Aromatic Amino Acids Tryptophan Tyrosine and Phenylalanine

These are also bulky and nonpolar, and they may interact with other hydrophobic molecules. The phenolic hydrogen of tyrosine is weakly acidic and can form hydrogen bonds to create cross-links or can be donated during catalysis. Tyrosine residues on certain membrane-bound receptors become phos-phorylated by tyrosine kinase domains, thereby initiating a signal transduction cascade. Tryptophan is important as a precursor of the neurotransmitter 5-hydroxytryptamine (serotonin) and of the nicotinamide-containing coenzymes NAD and NADP. Phenylalanine can be converted to tyrosine in the body, but not vice versa. Tyrosine is a precursor of the catecholamines and the thyroid hormones and also the pigment melanin.

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