Single-Enzyme Conversion of Tryptophan to Skatole and Cyanide Expands the Mechanistic Competence of Diiron Oxidases

Abstract

Skatole is a pungent heterocyclic compound derived from the essential amino acid l-tryptophan by bacteria in the mammalian digestive tract. The four-step anaerobic conversion of tryptophan to skatole is well-established; though, to date, no aerobic counterpart has been reported. Herein, we report the discovery of the oxygen-dependent skatole synthase SktA that single-handedly converts 5-bromo-l-tryptophan to 5-bromoskatole, obviating the need for a multienzyme process. SktA is part of a three-gene biosynthetic gene cluster (BGC) in the cyanobacterium Nostoc punctiforme NIES-2108 and functions as a nonheme diiron enzyme belonging to the heme oxygenase-like domain-containing oxidase (HDO) superfamily. Our detailed biochemical analyses revealed cyanide and bicarbonate as biosynthetic coproducts, while stopped-flow experiments showed the hallmark formation of a substrate-triggered peroxo Fe2(III) intermediate. Overall, this work unravels an alternative pathway for converting tryptophan to skatole while also expanding the functional repertoire of HDO enzymes.