Decarboxylase Activity of an Antibiotic Biosynthesis Monooxygenase Family Protein in the Biosynthesis of the Type II Polyketide Murayaquinone

Type II aromatic polyketides represent a structurally diverse class of natural products with medicinally relevant properties, and their biosynthesis usually involves biosynthetic intermediates with terminal carboxyl groups. In certain instances, terminal decarboxylation occurs, which can significantly impact the structural complexity. However, the enzymes and their involved mechanisms of terminal decarboxylation in type II aromatic polyketide biosynthesis have rarely been studied. This study has now shown that MrqO5, a member of the antibiotic biosynthesis monooxygenase (ABM) family, unexpectedly functions as a terminal decarboxylase involved in the biosynthesis of murayaquinone. Furthermore, an <i>in vitro</i> biochemical study demonstrated that two homologous proteins of MrqO5 exhibited similar decarboxylase activity. Therefore, the functional assignment and mechanistic investigation of this polyketide terminal decarboxylase elucidated an overlooked step in type II polyketide biosynthesis. Also, the discovery of this new family of decarboxylases expands the functions of the ABM superfamily proteins. Our structural characterizations, combined with site-directed mutagenesis studies, have unveiled the key residues involved in the decarboxylation and allowed an enzymatic decarboxylation mechanism to be proposed. Our studies advance the currently incomplete understanding of type II aromatic polyketide biosynthesis and gain the insight necessary for future engineering of these enzymes.