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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