Metabolic control analysis and engineering of the yeast sterol biosynthetic pathway

Sterols are essential components of eukaryotic membranes and contribute to important physiological characteristics, such as osmotic robustness and hormonal effects on growth (Daum et al., 1999), but most importantly, the sterol pathway has shown to be a highly effective target for antifungal drug development. The incidence of life-threatening fungal infections has been increased, particularly among patients who are immuno-compromised by Human Immunodeficiency Virus infection (HIV causing AIDS) and those receiving immuno-suppressive therapy for organ transplantation or chemotherapy for cancer. Resistance among various fungal pathogens in these patients to antifungal drugs has been a matter of concern. Therefore, development of more effective antifungal agents has become a priority. Our research focuses on the yeast sterol biosynthesis. The type of sterol observed varies between the Kingdoms of Life; in yeast it is ergosterol. Starting from 2-aceto-CoA, ergosterol is synthesized by a sequence of at least 21 enzymes, among them two different cytochrome-P450 heme oxygenases. One P450, lanosterol C-14α demethylase (sterol biosynthetic step 13), encoded by the yeast ERG11-gene, is specifically inhibited by the antifugal compound fluconazole. This particular step (13) is suggested to be important for fungal growth. However, how control on ergosterol biosynthesis in yeast is distributed among this and all other steps in the pathway is still unknown. In the current report, we present the first results of a systematic approach to quantify metabolic control of the ergosterol biosynthesis in the yeast Saccharomyces cerevisiae.