WordNet records no synonym and no antonym for phenylalanine. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an essential amino acid found in proteins and needed for growth of children and for protein metabolism in children and adults; abundant in milk and eggs; it is normally converted to tyrosine in the human body
Synonyms, antonyms and the broader and narrower relations for phenylalanine come from Princeton University’s WordNet. Full sources and licences.