Tryptophan Metabolism simplified

1. Serotonin pathway: Tryptophan is primarily metabolized along the serotonin pathway, also known as the tryptophan-serotonin pathway. In this pathway, tryptophan is converted into 5-hydroxytryptophan (5-HTP) by the enzyme tryptophan hydroxylase. Subsequently, 5-HTP is converted into serotonin (5-hydroxytryptamine) by the enzyme aromatic amino acid decarboxylase. Serotonin is a neurotransmitter involved in mood regulation, sleep, appetite, and other physiological processes.

2. Kynurenine pathway: Tryptophan can also be metabolized through the kynurenine pathway. In this pathway, tryptophan is converted into kynurenine by the enzyme tryptophan dioxygenase or indoleamine 2,3-dioxygenase (IDO). Kynurenine is further metabolized into various downstream products, including kynurenic acid, quinolinic acid, and picolinic acid. These metabolites have diverse roles in immune regulation, inflammation, and neurological processes.

3. Niacin synthesis: As mentioned earlier, tryptophan can be converted into niacin (vitamin B3) through a series of enzymatic reactions. Tryptophan is first converted into kynurenine via the kynurenine pathway, and then kynurenine is further metabolized to produce niacin. This pathway is important for maintaining adequate levels of niacin in the body, which is essential for energy production, DNA repair, and metabolism.

4.  Melatonin synthesis: Tryptophan is also involved in the synthesis of melatonin, a hormone that regulates the sleep-wake cycle. In this pathway, tryptophan is converted into serotonin, as mentioned earlier. Serotonin is then further metabolized into N-acetylserotonin by the enzyme serotonin N-acetyltransferase (SNAT). N-acetylserotonin is finally converted into melatonin by the enzyme acetylserotonin O-methyltransferase (ASMT).

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