ergosterol

Ergosterol is synthesized via a pathway that includes lanosterol demethylation by the enzyme 14-alpha-demethylase (a cytochrome P450 enzyme). This sterol is essential for fungal cell membrane structure and function, much like cholesterol is for human cells. Because ergosterol is absent in human cell membranes, it serves as an excellent selective target for antifungal pharmacotherapy, minimizing host toxicity.

Key antifungal drug classes targeting ergosterol pathway: 1. Azoles (fluconazole, itraconazole, voriconazole, ketoconazole) - inhibit 14-alpha-demethylase, blocking ergosterol synthesis from lanosterol, leading to toxic sterol accumulation. 2. Polyenes (amphotericin B, nystatin) - bind directly to ergosterol in the fungal membrane, forming pores that cause leakage of cellular contents and fungal cell death. 3. Allylamines (terbinafine) - inhibit squalene epoxidase, an earlier step in ergosterol synthesis, causing squalene accumulation (toxic) and ergosterol deficiency.

Clinical relevance: Understanding ergosterol's role explains the mechanism and selective toxicity of antifungals. Amphotericin B's affinity for ergosterol also explains its toxicity profile, as it has some lower affinity for cholesterol in human membranes (causing nephrotoxicity and infusion reactions). Resistance to azoles can occur via mutations in the 14-alpha-demethylase enzyme (ERG11 gene) or efflux pump upregulation.