BASES FOR BREADTH - INSIGHTS INTO HOW THE MECHANISM AND DYNAMICS OF NITROREDUCTASE CAN EXPLAIN THIS ENZYME\u27S BROAD SUBSTRATE REPERTOIRE

Nitroreductase from Enterobacter cloacae (NR) is a member of a large family of homologues represented in all branches of the tree of life. However the physiological roles of many of these enzymes remain unknown. NR has distinguished itself on the basis the diverse sizes and chemical types of substrates it is able to reduce (Koder et al 1998). This might be an evolved characteristic suiting NR for a role in metabolism of diverse occasional toxins. While there are numerous studies of determinants of substrate specificity, we know less about mechanisms by which enzymes can be inclusive. Therefore, we present a synthesis of NR\u27s dynamics, stability, ligand binding repertoire and kinetic mechanism. We find that NR reduces para-nitrobenzoic acid (p-NBA) via a simple mechanism limited by the chemical step in which the nitro group is reduced (Pitsawong et al 2014). Thus, for this substrate, NR\u27s mechanism dispenses with gating steps that in other enzymes can enforce substrate specificity. Our data demonstrate that substrate reduction is accomplished by rate-contributing hydride transfer from the flavin cofactor coupled to proton transfer from solvent, but do not identify specific amino acids with a role. This is consistent with our crystal structures, which reveal a spacious solvent-exposed active site bounded by a helix that moves to accommodate binding of substrate analogs (Haynes et al 2002). Because it is able to reduce TNT (trinitrotoluene), herbicides and pesticides, NR has important potential utility in bioremediation