This is the first biocatalytic synthesis route for indazoles that is crucially dependent on enzymatic activity. The direct two-electron reduction of nitro to nitroso compounds can be achieved by NRs. However, the high selectivity of the reaction compared to spontaneous indazole formation is remarkable and the selectivity is in good agreement with our previous observation that the reduction of nitroso intermediates is poorly performed by BaNTR1 . The fact that no over-reduction by-products 3 and 4 were isolated under the reaction conditions used, indicates that the spontaneous cyclization/aromatization of compounds 2 to 6 proceeds rapidly. This is consistent with the finding that nitroso compounds (2) were never detected in our reaction mixture. Although the instability of 2 precludes kinetic studies, the abinity of NfsA and BaNTR1 for 2 may be low, as suggested by kinetic studies with 1a. While the apparent Michaelis-Menten constant (KM,app) values for NAD(P)H of 58 ± 19 µM and 41 ± 9 µM for NfsA and BaNTR1 (Figures and), respectively, are in the typical range for NRs, the KM,app values for 1a are very high. A KM,app value of 10.8 ± 2.9 mM was determined for BaNTR1 (Figure ), while the determination for NfsA was not possible because the correlation between indazole formation and substrate concentration was still linear up to 20 mM (Figure ).