<prompt> Answer this question truthfullyWhat is the relationship between phenylalanine and tyrosine, and which enzyme and cofactor are involved in the conversion process?  <response>: <reason> Phenylalanine can be converted to tyrosine via the enzyme phenylalanine hydroxylase, which requires tetrahydrobiopterin as a cofactor. This conversion is an important step in the biosynthesis of tyrosine, which is an essential amino acid that plays a critical role in protein synthesis and other metabolic processes in the body. 