<i>Bacillus anthracis</i> Inosine 5′-Monophosphate Dehydrogenase in Action: The First
Bacterial Series of Structures of Phosphate Ion‑, Substrate‑,
and Product-Bound Complexes
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Abstract
Inosine 5′-monophosphate dehydrogenase (IMPDH)
catalyzes
the first unique step of the GMP branch of the purine nucleotide biosynthetic
pathway. This enzyme is found in organisms of all three kingdoms.
IMPDH inhibitors have broad clinical applications in cancer treatment,
as antiviral drugs and as immunosuppressants, and have also displayed
antibiotic activity. We have determined three crystal structures of <i>Bacillus anthracis</i> IMPDH, in a phosphate ion-bound (termed
“apo”) form and in complex with its substrate, inosine
5′-monophosphate (IMP), and product, xanthosine 5′-monophosphate
(XMP). This is the first example of a bacterial IMPDH in more than
one state from the same organism. Furthermore, for the first time
for a prokaryotic enzyme, the entire active site flap, containing
the conserved Arg-Tyr dyad, is clearly visible in the structure of
the apoenzyme. Kinetic parameters for the enzymatic reaction were
also determined, and the inhibitory effect of XMP and mycophenolic
acid (MPA) has been studied. In addition, the inhibitory potential
of two known <i>Cryptosporidium parvum</i> IMPDH inhibitors
was examined for the <i>B. anthracis</i> enzyme and compared
with those of three bacterial IMPDHs from <i>Campylobacter jejuni</i>, <i>Clostridium perfringens</i>, and <i>Vibrio cholerae</i>. The structures contribute to the characterization of the active
site and design of inhibitors that specifically target <i>B.
anthracis</i> and other microbial IMPDH enzymes