Tryptophan 7-halogenase which is involved in pyrrolnitrin biosynthesis is the first halogenating enzyme to be isolated that has substrate specificity and regioselectivity. This FADH2-dependent halogenase catalyzes the chlorination of its natural substrate tryptophan exclusively at the 7-position, a position at which direct chemical chlorination is not possible. Other substrates such as N-Ω-methyltryptamine, 5-methyltryptamine, 5-methylindole, 3-methylindole, or indole-3-acetonitrile are also chlorinated by the enzyme, whereas compounds like 1-methyltryptophan, indole-3-carboxylic acid, indole-3-acetic acid, or indole are not accepted as substrates. In addition, phenylpyrrole derivatives are also chlorinated by the enzyme. However, in contrast to tryptophan, the tryptophan and indole derivatives are chlorinated at positions 2 or/and 3 of the indole ring system and not at the 7-position. Chlorination of the phenylpyrrole derivatives also proceeds without regioselectivity and a mixture of mono- and dichlorinated products is obtained.
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Hölzer et al. (2001) studied this question.