Required to synthesize hydroxyphenylglycine, a recurring skeletal component of nonproteinogenic macrocyclic peptide antibiotics such as vancomycin. Catalyzes the conversion of p-hydroxyphenylpyruvate to p-hydroxymandelate. The decarboxylation and hydroxylation activities of HmaS show novel and distinct regioselectivity, compared to all other known p-hydroxyphenylpyruvate dioxygenases, by hydroxylating the benzylic position of the substrate instead of the phenyl ring. 3-(4-hydroxyphenyl)pyruvate + O2 = (S)-4-hydroxymandelate + CO2 Binds 1 Fe cation per subunit. Antibiotic biosynthesis; vancomycin biosynthesis. Monomer. Belongs to the 4HPPD family.