CB1954 is a cancer pro-drug that can be activated through reduction by
Escherichia coli nitro-reductases and quinone reductases. Human quinone
reductase 2 is very efficient in the activation of CB1954, approximately 3000
times more efficient than human QR1 in terms of k(cat)/K(m). We have solved the
three-dimensional structure of QR2 in complex with CB1954 to a nominal
resolution of 1.5A. The complex structure indicates the essentiality of the two
nitro groups: one nitro group forms hydrogen bonds with the side-chain of Asn161
of QR2 to hold the other nitro group in position for the reduction. We further
conclude that residue 161, an Asn in QR2 and a His in QR1, is critical in
differentiating the substrate specificities of these two enzymes. Mutation of
Asn161 to His161 in QR2 resulted in the total loss of the enzymatic activity
towards activation of CB1954, whereas the rates of reduction towards menadione
are not altered.
