Reference Table

The Twenty-Two Proteinogenic Amino Acids

Structure, dietary essentiality, chemical class, physiological role, and genetic codons for every amino acid incorporated into protein during translation — the twenty standard amino acids, plus selenocysteine (the 21st) and pyrrolysine (the 22nd).

22proteinogenic
21used by humans
9essential
6conditionally essential
6non-essential
61standard coding triplets
Showing all 20
Structure What it does Codons
Glycine Gly · G
R–H
 Conditional Nonpolar, aliphatic Smallest amino acid; provides the flexible hinge at every third residue of collagen. Precursor of creatine, glutathione, heme, and porphyrins; inhibitory neurotransmitter in the spinal cord.
DNAGGT GGC GGA GGG
mRNAGGU GGC GGA GGG
Alanine Ala · A
R–CH₃
 No Nonpolar, aliphatic Carries nitrogen from muscle to liver via the glucose–alanine cycle; a major gluconeogenic substrate released by muscle during fasting and catabolic illness.
DNAGCT GCC GCA GCG
mRNAGCU GCC GCA GCG
Valine Val · V
R–CH(CH₃)₂
 Yes Nonpolar, aliphatic (BCAA) Branched-chain amino acid oxidized directly within muscle for energy, sparing glucose during sustained exercise.
DNAGTT GTC GTA GTG
mRNAGUU GUC GUA GUG
Leucine Leu · L
R–CH₂CH(CH₃)₂
 Yes Nonpolar, aliphatic (BCAA) The principal nutritional trigger of mTORC1 and muscle protein synthesis; the key amino acid for overcoming the anabolic resistance of aging and chronic disease.
DNATTA TTG CTT CTC CTA CTG
mRNAUUA UUG CUU CUC CUA CUG
Isoleucine Ile · I
R–CH(CH₃)CH₂CH₃
 Yes Nonpolar, aliphatic (BCAA) Branched-chain amino acid oxidized in muscle as fuel; supports glucose uptake and hemoglobin synthesis.
DNAATT ATC ATA
mRNAAUU AUC AUA
Proline Pro · P
Rcyclic –(CH₂)₃– to α-N
 Conditional Nonpolar, cyclic imino acid Its ring imposes a rigid kink in the polypeptide chain. As hydroxyproline it is the structural core of collagen, underpinning tendon, fascia, and connective-tissue integrity.
DNACCT CCC CCA CCG
mRNACCU CCC CCA CCG
Methionine Met · M
R–CH₂CH₂SCH₃
 Yes Nonpolar, sulfur-containing The initiator amino acid for all protein synthesis (start codon AUG). Methyl donor via S-adenosylmethionine; precursor of cysteine and of creatine synthesis.
DNAATG
mRNAAUG
Phenylalanine Phe · F
R–CH₂–C₆H₅
 Yes Aromatic Precursor of tyrosine and thence the catecholamines (dopamine, norepinephrine, epinephrine). Accumulates toxically when hydroxylase activity is absent, as in phenylketonuria.
DNATTT TTC
mRNAUUU UUC
Tyrosine Tyr · Y
R–CH₂–C₆H₄–OH
 Conditional Aromatic Precursor of catecholamines, thyroid hormones (T₃/T₄), and melanin. Its hydroxyl group is a major site of protein phosphorylation in growth-factor signaling.
DNATAT TAC
mRNAUAU UAC
Tryptophan Trp · W
R–CH₂–indole
 Yes Aromatic Precursor of serotonin, melatonin, and niacin (NAD⁺). The least abundant amino acid; its gut metabolism generates indoxyl sulfate, a uremic toxin implicated in muscle wasting in CKD.
DNATGG
mRNAUGG
Serine Ser · S
R–CH₂OH
 No Polar, uncharged A principal phosphorylation site in signaling cascades including Akt and mTOR. Precursor of glycine, cysteine, and phospholipids; central to one-carbon metabolism.
DNATCT TCC TCA TCG AGT AGC
mRNAUCU UCC UCA UCG AGU AGC
Threonine Thr · T
R–CH(OH)CH₃
 Yes Polar, uncharged Phosphorylation site in regulatory proteins; required for mucin and immunoglobulin synthesis and thus for gut barrier integrity.
DNAACT ACC ACA ACG
mRNAACU ACC ACA ACG
Cysteine Cys · C
R–CH₂SH
 Conditional Polar, sulfur-containing Forms the disulfide bridges that stabilize protein tertiary structure. Rate-limiting precursor of glutathione, the principal intracellular antioxidant defending muscle against oxidative stress.
DNATGT TGC
mRNAUGU UGC
Asparagine Asn · N
R–CH₂CONH₂
 No Polar, uncharged (amide) Amide nitrogen carrier and the principal site of N-linked glycosylation, anchoring carbohydrate chains to proteins.
DNAAAT AAC
mRNAAAU AAC
Glutamine Gln · Q
R–CH₂CH₂CONH₂
 Conditional Polar, uncharged (amide) The most abundant free amino acid in muscle and plasma; primary fuel for enterocytes and immune cells and the body's main nitrogen shuttle. Depleted rapidly in critical illness and CKD catabolism.
DNACAA CAG
mRNACAA CAG
Lysine Lys · K
R–(CH₂)₄NH₃⁺
 Yes Basic (positively charged) Cross-links collagen via lysyl oxidase; precursor of carnitine, which transports fatty acids into mitochondria. Its ε-amino group is the attachment site for ubiquitin, tagging muscle protein for proteasomal degradation.
DNAAAA AAG
mRNAAAA AAG
Arginine Arg · R
R–(CH₂)₃NHC(=NH₂⁺)NH₂
 Conditional Basic (positively charged) The sole substrate for nitric oxide synthase, governing vasodilation and muscle blood flow. Urea cycle intermediate and precursor of creatine; becomes essential during stress and renal disease.
DNACGT CGC CGA CGG AGA AGG
mRNACGU CGC CGA CGG AGA AGG
Histidine His · H
R–CH₂–imidazole
 Yes Basic (positively charged) Its imidazole ring buffers intracellular pH — as carnosine it is a major muscle buffer during high-intensity effort. Precursor of histamine; notably depleted in dialysis patients.
DNACAT CAC
mRNACAU CAC
Aspartate Asp · D
R–CH₂COO⁻
 No Acidic (negatively charged) Urea cycle intermediate and a carrier in the malate–aspartate shuttle that moves reducing equivalents into mitochondria. Substrate for purine and pyrimidine synthesis.
DNAGAT GAC
mRNAGAU GAC
Glutamate Glu · E
R–CH₂CH₂COO⁻
 No Acidic (negatively charged) The principal excitatory neurotransmitter of the CNS and the hub of amino-acid nitrogen transfer via transamination. Precursor of glutamine, GABA, and glutathione.
DNAGAA GAG
mRNAGAA GAG
Selenocysteine Sec · U 21st
R–CH₂SeH
 No Polar, selenium-containing The 21st amino acid. Its selenol group is far more reactive than the thiol of cysteine, making it the catalytic residue of the glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases — enzymes central to antioxidant defense and thyroid hormone activation. Not obtained from the diet as an amino acid: it is built on its own tRNA from serine, but requires dietary selenium. Selenium deficiency is common in dialysis patients and blunts this antioxidant capacity.
DNATGA
mRNAUGA

