Lb. Sanchez et al., NAD(P)H : menadione oxidoreductase of the amitochondriate eukaryote Giardia lamblia: a simpler homologue of the vertebrate enzyme, MICROBIO-UK, 147, 2001, pp. 561-570
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.