Phenylalanine meta-Hydroxylase: A Single Residue Mediates Mechanistic Control of Aromatic Amino Acid Hydroxylation

The rare nonproteinogenic 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 coeruleorubidus. Insights from kinetic analyses of the wild-type enzyme and key mutants as well as of the biocatalytic conversion of synthetic isotopically labelled substrates and fluorinated substrate analogues advance understanding of the process by which meta-hydroxylation is mediated, revealing T202 to play an important role. In the case of the WT enzyme, a deuterium label at the 3-position is lost, whereas in in the T202A mutant 75 % retention is observed, with loss of stereospecificity. These data suggest that one of two possible mechanisms is at play; direct, enzyme-catalysed deprotonation following electrophilic aromatic substitution or stereospecific loss of one proton after a 1,2-hydride shift. 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.