU, UAC	3.2	Conditionally
Stop codon†	-	Term	UAA, UAG, UGA††	-	-

* UAG is normally the amber stop codon, but encodes pyrrolysine if a PYLIS element is present.
** UGA is normally the opal (or umber) stop codon, but encodes selenocysteine if a SECIS element is present.
† The stop codon is not an amino acid, but is included for completeness.
†† UAG and UGA do not always act as stop codons (see above).
‡ An essential amino acid cannot be synthesized in humans. It must be supplied in the diet. Conditionally essential amino acids are not normally required in the diet, but must be supplied to populations which do not make enough of it.