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Phenylalanine hydroxylase isa mononuclear non-heme iron protein that uses tetrahydropterin as the source of the two electrons needed to activate dioxygen for the hydroxylation of phenylalanine to tyrosine. Rapid-quench methods have been used to analyze the mechanism of a bacterial phenylalanine hydroxylase from Chromobacterium violaceum. Mössbauer spectra of samples prepared by freeze-quenching the reaction of the enzyme-57Fe(II)-phenylalanine-6-methyltetrahydropterin complex with O2 reveal the accumulation of an intermediate at short reaction times (20-100 ms). The Mössbauer parameters of the intermediate (δ = 028 mm/s, and ∣ΔEQ∣ = 1.26 mm/s) suggest that it is a high-spin Fe(IV) complex similar to those that have previously been detected in the reactions of other mononuclear Fe(II) hydroxylases, including a tetrahy- dropterin-dependent tyrosine hydroxylase. Analysis of the tyrosine content of acid-quenched samples from similar reactions establishes that the Fe(IV) intermediate is kinetically competent to be the hydroxylating intermediate. Similar chemical-quench analysis of a reaction allowed to proceed for several turnovers shows a burst of tyrosine formation, consistent with rate-limiting product release. All three data sets can be modeled with a mechanism in which the enzyme-substrate complex reacts with oxygen to form an Fe(IV)O intermediate with a rate constant of 19 mM-1 s-1, the Fe(IV)0 intermediate hydroxylates phenylalanine with a rate constant of 42 s-1, and rate-limiting product release occurs with a rate constant of 6 s at 5°C. [ABSTRACT FROM AUTHOR]