How Does Tryptophan Influence Health and Nutrition?
Tryptophan (Trp or W symbol) is an alpha-amino acid that is used in protein biosynthesis. Tryptophan has an alpha-amino group, an alpha-carboxylic acid group, and an indole side chain, making it a non-polar aromatic amino acid. In humans, it is essential, meaning that it can not be synthesized by the body and must be obtained from the diet. Tryptophan is the precursor of the neurotransmitter serotonin, the hormone melatonin, and vitamin B3 are all precursors of tryptophan. The codon UGG encodes it.
Frederick Hopkins first reported the isolation of tryptophan in 1901. Hopkins recovered tryptophan from hydrolyzed casein, recovering 4-8 g of tryptophan from 600 g of crude casein.
In this article, we will study tryptophan amino acids, source of tryptophan, and tryptophan and serotonin In detail.
Lysine and Tryptophan
A few similarities are shared between lysine and tryptophan. They are both amino acids used to make proteins — and they are important, meaning you have to get them from your diet because they can't be created by your body. You have a greater chance of not getting enough lysine and tryptophan than other amino acids, which makes them both amino acid-limiting. Otherwise, they each have functions that are unique.
Physical Properties of Tryptophan
It is a solid colour, slightly yellowish-white, with no odour and a flat taste. C₁₁H₁₂N₂O₂ is its chemical formula and has a molar mass of 204.229 g·mol−1. It is soluble in hot alcohol, alkali hydroxides, ethanol, acetic acid, but insoluble in chloroform and ethyl ether. Trp's melting point is 290.5 dec °C and has a pKa value equal to 25 °C at 7.38.
Source of Tryptophan
In most protein-based foods or dietary proteins, tryptophan is present. Chocolate, oats, dried dates, milk, yoghurt, cottage cheese, red meat, eggs, pork, poultry, sesame, chickpeas, almonds, sunflower seeds, pumpkin, buckwheat, spirulina, and peanuts are particularly abundant.
Production of Tryptophan
As an essential amino acid, tryptophan in humans and other animals is not synthesized from simpler compounds, so it needs to be present in the diet in the form of proteins containing tryptophan. Tryptophan is usually synthesized by plants and microorganisms from shikimic acid or anthranilate.
Anthranilate condenses with phosphoribosylpyrophosphate (PRPP), which as a by-product generates pyrophosphate.
The ring of the ribose moiety is opened and subjected to reductive decarboxylation, producing indole-3-glycerol phosphate; this, in turn, is transformed into indole.
The formation of tryptophan from indol and amino acid serine is catalyzed by tryptophan synthase in the last step.
Uses of Tryptophan
In the "anchoring" of the membrane, proteins inside the cell membrane, tryptophan and tyrosine residues play unique roles.
Tryptophan is also essential in glycan-protein interactions, along with other aromatic amino acids.
For the following substances, tryptophan acts as a biochemical precursor:
Serotonin (a neurotransmitter), synthesized by tryptophan hydroxylase.
Melatonin (a neurohormone) is in turn synthesized from serotonin, via N-acetyltransferase and 5-hydroxy indole-O-methyltransferase enzymes.
Niacin, also known as vitamin B3, is synthesized from tryptophan via kynurenine and quinolinic acids.
Auxins (a class of phytohormones) is synthesized from tryptophan.
Tryptophan Depression (acts as an antidepressant)
Low levels of tryptophan are responsible for depression and anxiety.
Since tryptophan is converted into 5-hydroxytryptophan (5-HTP), which is then converted into serotonin as a neurotransmitter, it has been suggested that tryptophan or 5-HTP intake can improve symptoms of depression by increasing the level of serotonin in the brain. In the United States and the United Kingdom as a dietary supplement for use as an antidepressant, anxiolytic, and sleep aid, tryptophan is marketed over the counter. It is also sold for the treatment of severe depression in several European countries as a prescription medication.
Did You Know?
High cellular levels of this amino acid activate a repressor protein that binds to the trp operon in bacteria that synthesize tryptophan. Binding this repressor to the tryptophan operon inhibits downstream DNA transcription that codes for the enzymes involved in tryptophan biosynthesis. So high tryptophan levels inhibit tryptophan synthesis via a negative feedback loop, and transcription from the trp operon resumes when the cell's tryptophan levels go down again. This allows for tightly regulated and rapid responses to changes in the inner and outer tryptophan levels of the cell.
