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Skeletal formula of L-isomer
Ball-and-stick model of L-isomer
IUPAC name
Tryptophan or (2S)-2-amino-3-(1H-indol-3-yl)propanoic acid
Other names
2-Amino-3-(1H-indol-3-yl)propanoic acid
73-22-3 YesY
ATC code N06AX02
ChemSpider 6066 YesY
DrugBank DB00150 YesY
Jmol interactive 3D Image
KEGG D00020 YesY
PubChem 6305
Molar mass 204.23 g·mol−1
Soluble: 0.23 g/L at 0 °C,

11.4 g/L at 25 °C,
17.1 g/L at 50 °C,
27.95 g/L at 75 °C

Solubility Soluble in hot alcohol, alkali hydroxides; insoluble in chloroform.
Acidity (pKa) 2.38 (carboxyl), 9.39 (amino)[1]
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Phase behaviour
N verify (what is YesYN ?)
Infobox references

Tryptophan (IUPAC-IUBMB abbreviation: Trp or W; IUPAC abbreviation: L-Trp or D-Trp; sold for medical use as Tryptan)[2] is one of the 22 standard amino acids and an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG. Only the L-stereoisomer of tryptophan is used in structural or enzyme proteins, but the R-stereoisomer is occasionally found in naturally produced peptides (for example, the marine venom peptide contryphan).[3] The distinguishing structural characteristic of tryptophan is that it contains an indole functional group.


The isolation of tryptophan was first reported by Frederick Hopkins in 1901[4] through hydrolysis of casein. From 600 grams of crude casein one obtains 4-8 grams of tryptophan.[5]

Biosynthesis and industrial production[edit]

Plants and microorganisms commonly synthesize tryptophan from shikimic acid or anthranilate.[6] The latter condenses with phosphoribosylpyrophosphate (PRPP), generating pyrophosphate as a by-product. After ring opening of the ribose moiety and following reductive decarboxylation, indole-3-glycerinephosphate is produced, which in turn is transformed into indole. In the last step, tryptophan synthase catalyzes the formation of tryptophan from indole and the amino acid serine.

Tryptophan biosynthesis (en).svg

The industrial production of tryptophan is also biosynthetic and is based on the fermentation of serine and indole using either wild-type or genetically modified bacteria such as B. amyloliquefaciens, B. subtilis, C. glutamicum or E. coli. These strains carry either mutations that prevent the reuptake of aromatic amino acids or multiple/overexpressed trp operons. The conversion is catalyzed by the enzyme tryptophan synthase.[7][8][9]


Metabolism of L-tryptophan into serotonin and melatonin (left) and niacin (right). Transformed functional groups after each chemical reaction are highlighted in red.

For many organisms (including humans), tryptophan is an essential amino acid. This means that it cannot be synthesized by the organism, it is needed to prevent illness or death, and it therefore must be part of their diet. Amino acids, including tryptophan, act as building blocks in protein biosynthesis, and proteins are required to sustain life. In addition, tryptophan functions as a biochemical precursor for the following compounds (see also figure to the right):

The disorder fructose malabsorption causes improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood,[15] and depression.[16] The authors did not find reduced tryptophan in cases of lactose maldigestion.[15]

In bacteria that synthesize tryptophan, high cellular levels of this amino acid activate a repressor protein, which binds to the trp operon.[17] Binding of this repressor to the tryptophan operon prevents transcription of downstream DNA that codes for the enzymes involved in the biosynthesis of tryptophan. So high levels of tryptophan prevent tryptophan synthesis through a negative feedback loop and, when the cell's tryptophan levels are reduced, transcription from the trp operon resumes. The genetic organisation of the trp operon thus permits tightly regulated and rapid responses to changes in the cell's internal and external tryptophan levels.

Dietary sources[edit]

Tryptophan is a routine constituent of most protein-based foods or dietary proteins. It is particularly plentiful in chocolate, oats, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, almonds, sunflower seeds, pumpkin seeds, spirulina, bananas, and peanuts. Contrary to the popular belief[18][19][20] that turkey has a particularly high amount of tryptophan, the amount of tryptophan in turkey is typical of most poultry.[21]

Tryptophan (Trp) Content of Various Foods[21][22]
Food Tryptophan
[g/100 g of food]
[g/100 g of food]
Tryptophan/Protein [%]
egg white, dried
spirulina, dried
cod, atlantic, dried
soybeans, raw
cheese, Parmesan
sesame seed
cheese, cheddar
sunflower seed
pork, chop
lamb, chop
perch, Atlantic
chickpeas, raw
wheat flour, white
baking chocolate, unsweetened
rice, white, medium-grain, cooked
quinoa, uncooked
quinoa, cooked
potatoes, russet

Turkey meat and drowsiness[edit]

A common assertion in the US is that heavy consumption of turkey meat results in drowsiness, due to high levels of tryptophan contained in turkey.[18][20] However, the amount of tryptophan in turkey is comparable to that contained in most other meats.[19][21] Furthermore, post-meal drowsiness may have more to do with what is consumed along with the turkey, carbohydrates in particular.[23] It has been demonstrated in both animal models[24] and humans[25][26][27] that ingestion of a meal rich in carbohydrates triggers release of insulin. Insulin in turn stimulates the uptake of large neutral branched-chain amino acids (BCAA), but not tryptophan (an aromatic amino acid) into muscle, increasing the ratio of tryptophan to BCAA in the blood stream. The resulting increased tryptophan ratio reduces competition at the large neutral amino acid transporter (which transports both BCAA and aromatic amino acids), resulting in more uptake of tryptophan across the blood–brain barrier into the cerebrospinal fluid (CSF).[28][29] Once in the CSF, tryptophan is converted into serotonin in the raphe nuclei by the normal enzymatic pathway.[24][26] The resultant serotonin is further metabolised into melatonin by the pineal gland.[12] Hence, this data suggests that "feast-induced drowsiness"— or postprandial somnolence — may be the result of a heavy meal rich in carbohydrates, which indirectly increases the production of sleep-promoting melatonin in the brain.[24][25][26][27]

