An NADPH-dependent cyclopropanase (CurF ER) crystal structure at 0.96 Å is reported
Two thioester enoyl reductases (JamJ and CurK ERs) are highly similar to CurF ER
Insertion of a CurF ER “cyclopropanase loop” confers the unusual activity on JamJ ER
A covalent adduct of NADPH and the enoyl thioester substrate is detected in CurK ER