An N-nitrosating metalloenzyme constructs the pharmacophore of streptozotocin

Small molecules containing the N-nitroso group, such as the bacterial natural product streptozotocin, are prominent carcinogens^1,2 and important cancer chemotherapeutics^3,4. Despite the considerable importance of this functional group to human health, enzymes dedicated to the assembly of the N-nitroso unit have not been identified. Here we show that SznF, a metalloenzyme from the biosynthesis of streptozotocin, catalyses an oxidative rearrangement of the guanidine group of N^ω-methyl-L-arginine to generate an N-nitrosourea product. Structural characterization and mutagenesis of SznF reveal two separate active sites that promote distinct steps in this transformation using different iron-containing metallocofactors. This biosynthetic reaction, which has little precedent in enzymology or organic synthesis, expands the catalytic capabilities of non-haem-iron-dependent enzymes to include N-N bond formation. We find that biosynthetic gene clusters that encode SznF homologues are widely distributed among bacteria-including environmental organisms, plant symbionts and human pathogens-which suggests an unexpectedly diverse and uncharacterized microbial reservoir of bioactive N-nitroso metabolites.