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Biochemistry, 4th Edition P28

Biochemistry, 4th Edition P28. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | 8.7 What Are Steroids and What Are Their Cellular Functions 233 8.7 What Are Steroids and What Are Their Cellular Functions Cholesterol A large and important class of terpene-based lipids is the steroids. This molecular family whose members affect an amazing array of cellular functions is based on a common structural motif of three 6-membered rings and one 5-membered ring all fused together. Cholesterol Figure 8.16 is the most common steroid in animals and the precursor for all other animal steroids. The numbering system for cholesterol applies to all such molecules. Many steroids contain methyl groups at positions 10 and 13 and an 8- to 10-carbon alkyl side chain at position 17. The polyprenyl nature of this compound is particularly evident in the side chain. Many steroids contain an oxygen at C-3 either a hydroxyl group in sterols or a carbonyl group in other steroids. Significantly the carbons at positions 10 and 13 and the alkyl group at position 17 are nearly always oriented on the same side of the steroid nucleus the -orientation. Alkyl groups that extend from the other side of the steroid backbone are in an -orientation. Cholesterol is a principal component of animal cell plasma membranes and smaller amounts of cholesterol are found in the membranes of intracellular organelles. The relatively rigid fused ring system of cholesterol and the weakly polar alcohol group at the C-3 position have important consequences for the properties of plasma membranes. Cholesterol is also a component of lipoprotein complexes in the blood and it is one of the constituents of plaques that form on arterial walls in atherosclerosis. Steroid Hormones Are Derived from Cholesterol Steroids derived from cholesterol in animals include five families of hormones the androgens estrogens progestins glucocorticoids and mineralocorticoids and bile acids Figure 8.17 . Androgens such as testosterone and estrogens such as estradiol mediate the development of sexual characteristics and sexual .