What is the Difference Between Tryptophan and L-Tryptophan

The main difference between tryptophan and L-tryptophan is that tryptophan is an amino acid required for the normal growth in infants and maintenance of proteins, muscles, enzymes, and neurotransmitters of the body, whereas L-tryptophan is an α-amino acid required for the synthesis of proteins.

Tryptophan and L-tryptophan are two types of amino acids important for the body. They contain an indole side chain. They are essential amino acids.

Key Areas Covered

1. What is Tryptophan
     – Definition, Structure, Function
2. What is L-Tryptophan
     – Definition, Structure, Function
3. Similarities Between Tryptophan and L-Tryptophan
     – Outline of Common Features
4. Difference Between Tryptophan and L-Tryptophan
     – Comparison of Key Differences

Key Terms

L-Tryptophan, Tryptophan

What is Tryptophan

Tryptophan is one of the 20 standard amino acids and an essential amino acid in the human diet. The codon UUG encodes for the tryptophan in the standard genetic code. Most importantly, tryptophan contains two stereoisomers, D-tryptophan and L-tryptophan. Its D-stereoisomer occasionally occurs in many peptides; for example, the marine venom peptide contryphan. Another importance of D-tryptophan is the presence of an indole side group. However, the body does not produce tryptophan through normal metabolic processes. Therefore, it is an essential amino acid that has to be taken through the diet.

Figure 1: Tryptophan Metabolism

Furthermore, tryptophan from the diet converts into 5-HTP, and then that converts into serotonin, a neurotransmitter. Meanwhile, some tryptophan converts into nicotinamide adenine dinucleotide (NAD), a dietary source of niacin. Apart from that, the only difference between D-tryptophan and L-tryptophan is their orientation. Tryptophan occurs in many food sources that are rich in protein, including chicken, eggs, cheese, fish, peanuts, milk, turkey, and tofu.

What is L-Tryptophan

L-tryptophan is the L-stereoisomer of tryptophan. It is an α-amino acid used in protein synthesis. Therefore, L-tryptophan is a proteogenic amino acid. L-tryptophan contains an α-carboxylic acid group, α-amino group, and an indole side chain. Due to the presence of an indole side chain, L-tryptophan is a polar molecule with a nonpolar aromatic beta-carbon substituent. 

Figure 2: L-Tryptophan

Apart from its pathogenicity, tryptophan is a precursor for the hormone melatonin, vitamin B3, and the neurotransmitter serotonin. Moreover, the other compounds of precursor L-tryptophan include kynurenine, niacin, and auxins.   

Similarities Between Tryptophan and L-Tryptophan

• Tryptophan and L-tryptophan are two types of amino acids the body needs.
• They are important for the normal growth and the production of proteins.
• They are essential amino acids that the body cannot synthesize through metabolic processes.
• Moreover, they are polar molecules with a nonpolar, aromatic beta-carbon substituent.

Difference Between Tryptophan and L-Tryptophan

Definition

Tryptophan refers to an amino acid that is a constituent of most proteins, an essential nutrient in the diet of vertebrates, while L-tryptophan refers to an α-amino acid that is used in the biosynthesis of proteins.

Nature 

Tryptophan is an essential amino acid in the human diet, while L-tryptophan is a polar molecule with a nonpolar, aromatic beta-carbon substituent.

Importance of Tryptophan and L-Tryptophan

Moreover, tryptophan is an amino acid required for normal growth in infants and for the maintenance of the proteins, muscles, enzymes, and neurotransmitters of the body, whereas L-tryptophan is an α-amino acid required for the synthesis of proteins.

Proteogenic or Nonproteogenic Amino Acids 

D-tryptophan is not a proteogenic amino acid, while L-tryptophan is a proteogenic amino acid.

Conclusion

In brief, tryptophan and L-tryptophan are two types of amino acids that are essential for the growth of the human body. Tryptophan is a constituent of most proteins, and it is an essential amino acid for the human diet. It is important for the normal growth of infants and the maintenance of proteins, muscles, enzymes, and neurotransmitters. However, D-tryptophan is not a proteogenic amino acid. In comparison, L-tryptophan is an alpha amino acid with a side chain of indole. It is a proteogenic amino acid involved in the process of protein synthesis. Therefore, it is a proteogenic amino acid. Hence, the main difference between tryptophan and L-tryptophan is their structure.

References:                     

1. “What Is Tryptophan?” Healthline. 
2. “L-tryptophan” MedlinePlus Supplements. S. National Library of Medicine.

Image Courtesy:

1. “Microbiota-derived 3-Indolepropionic acid” By Slashme– Own work (CC-BY SA 4.0) via Commons Wikimedia
2. “L-Tryptophan – L-Tryptophan” By NEUROtiker – Own Work (Public Domain) via Commons Wikimedia