Three-dimensional structure of AzoR from Escherichia coli. An oxidereductase conserved in microorganisms.

The crystal structure of AzoR (azoreductase) has been determined in complex with FMN for two different crystal forms at 1.8 and 2.2 A resolution. AzoR is an oxidoreductase isolated from Escherichia coli as a protein responsible for the degradation of azo compounds. This enzyme is an FMN-dependent NADH-azoreductase and catalyzes the reductive cleavage of azo groups by a ping-pong mechanism. The structure suggests that AzoR acts in a homodimeric state forming the two identical catalytic sites to which both monomers contribute. The structure revealed that each monomer of AzoR has a flavodoxin-like structure, without the explicit overall amino acid sequence homology. Superposition of the structures from the two different crystal forms revealed the conformational change and suggested a mechanism for accommodating substrates of different size. Furthermore, comparison of the active site structure with that of NQO1 complexed with substrates provides clues to the possible substrate-binding mechanism of AzoR.