FAQs on Tryptophan: Properties, Benefits, and Sources
1. What is tryptophan and what is its primary role in chemistry and biology?
Tryptophan is an α-amino acid used in the biosynthesis of proteins. It is unique due to its indole side chain, making it an aromatic amino acid. Biologically, it's an essential amino acid, meaning the human body cannot synthesize it and must obtain it from the diet. Its primary roles include protein synthesis and serving as a metabolic precursor for serotonin, melatonin, and niacin (vitamin B3).
2. What are the key chemical and physical properties of tryptophan?
Tryptophan exhibits the following properties:
- Chemical Formula: C₁₁H₁₂N₂O₂
- Molar Mass: 204.229 g/mol
- Appearance: A slightly yellowish-white solid with a flat taste.
- Solubility: It is soluble in hot alcohol and alkali hydroxides but is poorly soluble in water and insoluble in chloroform and ether.
- Structure: It contains both an α-amino group and an α-carboxylic acid group, and a side chain containing an indole functional group, which classifies it as an aromatic amino acid.
3. From a chemical perspective, why is tryptophan classified as an essential amino acid?
Tryptophan is classified as an essential amino acid because human metabolic pathways lack the necessary enzymes to synthesize its complex indole ring structure from simpler metabolic precursors. While plants and microorganisms can produce tryptophan through the shikimate pathway, humans cannot. Therefore, it must be ingested through diet to meet the body's requirements for protein synthesis and the production of other crucial biomolecules.
4. What are the major biological benefits of tryptophan for the human body?
The primary biological benefits of tryptophan stem from its role as a precursor to several key molecules:
- Serotonin Production: Tryptophan is converted into the neurotransmitter serotonin, which regulates mood, appetite, and sleep.
- Melatonin Synthesis: Serotonin can be further converted into melatonin, the hormone responsible for regulating the sleep-wake cycle.
- Niacin (Vitamin B3) Formation: The liver can use tryptophan to produce niacin, which is vital for energy metabolism and DNA repair.
- Protein Building Block: As an amino acid, it is a fundamental component for building and maintaining proteins throughout the body.
5. How is tryptophan metabolised in the body to produce vital compounds like serotonin and niacin?
Tryptophan is metabolised through a pathway called the kynurenine pathway, where most of it is converted. However, a small but critical fraction is hydroxylated by the enzyme tryptophan hydroxylase to form 5-hydroxytryptophan (5-HTP). This intermediate is then decarboxylated to produce the neurotransmitter serotonin. Separately, the liver can convert tryptophan into kynurenine, which eventually leads to the synthesis of niacin (vitamin B3).
6. What are some common food sources rich in tryptophan?
Tryptophan is found in most protein-rich foods. Some of the most common sources include:
- Animal Products: Turkey, chicken, cheese, fish (like tuna), milk, and eggs.
- Plant Products: Oats, bananas, nuts (especially peanuts and almonds), seeds (pumpkin and sesame), soybeans (tofu, edamame), and chocolate.
7. Why is the idea that eating turkey makes you sleepy due to tryptophan often considered a myth?
While turkey does contain tryptophan, the effect is minimal because it also contains many other amino acids that compete with tryptophan for transport across the blood-brain barrier. A large, carbohydrate-rich meal eaten alongside the turkey is a more likely cause of sleepiness. The insulin spike from carbohydrates helps clear other amino acids from the blood, giving tryptophan a better chance to enter the brain and be converted to serotonin.
8. How does the aromatic structure of tryptophan's indole side chain influence its properties compared to a non-aromatic amino acid like alanine?
Tryptophan's aromatic indole side chain gives it unique properties compared to alanine, which has a simple methyl side chain:
- Size and Hydrophobicity: Tryptophan's large, bulky indole ring makes it significantly more hydrophobic, influencing how proteins fold to bury it away from water. Alanine is small and much less hydrophobic.
- UV Absorbance: The conjugated ring system in tryptophan allows it to strongly absorb ultraviolet (UV) light at a wavelength of approximately 280 nm, a property used to quantify protein concentration. Alanine does not absorb UV light in this range.
- Reactivity: The indole ring can participate in hydrogen bonding and pi-stacking interactions, which are crucial for protein structure and function, unlike alanine's simple alkyl group.
9. What are the chemical implications of having excessively high levels of tryptophan in the blood?
Excessively high levels of tryptophan in the blood, a rare metabolic condition known as hypertryptophanemia, indicates a defect in the enzyme responsible for its breakdown (tryptophan 2,3-dioxygenase). Chemically, this means the first step of the kynurenine pathway is blocked. While often asymptomatic, the buildup of tryptophan and its metabolites can sometimes lead to neurological or developmental issues, as the delicate balance of neurotransmitter synthesis can be disrupted.
