Structural basis for the erythro-stereospecificity of the L-arginine oxygenase VioC in viomycin biosynthesis.

The nonheme iron oxygenase VioC from Streptomyces vinaceus catalyzes Fe(II)-dependent and alpha-ketoglutarate-dependent Cbeta-hydroxylation of L-arginine during the biosynthesis of the tuberactinomycin antibiotic viomycin. Crystal structures of VioC were determined in complexes with the cofactor Fe(II), the substrate L-arginine, the product (2S,3S)-hydroxyarginine and the coproduct succinate at 1.1-1.3 A resolution. The overall structure reveals a beta-helix core fold with two additional helical subdomains that are common to nonheme iron oxygenases of the clavaminic acid synthase-like superfamily. In contrast to other clavaminic acid synthase-like oxygenases, which catalyze the formation of threo diastereomers, VioC produces the erythro diastereomer of Cbeta-hydroxylated L-arginine. This unexpected stereospecificity is caused by conformational control of the bound substrate, which enforces a gauche(-) conformer for chi(1) instead of the trans conformers observed for the asparagine oxygenase AsnO and other members of the clavaminic acid synthase-like superfamily. Additionally, the substrate specificity of VioC was investigated. The side chain of the L-arginine substrate projects outwards from the active site by undergoing interactions mainly with the C-terminal helical subdomain. Accordingly, VioC exerts broadened substrate specificity by accepting the analogs L-homoarginine and L-canavanine for Cbeta-hydroxylation.