(4-Hydroxyphenyl)pyruvate Dioxygenase from Streptomyces avermitilis: The Basis for Ordered Substrate Addition

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Abstract
(4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) catalyzes the second step in the pathway for the catabolism of tyrosine, the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate (HG). This reaction involves decarboxylation, substituent migration, and aromatic oxygenation. HPPD is a member of the α-keto acid dependent oxygenases that require Fe(II) and an α-keto acid substrate to oxygenate an organic molecule. We have examined the binding of ligands to HPPD from Streptomyces avermitilis. Our data show that HPP binds to the apoenzyme and that the apo-HPPD·HPP complex does not bind Fe(II) to generate active holoenzyme. The binding of HPP, phenylpyruvate (PPA), and pyruvate to the holoenzyme produces a weak ligand charge-transfer band at ∼500 nm that is indicative of bidentate binding of the 1-carboxylate and 2-keto pyruvate oxygen atoms to the active site metal ion. For HPPD from this organism the 4-hydroxyl group of (4-hydroxyphenyl)pyruvate is a requirement for catalysis; no turnover is observed in the presence of phenylpyruvate. The rate constant for the dissociation of Fe(II) from the holoenzyme is 0.0006 s-1 and indicates that this phenomenon is not significantly relevant in steady-state turnover. The addition of HPP and molecular oxygen to the holoenzyme is formally random. The basis of the ordered bi bi steady-state kinetic mechanism previously observed by Rundgren (Rundgren, M. (1977) J. Biol. Chem.252, 5094−9) is the 3600-fold increase in oxygen reactivity when holo-HPPD is in complex with HPP. This complex reacts with molecular oxygen with a second-order rate constant of 1.4 × 105 M-1 s-1 inducing the formation of an intermediate that decays at the catalytically relevant rate of 7.8 s-1.

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