NAD(P)H/quinone acceptor oxidoreductase type 1 (QR1) protects cells from cytotoxic and neoplastic effects of quinones though two-electron reduction. Kinetic experiments, docking, and binding affinity calculations were performed on a series of structurally varied quinone substrates. A good correlation between calculated and measured binding affinities from kinetic determinations was obtained. The experimental and theoretical studies independently support a model in which quinones (with one to three fused aromatic rings) bind in the QR1 active site utilizing a π-stacking interaction with the isoalloxazine ring of the FAD cofactor. NAD(P)H:quinone acceptor oxidoreductase type 1 (QR1, 1 EC 1.6.99.2), a homodimeric flavoprotein of 273 residues (M ) 30815) per monomer, catalyzes an obligatory twoelectron reduction of quinones using either NADH or NADPH as electron donors (1, 2). QR1 affords protection against the cytotoxic and neoplastic effects of electrophilic quinones and partially reduced semiquinones that can undergo redox cycling to generate active oxygen species (3). QR1, found in many solid tumors at elevated levels, can be used to target the tumor cells through bioreductive activation of quinone-based chemotherapeutic agents (3).The crystal structures of quinone reductases from rat (PDB ID: 1QRD), mouse (1DXQ), and human (1DXO, 1D4A), reflecting a high degree of sequence identity, show very small differences in the positions of the 220 R-carbons in the catalytic domain and possess nearly identical active sites (1,4,5). The crystal structure for the rat enzyme (1QRD) contains duroquinone, FAD, and the NAD(P)H analogue cibracon blue, but the latter ligand sterically intrudes into the active site, affecting the position of at least one key residue, Tyr128 (1, 4). The human enzyme, whose crystal structure (1DXO) contains duroquinone and FAD but no cibracon blue, differs from the rat enzyme by one amino acid (Gln104 in human, Tyr104 in rat) in the active site, resulting in a slight change (0.7 Å) in the positioning of the flavin ring (1).The quinone reductase active site, produced by residues from each subunit of the dimeric protein, has the following features: (1) the duroquinone overlays the FAD isoalloxazine ring that is bound to one subunit, with the quinoid carbonyl oxygens oriented roughly parallel to the length of the flavin ring; (2) a loop from the second subunit, which forms a lid for the active site above the bound quinone, provides two tyrosine residues, Tyr126 and Tyr128, that hydrogen bond directly, or indirectly via a water molecule, to the quinone carbonyl groups; and (3) three aromatic residues provide hydrophobic contacts with the quinone, and one of these, Trp105, forms a wall bordering one side of the active site (1). A depiction of the human quinone reductase (1DXO) active site is shown in Figure 1.Mechanistically, a hydride ion from NAD(P)H is thought to be transferred to the flavin nitrogen, N 5 , with possible charge stabilization from a catalytic triad involving the FAD, Tyr155, ...