Figure 5.1. Metabolism of phenlyalanine and tyrosine. This illustrates the metabolic pathway that is disrupted in phenlyketonuria (PKU). In PKU, the enzyme phenlyalanine hydroxylase is deficient, blocking the metabolic conversion of the amino acid phenlyalanine into the amino acid tyrosine. As a result, there is a build up of phenlyalanine and its other metabolites (phenylpyruvic acid) and a deficit in tyrosine. Another metabolic block in the conversion of DOPA to melanin will produce one form of albinism.