Phosphoenolpyruvate (PEP) and erythrose-4-phosphate are precursors to the aromatic amino acids phenylalanine, tytrosine, and tryptophan (Figure 14-8). Humans cannot synthesize Phe or Trp by themselves, but if adequate amounts of Phe are present in the diet, humans can convert this amino acid to Tyr. An important genetic disease, phenylketonuria (discussed later in this chapter), involves this latter conversion.