How does labelled N from tyrosine appear in phenylalanine if the latter is an essential amino acid?
Labelled nitrogen (N) from tyrosine does not appear in phenylalanine.
Phenylalanine is indeed an essential amino acid, meaning that it cannot be synthesized by the human body and must be obtained from dietary sources. Tyrosine, on the other hand, is a non-essential amino acid, meaning that it can be synthesized by the body from other amino acids
During the process of protein synthesis, phenylalanine and tyrosine have different pathways. Phenylalanine is incorporated into proteins directly, while tyrosine is synthesized from phenylalanine. The conversion of phenylalanine to tyrosine occurs through a specific enzymatic reaction called hydroxylation, which is catalyzed by the enzyme phenylalanine hydroxylase
In this reaction, phenylalanine hydroxylase adds a hydroxyl group (-OH) to the phenylalanine molecule, resulting in the formation of tyrosine. This enzymatic reaction occurs in the liver, and it requires the presence of several cofactors such as oxygen, iron, and tetrahydrobiopterin (BH4). It is important to note that the labelled nitrogen (N) from tyrosine does not appear in phenylalanine during this process
Therefore, any labelled nitrogen in the form of isotopes (e. g. , 15N) that is present in tyrosine will remain in tyrosine and will not be transferred to phenylalanine through this enzymatic reaction. The labelled nitrogen from tyrosine will only appear in the downstream metabolites, such as dopa, dopamine, and other catecholamine neurotransmitters synthesized from tyrosine
In summary, labelled nitrogen from tyrosine does not appear in phenylalanine due to the distinct metabolic pathways of these amino acids
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