Balance between photoreduction efficiency, cofactor affinity, and allosteric coupling of halogenase flavoenzymes.

Flavin-dependent halogenases (FDHs) are promising candidates for the sustainable production of halogenated organic molecules by biocatalysis. FDHs require only oxygen, halide and a fully reduced flavin adenine dinucleotide (FADH) cofactor to generate the reactive HOX that diffuses 10 Å to the substrate binding pocket and enables regioselective oxidative halogenation. A key challenge for the application of FDHs is the regeneration of the FADH.

Structure-based switch of regioselectivity in the flavin-dependent tryptophan 6-halogenase Thal.

Flavin-dependent halogenases increasingly attract attention as biocatalysts in organic synthesis, facilitating environmentally friendly halogenation strategies that require only FADH, oxygen, and halide salts. Different flavin-dependent tryptophan halogenases regioselectively chlorinate or brominate trypto-phan's indole moiety at C5, C6, or C7. Here, we present the first substrate-bound structure of a tryptophan 6-halogenase, namely Thal, also known as ThdH, from the bacterium at 2.