What Is Tryptophan Definition Chemical Structure Functions and Sources
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 Structure Properties and Biological Role
1. What is tryptophan?
Tryptophan is an essential α-amino acid with the molecular formula C11H12N2O2 that contains an indole side chain. It is classified as an aromatic amino acid and must be obtained from the diet because the human body cannot synthesize it. In proteins, tryptophan is represented by the three-letter code Trp and one-letter code W. Chemically, it contains both an amino group (–NH2) and a carboxyl group (–COOH) attached to the same α-carbon.
2. What is the chemical structure of tryptophan?
The chemical structure of tryptophan consists of an α-amino acid backbone attached to an indole ring side chain. Its condensed structural formula can be written as HOOC–CH(NH2)–CH2–C8H6N. Key structural features include:
- An α-carbon bonded to –NH2, –COOH, –H, and a side chain.
- A bicyclic indole ring (a benzene ring fused to a pyrrole ring).
- Aromaticity due to conjugated π-electrons in the indole system.
3. Why is tryptophan called an essential amino acid?
Tryptophan is called an essential amino acid because humans cannot synthesize it and must obtain it from dietary sources. From a biochemical perspective:
- Its complex indole ring cannot be formed in human metabolic pathways.
- It must be supplied through protein-rich foods.
- It is required for protein synthesis and production of biologically important molecules.
4. What is the molar mass of tryptophan?
The molar mass of tryptophan (C11H12N2O2) is approximately 204.23 g·mol-1. It is calculated by summing atomic masses:
- 11 × 12.01 (C) = 132.11
- 12 × 1.008 (H) = 12.10
- 2 × 14.01 (N) = 28.02
- 2 × 16.00 (O) = 32.00
5. What type of amino acid is tryptophan?
Tryptophan is a nonpolar, aromatic amino acid with a hydrophobic indole side chain. It is classified as:
- Aromatic – due to its conjugated indole ring.
- Hydrophobic – tends to be located inside protein structures.
- Neutral at physiological pH (no net side-chain charge).
6. What is the isoelectric point (pI) of tryptophan?
The isoelectric point (pI) of tryptophan is approximately 5.9, which is the pH at which it has no net electrical charge. At this pH:
- The amino group exists mainly as –NH3+.
- The carboxyl group exists mainly as –COO-.
- The molecule forms a zwitterion with balanced charges.
7. How does tryptophan exist in aqueous solution?
In aqueous solution near neutral pH, tryptophan exists predominantly as a zwitterion with both positive and negative charges. Its ionic form can be represented as +H3N–CH(CH2–indole)–COO-. This occurs because:
- The amino group is protonated (–NH3+).
- The carboxyl group is deprotonated (–COO-).
8. Why does tryptophan absorb UV light at 280 nm?
Tryptophan absorbs UV light at around 280 nm because of π→π* electronic transitions in its aromatic indole ring. The conjugated double bonds allow delocalized electrons to absorb ultraviolet energy. Key points include:
- Strong UV absorbance compared to most other amino acids.
- Used in protein quantification by UV spectroscopy.
- Absorbance follows the Beer–Lambert law: A = εcl.
9. What is the difference between tryptophan and phenylalanine?
The main difference between tryptophan and phenylalanine is that tryptophan contains an indole ring with nitrogen, while phenylalanine contains a benzene ring only. Comparison:
- Tryptophan: C11H12N2O2, larger, more complex aromatic system.
- Phenylalanine: C9H11NO2, simpler phenyl side chain.
- Tryptophan shows stronger UV absorption at 280 nm.
10. What are the chemical reactions of tryptophan in proteins?
In proteins, tryptophan participates mainly in peptide bond formation and aromatic interactions. Important reactions include:
- Peptide bond formation: –COOH of one amino acid reacts with –NH2 of another, forming –CONH– and releasing H2O.
- Oxidation reactions: The indole ring can be oxidized under strong chemical conditions.
- Electrophilic substitution: Possible on the aromatic indole ring.
