Residue Phe42 is critical for the catalytic activity of Escherichia coli major nitroreductase NfsA

The major O₂-insensitive nitroreductase (NfsA) of Escherichia coli shares low sequence homology but similar biochemical and structural features with NfsB, the E. coli minor O₂-insensitive nitroreductase. A structural comparison revealed Phe42 was present in the active site of NfsA but not NfsB. F42Y, F42N and F42A were generated and had decreased activity toward nitrofurazone by 52, 96, and 99 %, respectively. The kinetic parameters for other nitroaromatic substrates were also determined. Compared to wild type, the mutants did not have significantly altered K _ms, but had dramatically decreased k _cat and k _cat/K _m values. Far-UV CD spectral analysis of the mutants suggested that there were no significant conformational changes however F42A and F42N had changes from 208 to 222 nm, which was attributed to loss of helix content. These findings revealed that Phe42 is important for maintaining NfsA activity and structure.