Acanthamoeba castellanii is a free-living
amoeba that can cause severe eye and brain infections in humans. At
present, there is no uniformly effective treatment for any of these
infections. However, sterol 14α-demethylases (CYP51s), heme-containing
cytochrome P450 enzymes, are known to be validated drug targets in
pathogenic fungi and protozoa. The catalytically active P450 form
of CYP51 from A. castellanii (AcCYP51)
is stabilized against conversion to the inactive P420 form by dimerization.
In contrast, Naegleria fowleri CYP51
(NfCYP51) is monomeric in its active P450 and inactive P420 forms.
For these two CYP51 enzymes, we have investigated the interplay between
the enzyme activity and oligomerization state using steady-state and
time-resolved UV–visible absorption spectroscopy. In both enzymes,
the P450 → P420 transition is favored under reducing conditions.
The transition is accelerated at higher pH, which excludes a protonated
thiol as the proximal ligand in P420. Displacement of the proximal
thiolate ligand is also promoted by adding exogenous nitrogenous ligands
(N-ligands) such as imidazole, isavuconazole, and clotrimazole that
bind at the opposite, distal heme side. In AcCYP51, the P450 →
P420 transition is faster in the monomer than in the dimer, indicating
that the dimeric assembly is critical for stabilizing thiolate coordination
to the heme and thus for sustaining AcCYP51 activity. The spectroscopic
experiments were complemented with size-exclusion chromatography and
X-ray crystallography studies. Collectively, our results indicate
that effective inactivation of the AcCYP51 function by azole drugs
is due to synergistic interference with AcCYP51 dimerization and promoting
irreversible displacement of the proximal heme–thiolate ligand.