The remarkable site-selectivity and broad substrate scope of flavin dependent halogenases (FDHs) has led to much interest in their potential as biocatalysts. Multiple engineering efforts have demonstrated that FDHs can be tuned for non-native substrate scope and site-selectivity. FDHs have also proven useful as in vivo biocatalysts and have been successfully incorporated into biosynthetic pathways to build new chlorinated aromatic compounds in several heterologous organisms. In both cases, reduced flavin cofactor, usually supplied by a separate flavin reductase (FR), is required. Here, we report functional synthetic, fused FDH-FR proteins, containing various FDHs and FRs, joined by different linkers. We show that FDH-FR fusion proteins can increase product titers compared to the individual components for in vivo biocatalysis in E. coli.

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Flavin-dependent halogenases catalyze selective halogenation of electron-rich aromatic compounds without the need for harsh oxidants required by conventional oxidative halogenation reactions. Predictive models for halogenase site selectivity would greatly improve their utility for chemical synthesis. Toward this end, we analyzed the structures and selectivity of three halogenase variants evolved to halogenate tryptamine with orthogonal selectivity. Crystal structures and reversion mutations revealed key residues involved in altering halogenase selectivity. Density functional theory calculations and molecular dynamics simulations are both consistent with hypohalous acid as the active halogenating species in FDH catalysis. This model was used to accurately predict the site selectivity of halogenase variants toward different synthetic substrates, providing a valuable tool for implementing halogenases in biocatalysis efforts.