Infections by fungal pathogens such as Candida albicans and Aspergillus fumigatus and their resistance to triazole drugs are major concerns. Fungal lanosterol 14α-demethylase belongs to the CYP51 class in the cytochrome P450 superfamily of enzymes. This monospanning bitopic membrane protein is involved in ergosterol biosynthesis and is the primary target of azole antifungal drugs, including fluconazole. The lack of high-resolution structural information for this drug target from fungal pathogens has been a limiting factor for the design of modified triazole drugs that will overcome resistance. Here we report the X-ray structure of full-length Saccharomyces cerevisiae lanosterol 14α-demethylase in complex with fluconazole at a resolution of 2.05 Å. This structure shows the key interactions involved in fluconazole binding and provides insight into resistance mechanisms by revealing a water-mediated hydrogen bonding network between the drug and tyrosine 140, a residue frequently found mutated to histidine or phenylalanine in resistant clinical isolates.

Emergence of fungal strains showing resistance to triazole drugs can make treatment of fungal disease problematic. Triazole resistance can arise due to single mutations in the drug target lanosterol 14α-demethylase (Erg11p/CYP51). We have determined how commonly occurring single site mutations in pathogenic fungi affect triazole binding using Saccharomyces cerevisiae Erg11p (ScErg11p) as a target surrogate. The mutations Y140F/H were introduced into full-length hexahistidine-tagged ScErg11p. Phenotypes and high-resolution X-ray crystal structures were determined for the mutant enzymes complexed with short-tailed (fluconazole and voriconazole) or long-tailed (itraconazole and posaconazole) triazoles and wild type enzyme complexed with voriconazole. The mutations disrupted a water-mediated hydrogen bond network involved in binding of short-tailed triazoles, which contain a tertiary hydroxyl not present in long-tailed triazoles. This appears to be the mechanism by which resistance to these short chain azoles occurs. Understanding how these mutations affect drug affinity will aid the design of azoles that overcome resistance.

Targeting lanosterol 14α-demethylase (LDM) with azole drugs provides prophylaxis and treatments for superficial and disseminated fungal infections but cure rates are not optimal for immune compromised patients and individuals with co-morbidities. The efficacy of azole drugs has also been reduced due to the emergence of drug resistant fungal pathogens. We have addressed the need to improve the potency, spectrum and specificity for azoles by expressing in Saccharomyces cerevisiae functional, recombinant, hexahistidine-tagged, full-length Candida albicans LDM (CaLDM6×His) and Candida glabrata LDM (CgLDM6×His) and determining their X-ray crystal structures. The crystal structures of CaLDM6×His, CgLDM6×His and ScLDM6×His have the same fold and bind itraconazole in near identical conformations. The catalytic domains of the full-length LDMs have the same fold as the CaLDM6×His catalytic domain in complex with posaconazole, with minor structural differences within the ligand binding pocket. Our structures give insight into the LDM reaction mechanism and phenotypes of single site CaLDM mutations. This study provides a practical basis for the structure-directed discovery of novel antifungals that target LDMs of fungal pathogens.

A series of metallosupramolecular [Fe₂L₃](BF₄)₄ “click” cylinders have been synthesized in excellent yields (90%–95%) from [Fe(H₂O)₆](BF₄)₂ and bis(bidentate) pyridyl-1,2,3-triazole ligands. All complexes were characterized by elemental analysis, IR, UV-vis, ¹H-, ¹³C- and DOSY-NMR spectroscopies and, in four cases, the structures confirmed by X-ray crystallography. Molecular modeling indicated that some of these “click” complexes were of similar size and shape to related biologically active pyridylimine-based iron(II) helicates and suggested that the “click” complexes may bind both duplex and triplex DNA. Cell-based agarose diffusion assays showed that the metallosupramolecular [Fe₂L₃](BF₄)₄ “click” cylinders display no antifungal activity against S. cerevisiae. This observed lack of antifungal activity appears to be due to the poor stability of the “click” complexes in DMSO and biological media.

Azole antifungals, known as demethylase inhibitors (DMIs), target sterol 14α-demethylase (CYP51) in the ergosterol biosynthetic pathway of fungal pathogens of both plants and humans. DMIs remain the treatment of choice in crop protection against a wide range of fungal phytopathogens that have the potential to reduce crop yields and threaten food security. We used a yeast membrane protein expression system to overexpress recombinant hexahistidine-tagged S. cerevisiae lanosterol 14α-demethylase and the Y140F or Y140H mutants of this enzyme as surrogates in order characterize interactions with DMIs. The whole-cell antifungal activity (MIC₅₀ values) of both the R- and S-enantiomers of tebuconazole, prothioconazole (PTZ), prothioconazole-desthio, and oxo-prothioconazole (oxo-PTZ) as well as for fluquinconazole, prochloraz and a racemic mixture of difenoconazole were determined. In vitro binding studies with the affinity purified enzyme were used to show tight type II binding to the yeast enzyme for all compounds tested except PTZ and oxo-PTZ. High resolution X-ray crystal structures of ScErg11p6×His in complex with seven DMIs, including four enantiomers, reveal triazole-mediated coordination of all compounds and the specific orientation of compounds within the relatively hydrophobic binding site. Comparison with CYP51 structures from fungal pathogens including Candida albicans, Candida glabrata and Aspergillus fumigatus provides strong evidence for a highly conserved CYP51 structure including the drug binding site. The structures obtained using S. cerevisiae lanosterol 14α-demethylase in complex with these agrochemicals provide the basis for understanding the impact of mutations on azole susceptibility and a platform for the structure-directed design of the next-generation of DMIs.