What is the Origin of the Regioselective C3‐Hydroxylation of L‐Arg by the Nonheme Iron Enzyme Capreomycin C?

The nonheme iron dioxygenase capreomycin C (CmnC) hydroxylates a free L‐arginine amino acid regio‐ and stereospecifically at the C 3 ‐position as part of the capreomycin antibiotics biosynthesis. Little is known on its structure, catalytic cycle and substrate specificity and, therefore, a comprehensive computational study was performed. A large QM cluster model of CmnC was created of 297 atoms and the mechanisms for C 3 −H, C 4 −H and C 5 −H hydroxylation and C 3 −C 4 desaturation were investigated. All low‐energy pathways correspond to radical reaction mechanisms with an initial hydrogen atom abstraction followed by OH rebound to form alcohol product complexes. The work is compared to alternative L‐Arg hydroxylating nonheme iron dioxygenases and the differences in active site polarity are compared. We show that a tight hydrogen bonding network in the substrate binding pocket positions the substrate in an ideal orientation for C 3 −H activation, whereby the polar groups in the substrate binding pocket induce an electric field effect that guides the selectivity.