Intrinsic isotope effects on benzylic hydroxylation by the aromatic amino acid hydroxylases: Evidence for hydrogen tunneling, coupled motion, and similar reactivities

Jorge Alex Pavon, Paul F. Fitzpatrick

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Deuterium kinetic isotope effects for hydroxylation of the methyl group of 4-methylphenylalanine have been used as a probe of the relative reactivities of the hydroxylating intermediates in the aromatic amino acid hydroxylases phenylalanine, tyrosine, and tryptophan hydroxylase. When there are three deuterium atoms in the methyl group, all three enzymes exhibit an intrinsic isotope effect of about 13. The temperature dependence of the isotope effect is consistent with moderate tunneling, with the extent of tunneling identical for all three enzymes. In the case of phenylalanine hydroxylase, the presence of the regulatory domain has no effect on the values. The intrinsic primary and secondary isotope effects were determined using 4-methylphenylalanine containing one or two deuterium atoms in the methyl group. With one deuterium atom, the intrinsic primary and secondary effects have average values of 10 and 1.1, respectively. With two deuterium atoms, the primary effects decrease to 7.4 and the secondary effect increases to 1.3, consistent with coupled motion of the primary and secondary hydrogens. The results with all three enzymes are consistent with a hydrogen abstraction mechanism. The similarities of the isotope effects and extent of tunneling establish that the reactivities of the hydroxylating intermediates in the three enzymes are essentially identical.

Original languageEnglish (US)
Pages (from-to)16414-16415
Number of pages2
JournalJournal of the American Chemical Society
Issue number47
StatePublished - Nov 30 2005


ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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