PKU is the abbreviation for the disease phenylketonuria. The liver of a person with phenylketonuria cannot readily convert phenylalanine, an essential amino acid, to tyrosine, a nonessential amino acid. Insufficient enzyme action causes this defect.
The inability to metabolize excess phenylalanine to tyrosine leads to the formation of abnormal products that arise from alternative metabolic pathways; these products can cause mental retardation. Thus, it is vital to determine which infants have PKU, since the amount of phenylalanine in their diets must be monitored. However, because it is an essential amino acid, some phenylalanine must be consumed. Since phenylalanine cannot be sufficiently metabolized to tyrosine, the latter must now be considered an essential amino acid for people with PKU.
Protein synthesis is a complex process by which a specific sequence and number of amino acids determine the primary structure of a protein. If a given amino acid is not present during protein synthesis, production will stop. In other words, protein is an all-or-none process: All of the amino acids necessary to make the protein must be available, or the protein will not be made at all. A mixed diet of plant products will likely contain enough of all nine essential amino acids, so the all-or-none principle will not typically be an issue in diet planning, even in vegetarianism.
