Crystal structure of human quinone reductase type 2, a metalloflavoprotein

In mammals, two separate but homologous cytosolic quinone reductases have b een identified: NAD(P)H:quinone oxidoreductase type 1 (QR1) (EC 1.6.99.2) a nd quinone reductase type 2 (QR2). Although QR1 and QR2 are nearly 50% iden tical in protein sequence, they display markedly different catalytic proper ties and substrate specificities. We report here two crystal structures of QR2: in its native form and bound to menadione (vitamin K-3), a physiologic al substrate. Phases were obtained by molecular replacement, using our prev iously determined rat QR1 structure as the search model. QR2 shares the ove rall fold of the major catalytic domain of QR1, but lacks the smaller C-ter minal domain. The FAD binding sites of QRI and QR2 are very similar, but th eir hydride donor binding sites are considerably different. Unexpectedly, w e found that QR2 contains a specific metal binding site, which is not prese nt in QR1. Two histidine nitrogens, one cysteine thiol, and a main chain ca rbonyl group are involved in metal coordination. The metal binding site is solvent-accessible, and is separated from the FAD cofactor by a distance of about 13 Angstrom.