citrulline vs amino acid

citrulline and amino acid both are nouns.

Word	Noun	Adjective	Verb	Adverb
citrulline	Yes	No	No	No
amino acid	Yes	No	No	No

As nouns, amino acid is a hypernym of citrulline; that is, amino acid is a word with a broader meaning than citrulline:

citrulline: an amino acid that does not occur in proteins but is an intermediate in the conversion of ornithine to arginine
amino acid: organic compounds containing an amino group and a carboxylic acid group

Other hypernyms of citrulline include aminoalkanoic acid.

citrulline (noun)	amino acid (noun)
an amino acid that does not occur in proteins but is an intermediate in the conversion of ornithine to arginine	organic compounds containing an amino group and a carboxylic acid group