NAD(P)H : menadione oxidoreductase of the amitochondriate eukaryote Giardia lamblia: a simpler homologue of the vertebrate enzyme

The amitochondriate eukaryote Giardia lamblia contains an NAD(P)H:menadione oxidoreductase (EC 1.6.99.2) (gIQR) that catalyses the two-electron transf er oxidation of NAD(P)H with a quinone as acceptor. The gene encoding this protein in G. lamblia was expressed in Escherichia coli. The purified recom binant protein had an NAD(P)H oxidoreductase activity, with NADPH being a m ore efficient electron donor than NADH. Menadione, naphthoquinone and sever al artificial electron accepters served as substrate for the enzyme. gIQR s hows high amino acid similarity to its homologues in vertebrates and also t o a series of hypothetical proteins from bacteria. Although gIQR is conside rably smaller than the mammalian enzymes, three-dimensional modelling shows similar arrangement of the secondary structural elements. Most amino acid residues of the mammalian enzymes that participate in substrate binding or catalysis are conserved. Conservation of these features and the similarity in substrate specificity and in susceptibility to inhibitors establish gIQR as an authentic member of this protein family.