UGA recoded by a SECIS element in the 3′ UTR (otherwise a stop codon)
Pyrrolysine Pyl · O 22nd
R–(CH₂)₄NHCO–(methylpyrroline)
 Not in humans Basic, lysine-derived The 22nd amino acid, found in certain methanogenic archaea and a few bacteria but absent from humans. It forms the catalytic residue of methylamine methyltransferases, allowing these organisms to use methylamines as a carbon and energy source.
DNATAG
mRNAUAG

UAG recoded by a PYLIS element (otherwise a stop codon)

Notes

Structures are drawn in the neutral form with L-stereochemistry at the α-carbon — all except glycine, which is achiral. R denotes the side chain attached to the α-carbon. Nitrogen, oxygen, and sulfur atoms are colored; unlabeled vertices are carbon.

Essential means the amino acid must be obtained from the diet. Conditional means it is normally synthesized in adequate amounts but becomes dietarily essential during growth, illness, catabolic stress, or organ failure. Not essential means it is synthesized de novo in sufficient quantity.

Codons are given first as the DNA coding (sense) strand and second as the corresponding mRNA codon; they differ only in that thymine (T) replaces uracil (U). The twenty standard amino acids are specified by 61 of the 64 possible triplets — the remaining three (DNA TAA, TAG, TGA) are stop codons. ATG / AUG serves as both the methionine codon and the universal start codon.

Selenocysteine and pyrrolysine have no codons of their own. Each is inserted at what would otherwise be a stop codon, recoded by a stem-loop element elsewhere in the same mRNA: UGA is read as selenocysteine when a SECIS element sits in the 3′ untranslated region, and UAG is read as pyrrolysine when a PYLIS element follows the codon. Without that element the same triplet terminates translation — so these two are specified by context, not by the triplet alone.

Humans use 21 of the 22: the twenty standard amino acids plus selenocysteine, which is built on its own tRNA from serine and requires dietary selenium rather than being absorbed as an amino acid. Pyrrolysine occurs only in certain methanogenic archaea and a few bacteria, and is included here for completeness of the proteinogenic set.

Branched-chain amino acids (valine, leucine, isoleucine) are oxidized directly within skeletal muscle rather than in the liver.