O

Vitamin D

membrane (almost equimolar with all phospholipids). Between 50 and 90 percent of the cholesterol in most mammalian cells is present in the plasma membrane and related endocytic vesicle membranes. Cholesterol is also critical for intercellular signaling and has other functions to be described shortly. The structures of the principal yeast sterol (ergosterol) and plant phytosterols (e.g., stigmasterol) differ slightly from that of cholesterol (Figure 18-6). The small differences in the biosynthetic pathways of fungal and animal sterols and in their structures are the basis of most antifungal drugs currently in use.

Cholesterol Is Synthesized by Enzymes in the Cytosol and ER Membrane

Figure 18-7 summarizes the complex series of reactions that yield cholesterol and several other related biomolecules. The basic features of this pathway are important in the synthesis of other important lipids, and familiarity with these features helps in understanding lipid regulation, discussed later. The first steps of cholesterol synthesis (acetyl CoA ^ HMG CoA) take place in the cytosol. The conversion of HMG CoA into meval-onate, the key rate-controlling step in cholesterol biosynthesis,

Figure 18-7 summarizes the complex series of reactions that yield cholesterol and several other related biomolecules. The basic features of this pathway are important in the synthesis of other important lipids, and familiarity with these features helps in understanding lipid regulation, discussed later. The first steps of cholesterol synthesis (acetyl CoA ^ HMG CoA) take place in the cytosol. The conversion of HMG CoA into meval-onate, the key rate-controlling step in cholesterol biosynthesis,

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