Structures of recombinant human and mouse NAD(P)H : quinone oxidoreductases: Species comparison and structural changes with substrate binding and release

NAD(P)H/quinone acceptor oxidoreductase (QR1, NQO1, formerly DT-diaphorase; EC 1.6.99.2) protects animal cells from the deleterious and carcinogenic e ffects of quinones and other electrophiles. In this paper we report the apo enzyme structures of human (at 1.7-Angstrom resolution) and mouse (2.8 Angs trom) QR1 and the complex of the human enzyme with the substrate duroquinon e (2.5 Angstrom) (2,3,5,6-tetramethyl-p-benzoquinone). In addition to provi ding a description and rationale of the structural and catalytic difference s among several species, these structures reveal the changes that accompany substrate or cofactor (NAD) binding and release. Tyrosine-128 and the loop spanning residues 232-236 close the binding site, partially occupying the space left vacant by the departing molecule (substrate or cofactor). These changes highlight the exquisite control of access to the catalytic site tha t is required by the ping-pong mechanism in which, after reducing the flavi n, NAD(P)(+) leaves the catalytic site and allows substrate to bind at the vacated position. In the human QR1-duroquinone structure one ring carbon is significantly closer to the flavin N5, suggesting a direct hydride transfe r to this atom.