Cryptic chlorination by a non-haem iron enzyme during cyclopropyl amino acid biosynthesis

Enzymatic incorporation of chlorine, bromine or iodine atoms occurs during the biosynthesis of more than 4,000 natural products(1). Halogenation can have significant consequences for the bioactivity of these products so there is great interest in understanding the biological catalysts that perform these reactions. Enzymes that halogenate unactivated aliphatic groups have not previously been characterized. Here we report the activity of five proteins - CmaA, CmaB, CmaC, CmaD and CmaE - in the construction of coronamic acid (CMA; 1-amino-1-carboxy-2-ethylcyclopropane), a constituent of the phytotoxin coronatine synthesized by the phytopathogenic bacterium Pseudomonas syringae(2). CMA derives from L-allo-isoleucine, which is covalently attached to CmaD through the actions of CmaA, a non-ribosomal peptide synthetase module, and CmaE, an unusual acyltransferase. We show that CmaB, a member of the non-haem Fe2+, alpha-ketoglutarate-dependent enzyme superfamily, is the first of its class to show halogenase activity, chlorinating the gamma-position of L-allo-isoleucine. Another previously undescribed enzyme, CmaC, catalyses the formation of the cyclopropyl ring from the gamma-Cl-L-allo-isoleucine product of the CmaB reaction. Together, CmaB and CmaC execute gamma-halogenation followed by intramolecular gamma-elimination, in which biological chlorination is a cryptic strategy for cyclopropyl ring formation.