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Tryptophan halogenases are found in diverse organisms and catalyze regiospecific halogenation. They play an important role in the biosynthesis of halogenated indole alkaloids, which are biologically active and of therapeutic importance. Here, a tryptophan 6‐halogenase (SatH) from Streptomyces albus was characterized by using a whole‐cell reaction system in Escherichia coli. SatH showed substrate specificity for chloride and bromide ions, leading to regiospecific halogenation at the C6‐position of l‐tryptophan. In addition, SatH exhibited higher performance in bromination than that of previously reported tryptophan halogenases in the whole‐cell reaction system. Through structure‐based protein mutagenesis, it has been revealed that two consecutive residues, A78/V79 in SatH and G77/I78 in PyrH, are key determinants in the regioselectivity difference between tryptophan 6‐ and 5‐halogenases. Substituting the AV with GI residues switched the regioselectivity of SatH by moving the orientation of tryptophan. These data contribute to an understanding of the key residues that determine the regioselectivity of tryptophan halogenases.

Melanin nanoparticles (MNPs) used for biomedical applications are often synthesized via the chemical auto-oxidation of catecholic monomers such as dopamine and 3,4-dihydroxyphenylalanine (DOPA) under alkaline conditions.

No abstract

Changing nature: Tryptophan halogenases, which play an important role in the biosynthesis of halogenated indole alkaloids, are found in diverse organisms and catalyze regiospecific halogenation. A tryptophan 6‐halogenase from S. albus is identified, and key residues that determine the regioselectivity difference between tryptophan 6‐ and 5‐halogenases are investigated. More information can be found in the full paper by B.‐G. Kim et al. on page 1446 in Issue 10, 2020 (DOI: 10.1002/cbic.201900723).