enhancement: The X‐ray crystal structure of an arylmalonatedecarboxylase (AMDase) with a mechanism‐based inhibitor bound to an active‐site dioxyanion hole provides insight into the mechanism of this intriguing enzyme. The structure also guided the extension of the AMDase biocatalytic repertoire to include a wide range of α‐alkenyl as well as α‐arylmalonates.