Tryptophan

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. 

1. Anthranilate condenses with phosphoribosylpyrophosphate (PRPP), which as a by-product generates pyrophosphate. 

2. 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. 

3. The formation of tryptophan from indol and amino acid serine is catalyzed by tryptophan synthase in the last step.

Uses of Tryptophan

1. In the "anchoring" of the membrane, proteins inside the cell membrane, tryptophan and tyrosine residues play unique roles.

2. Tryptophan is also essential in glycan-protein interactions, along with other aromatic amino acids.

3. 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.