Garrod's information about the enzyme alteration resulting in alkaptonuria led to further understanding of these types of metabolic pathways in humans. phenylketonuria (pku) occurs when another enzyme in the pathway is altered or missing, resulting in a failure of phenylalanine (phe) to be metabolized to another amino acid: tyrosine. tyrosine is an earlier substrate in the pathway altered in alkaptonuria. how might pku affect the presence or absence of alkaptonuria?