Steroids Play Many Physiologically Important Roles

Cholesterol is probably the best known steroid because of its association with atherosclerosis. However, biochemically it is also of significance because it is the precursor of a large number of equally important steroids that include the bile acids, adrenocortical hormones, sex hormones, D vitamins, cardiac glycosides, sitosterols of the plant kingdom, and some alkaloids.

All of the steroids have a similar cyclic nucleus resembling phenanthrene (rings A, B, and C) to which a cyclopentane ring (D) is attached. The carbon positions on the steroid nucleus are numbered as shown in Figure 14-14. It is important to realize that in structural formulas of steroids, a simple hexagonal ring denotes a completely saturated six-carbon ring with all valences satisfied by hydrogen bonds unless shown otherwise; ie, it is not a benzene ring. All double bonds are shown as such. Methyl side chains are shown as single bonds unattached at the farther (methyl) end. These occur typically at positions 10 and 13 (constituting C atoms 19 and 18). A side chain at position 17 is usual (as in cholesterol). If the compound has one or more hydroxyl groups and no carbonyl or carboxyl groups, it is a sterol, and the name terminates in -ol.

Because of Asymmetry in the Steroid Molecule, Many Stereoisomers Are Possible

Each of the six-carbon rings of the steroid nucleus is capable of existing in the three-dimensional conformation either of a "chair" or a "boat" (Figure 14-15). In naturally occurring steroids, virtually all the rings are in the "chair" form, which is the more stable conformation. With respect to each other, the rings can be either cis or trans (Figure 14-16). The junction between the A and B rings can be cis or trans in naturally occurring steroids. That between B and C is trans, as is usually the C/D junction. Bonds attaching substituent groups above the plane of the rings (P bonds) are shown with bold solid lines, whereas those bonds attaching groups below (a bonds) are indicated with broken lines. The A ring of a 5a steroid is always trans to the B ring, whereas it is cis in a 5P steroid. The methyl groups attached to C10 and C^ are invariably in the P configuration.

Ceramide — Glucose — Galactose — N-Acetylgalactosamine

sphingo-sine)

NeuAc

Galactose

NeuAc

Figure 14-13. GM1 ganglioside, a monosialoganglio-side, the receptor in human intestine for cholera toxin.

Figure 14-14. The steroid nucleus.

Figure 14-14. The steroid nucleus.

"Chair" form "Boat" form

Figure 14-15. Conformations of stereoisomers of the steroid nucleus.

"Chair" form "Boat" form

Figure 14-15. Conformations of stereoisomers of the steroid nucleus.

Cholesterol Is a Significant Constituent of Many Tissues

Cholesterol (Figure 14-17) is widely distributed in all cells of the body but particularly in nervous tissue. It is a major constituent of the plasma membrane and of plasma lipoproteins. It is often found as cholesteryl ester, where the hydroxyl group on position 3 is esteri-fied with a long-chain fatty acid. It occurs in animals but not in plants.

Ergosterol Is a Precursor of Vitamin D

Ergosterol occurs in plants and yeast and is important as a precursor of vitamin D (Figure 14-18). When irradiated with ultraviolet light, it acquires antirachitic properties consequent to the opening of ring B.

Polyprenoids Share the Same Parent Compound as Cholesterol

Although not steroids, these compounds are related because they are synthesized, like cholesterol (Figure 26-2), from five-carbon isoprene units (Figure 14-19). They include ubiquinone (Chapter 12), a member of the respiratory chain in mitochondria, and the long-

chain alcohol dolichol (Figure 14-20), which takes part in glycoprotein synthesis by transferring carbohydrate residues to asparagine residues of the polypeptide (Chapter 47). Plant-derived isoprenoid compounds include rubber, camphor, the fat-soluble vitamins A, D, E, and K, and P-carotene (provitamin A).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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