Phenylalanine meta-hydroxylase: a single residue mediates mechanistic control of aromatic amino acid hydroxylation

The rare non-proteinogenic amino acid, meta- L-tyrosine is biosynthetically intriguing. Whilst the biogenesis of tyrosine from phenylalanine is well characterised, the mechanistic basis for meta-hydroxylation is unknown. Herein, we report the analysis of 3-hydroxylase (Phe3H) from Streptomyces coeruleorbidus. Insight from kinetic analyses, of both the wild-type enzyme and key mutants, of the biocatalytic conversion of synthetic isotopically labelled substrates and fluorinated substrate analogues advances understanding of the process by which meta-hydroxylation is mediated, revealing T202 to play an important role. In contrast to the established mechanism of tyrosine biogenesis, which proceeds via NIH shift, our data support direct, enzyme catalysed deprotonation following electrophilic aromatic substitution. We demonstrate that T202 is responsible for this shift in mechanism, with mutation to alanine resulting in a switch to the NIH shift mechanism and loss of regiospecificity. Furthermore, our kinetic parameters for Phe3H show efficient regiospecific generation of meta-L-tyrosine from phenylalanine and demonstrate the enzyme's ability to regiospecifically hydroxylate unnatural fluorinated substrates.