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Biosynthesis of Cholesterol, Steroids, and Isoprenoids:- Intermediates in Cholesterol Biosynthesis Have Many Alternative Fates

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  828

2026-07-02

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Biosynthesis of Cholesterol, Steroids, and Isoprenoids:- Intermediates in Cholesterol Biosynthesis Have Many Alternative Fates

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with di verse biological roles (Fig. 21–48). They include vita mins A, E, and K; plant pigments such as carotene and the phytol chain of chlorophyll; natural rubber; many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk); insect juvenile hormone, which controls metamorphosis; dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis; and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than 20,000 different isoprenoid molecules have been dis covered in nature, and hundreds of new ones are reported each year. Prenylation (covalent attachment of an isoprenoid; see Fig. 27–30) is a common mechanism by which proteins are anchored to the inner surface of cellular mem branes in mammals (see Fig. 11–14). In some of these proteins the attached lipid is the 15-carbon farnesyl group; others have the 20-carbon geranylgeranyl group. Different enzymes attach the two types of lipids. It is possible that prenylation reactions target proteins to different membranes, depending on which lipid is at tached. Protein prenylation is another important role for the isoprene derivatives of the pathway to cholesterol.

FIGURE 21–48 Overview of isoprenoid biosynthesis. The structures of most of the end products shown here .