CYP51

CYP51 is a member of the cytochrome P450 superfamily found in fungi (and other organisms) that catalyzes the demethylation of lanosterol at the C14 position, a key step in the ergosterol biosynthesis pathway. Ergosterol is analogous to cholesterol in mammalian cells and is required for maintaining fungal cell membrane fluidity, integrity, and function of membrane-bound enzymes.

Azole antifungals (e.g., fluconazole, itraconazole, voriconazole, posaconazole, and topical azoles like clotrimazole/ketoconazole) work by binding to the heme iron of CYP51, inhibiting its enzymatic activity. This blocks ergosterol synthesis and causes accumulation of toxic 14α-methylated sterols, leading to disrupted membrane structure, increased permeability, and inhibition of fungal growth (fungistatic effect, though higher concentrations can be fungicidal against some species).

Clinical relevance: - Azole resistance often arises from mutations in the CYP51 gene (ERG11 in yeast nomenclature), overexpression of CYP51, or upregulation of efflux pumps that reduce intracellular azole concentration. - Selectivity of azoles for fungal CYP51 over human CYP450 enzymes (like CYP3A4) is imperfect, which explains many of the significant drug-drug interactions and side effects (e.g., ketoconazole's strong inhibition of human steroidogenesis and CYP3A4). - CYP51 homologs also exist in plants and mammals, but fungal CYP51 has structural differences that allow for selective drug targeting, though newer azoles have improved selectivity.

This enzyme is a classic board exam topic connecting fungal cell biology, pharmacology mechanism of action, and antifungal resistance mechanisms.