Cystalysin from Treponema denticola is a
pyridoxal 5'-phosphate dependent
lyase that catalyzes the formation of
pyruvate,
ammonia, and
sulfide from
cysteine. It is a
virulence factor in
adult periodontitis because its reaction contributes to
hemolysis, which sustains the pathogen. Therefore, it was proposed as a potential antimicrobial target. To identify specific inhibitors by structure-based in silico methods, we first validated the crystal structure of
cystalysin as a reliable starting point for the design of
ligands. By using single-crystal absorption microspectrophotometry, we found that the
enzyme in the crystalline state, with respect to that in
solution, exhibits: 1) the same absorption spectra for the catalytic intermediates, 2) a close pKa value for the residue controlling the keto enamine ionization, and 3) similar reactivity with
glycine,
L-serine,
L-methionine, and the nonspecific irreversible inhibitor
aminoethoxyvinylglycine. Next, we screened in silico a library of 9357 compounds with the Fingerprints for
Ligands and
Proteins (FLAP) software, by using the three-dimensional structure of
cystalysin as a template. From the library, 17 compounds were selected and experimentally evaluated by
enzyme assays and spectroscopic methods. Two compounds were found to competitively inhibit recombinant T. denticola
cystalysin, with inhibition constant (Ki ) values of 25 and 37 μM. One of them exhibited a minimum inhibitory concentration (MIC) value of 64 μg mL(-1) on Moraxella catarrhalis ATCC 23246, which proves its ability to cross bacterial membranes.