Figure 21.12 provides an overview of the biosynthesis of aromatic amino acids and histidine. All of the carbons in phenylalanine and tyrosine are derived from erythrose-4-phosphate and phosphoenolpyruvate. A key intermediate in synthesis of virtually all aromatic compounds (including p-aminobenzoic acid) in plant and bacterial cells is shikimic acid. Shikimic acid gives rise to chorismate (Figure 21.13), which is a precursor of p-hydroxybenzoic acid (Figure 21.14). p-Hydroxybenzoic acid, in turn, can be converted to coenzyme Q.