Use as a dietary supplement[edit]

Tryptophan is sold over the counter in the United States, Canada, and the United Kingdom as a dietary supplement for use as an antidepressant, anxiolytic, and sleep aid. It is also marketed in some European countries for the indication of major depression under various trade names.

Since tryptophan is converted into 5-hydroxytryptophan (5-HTP) which is subsequently converted into the neurotransmitter serotonin, it has been proposed that consumption of tryptophan or 5-HTP may therefore improve depression symptoms by increasing the level of serotonin in the brain. In 2001 a Cochrane Review of the effect of 5-HTP and tryptophan on depression was published. The authors included only studies of a high rigor and included both 5-HTP and tryptophan in their review because of the limited data on either. Of 108 studies of 5-HTP and tryptophan on depression published between 1966 and 2000, only two met the authors' quality standards for inclusion, totaling 64 study participants. The substances were more effective than placebo in the two studies included but the authors state that, "the evidence was of insufficient quality to be conclusive," and note, "because alternative antidepressants exist which have been proven to be effective and safe, the clinical usefulness of 5-HTP and tryptophan is limited at present."[30] The use of tryptophan as an adjunctive therapy in addition to standard treatment for mood and anxiety disorders is not supported by the scientific evidence.[31] Due to the lack of high quality studies and preliminary nature of studies showing effectiveness and the lack of adequate study on their safety, the use of tryptophan and 5-HTP is not highly recommended or thought to be clinically useful.[30][31]

There is evidence that blood tryptophan levels are unlikely to be altered by changing the diet,[32] but tryptophan is available in health food stores as a dietary supplement.[33] Consuming purified tryptophan increases brain serotonin whereas eating foods containing tryptophan does not.[34] This is because the transport system which brings tryptophan across the blood-brain barrier is also selective for the other amino acids which are contained in protein food sources.[35] High plasma levels of other large neutral amino acids prevent the plasma concentration of tryptophan from increasing brain concentration levels.[35]

Side effects[edit]

Potential side effects of tryptophan include nausea, diarrhea, drowsiness, lightheadedness, headache, dry mouth, blurred vision, sedation, euphoria, and nystagmus (involuntary eye movements).[36][37] Because tryptophan has not been thoroughly studied in a clinical setting, possible side effects and interactions with other drugs are not well known.[30]


Tryptophan has the potential to cause serotonin syndrome when combined with antidepressants of the MAOI or SSRI class or other strongly serotonergic drugs.[37]


In 1912 Felix Ehrlich demonstrated that yeast attacks the natural amino acids essentially by splitting off carbon dioxide and replacing the amino group with hydroxyl. By this reaction, tryptophan gives rise to tryptophol.[38]

Tryptophan affects brain serotonin synthesis when given orally in a purified form and is used to modify serotonin levels for research in psychology.[34] Low brain serotonin is induced by administration of tryptophan-poor protein in a technique called 'acute tryptophan depletion'.[39] Studies using this method have evaluated the effect of serotonin on mood and social behavior, finding that serotonin reduces aggression and increases agreeableness.[40]


Tryptophan is an important intrinsic fluorescent probe (amino acid), which can be used to estimate the nature of microenvironment of the tryptophan. Most of the intrinsic fluorescence emissions of a folded protein are due to excitation of tryptophan residues.

Society and culture[edit]

Eosinophilia–myalgia syndrome[edit]

There was a large outbreak of eosinophilia-myalgia syndrome (EMS) in the U.S. in 1989, with more than 1,500 cases reported to the CDC and at least 37 deaths. After preliminary investigation revealed that the outbreak was linked to intake of tryptophan, the U.S. Food and Drug Administration (FDA) banned most tryptophan from sale in the US in 1991, and other countries followed suit.[41]

Subsequent epidemiological studies suggested that EMS was linked to specific batches of L-tryptophan supplied by a single large Japanese manufacturer, Showa Denko.[41][42][43][44] It eventually became clear that recent batches of Showa Denko's L-tryptophan were contaminated by trace impurities, which were subsequently thought to be responsible for the 1989 EMS outbreak.[41][45][46] However, other evidence suggests that tryptophan itself may be a potentially major contributory factor in EMS.[47]

The FDA loosened its restrictions on sales and marketing of tryptophan in February 2001, but continued to limit the importation of tryptophan not intended for an exempted use until 2005.[41]

The fact that the Showa Denko facility used genetically engineered bacteria to produce the contaminated batches of L-tryptophan later found to have caused the outbreak of eosinophilia-myalgia syndrome has been cited as evidence of a need for "close monitoring of the chemical purity of biotechnology-derived products."[48] Those calling for purity monitoring have, in turn, been criticized as anti-GMO activists who overlook possible non-GMO causes of contamination and threaten the development of biotech.[49]

See also[edit]


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