Bacillus anthracis causes
anthrax disease and exerts its deleterious effects by the release of three
exotoxins: lethal factor, protective
antigen, and
edema factor (EF), a highly active
calmodulin-dependent
adenylyl cyclase (AC). However, conventional
antibiotic treatment is ineffective against either
toxemia or
antibiotic-resistant strains. Thus, more effective drugs for
anthrax treatment are needed. Previous studies from our laboratory showed that mammalian membranous AC (mAC) exhibits broad specificity for
purine and
pyrimidine nucleotides ( Mol Pharmacol 70: 878-886, 2006 ). Here, we investigated structural requirements for EF inhibition by natural
purine and
pyrimidine nucleotides and
nucleotides modified with N-methylanthraniloyl (MANT)- or anthraniloyl groups at the 2'(3')-O-ribosyl position. MANT-
CTP was the most potent EF inhibitor (K(i), 100 nM) among 16 compounds studied. MANT-
nucleotides inhibited EF competitively. Activation of EF by
calmodulin resulted in effective fluorescence resonance energy transfer (FRET) from
tryptophan and
tyrosine residues located in the vicinity of the catalytic site to
MANT-ATP, but FRET to MANT-
CTP was only small. Mutagenesis studies revealed that Phe586 is crucial for FRET to
MANT-ATP and MANT-
CTP and that the mutations N583Q, K353A, and K353R differentially alter the inhibitory potencies of
MANT-ATP and MANT-
CTP. Docking approaches relying on crystal structures of EF indicate similar binding modes of the MANT
nucleotides with subtle differences in the region of the nucleobases. In conclusion, like mAC, EF accommodates both
purine and
pyrimidine nucleotides. The unique preference of EF for the base
cytosine offers an excellent starting point for the development of potent and selective EF inhibitors.