Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones

Figure 26.21. An Atherosclerotic Plaque. A plaque (marked by an arrow) blocks most of the lumen of this blood
vessel. The plaque is rich in cholesterol. [Courtesy of Dr. Jeffrey Sklar.]
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.3. The Complex Regulation of Cholesterol Biosynthesis Takes Place at Several Levels
Figure 26.22. Lovastatin, a Competitive Inhibitor of HMG-CoA Reductase. The part of the structure that resembles
the 3-hydroxy-3-methylglutaryl moiety is shown in red.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Cholesterol is a precursor for other important steroid molecules: the bile salts, steroid hormones, and vitamin D.
Bile Salts.
As polar derivatives of cholesterol, bile salts are highly effective detergents because they contain both polar and
nonpolar regions. Bile salts are synthesized in the liver, stored and concentrated in the gall bladder, and then released
into the small intestine. Bile salts, the major constituent of bile, solubilize dietary lipids (Section 22.1.1). Solubilization
increases in the effective surface area of lipids with two consequences: more surface area is exposed to the digestive
action of lipases and lipids are more readily absorbed by the intestine. Bile salts are also the major breakdown products
of cholesterol.
Cholesterol is converted into trihydroxycoprostanoate and then into cholyl CoA, the activated intermediate in the
synthesis of most bile salts (Figure 26.23). The activated carboxyl carbon of cholyl CoA then reacts with the amino
group of glycine to form glycocholate or it reacts with the amino group of taurine (H2NCH2CH2SO3 -), derived from
cysteine, to form taurocholate. Glycocholate is the major bile salt.
Steroid Hormones.
Cholesterol is the precursor of the five major classes of steroid hormones: progestagens, glucocorticoids,
mineralocorticoids, androgens, and estrogens (Figure 26.24). These hormones are powerful signal molecules that
regulate a host of organismal functions. Progesterone, a progestagen, prepares the lining of the uterus for implantation
of an ovum. Progesterone is also essential for the maintenance of pregnancy. Androgens of male secondary sex
characteristics, whereas estrogens (such as estrone) are required for the development of female secondary sex
characteristics. Estrogens, along with progesterone, also participate in the ovarian cycle. Glucocorticoids (such as
cortisol) promote gluconeogenesis and the formation of glycogen, enhance the degradation of fat and protein, and inhibit
the inflammatory response. They enable animals to respond to stress indeed, the absence of glucocorticoids can be
fatal. Mineralocorticoids (primarily aldosterone) act on the distal tubules of the kidney to increase the reabsorption of Na
+ and the excretion of K+ and H+, which leads to an increase in blood volume and blood pressure. The major sites of
synthesis of these classes of hormones are the corpus luteum, for progestagens; the ovaries, for estrogens; the testes, for
androgens; and the adrenal cortex, for glucocorticoids and mineralocorticoids.
Steroid hormones bind to and activate receptor molecules that serve as transcription factors to regulate gene expression
(Section 31.3.1). These small, relatively similar molecules are able to have greatly differing effects because the slight
structural differences among them allow interactions with specific receptor molecules.
26.4.1. The Nomenclature of Steroid Hormones
Carbon atoms in steroids are numbered as shown for cholesterol in (Figure 26.25). The rings in steroids are denoted by
the letters A, B, C, and D. Cholesterol contains two angular methyl groups: the C-19 methyl group is attached to C-10,
and the C-18 methyl group is attached to C-13. The C-18 and C-19 methyl groups of cholesterol lie above the plane
containing the four rings. A substituent that is above the plane is termed β oriented, whereas a substituent that is below
the plane is α oriented.
If a hydrogen atom is attached to C-5, it can be either α or β oriented. The A and B steroid rings are fused in a trans
conformation if the C-5 hydrogen is < oriented, and cis if it is < oriented. The absence of a Greek letter for the C-5
hydrogen atom on the steroid nucleus implies a trans fusion. The C-5 hydrogen atom is α oriented in all steroid
hormones that contain a hydrogen atom in that position. In contrast, bile salts have a β-oriented hydrogen atom at C-5.
Thus, a cis fusion is characteristic of the bile salts, whereas a trans fusion is characteristic of all steroid hormones that
possess a hydrogen atom at C-5. A trans fusion yields a nearly planar structure, whereas a cis fusion gives a buckled
structure.
26.4.2. Steroids Are Hydroxylated by Cytochrome P450 Monooxygenases That Utilize
NADPH and O2
Hydroxylation reactions play a very important role in the synthesis of cholesterol from squalene and in the conversion of
cholesterol into steroid hormones and bile salts. All these hydroxylations require NADPH and O . The oxygen atom of
2
the incorporated hydroxyl group comes from O2 rather than from H2O. While one oxygen atom of the O2 molecule goes
into the substrate, the other is reduced to water. The enzymes catalyzing these reactions are called monooxygenases (or
mixed-function oxygenases). Recall that a monooxygenase also participates in the hydroxylation of aromatic amino acids
(Section 23.5.7).
Hydroxylation requires the activation of oxygen. In the synthesis of steroid hormones and bile salts, activation is
accomplished by a cytochrome P450, a family of cytochromes that absorb light maximally at 450 nm when complexed in
vitro with exogenous carbon monoxide. These membraneanchored proteins (~50 kd) contain a heme prosthetic group.
Because the hydroxylation reactions promoted by P450 enzymes are oxidation reactions, it is at first glance surprising
that they also consume the reductant NADPH. NADPH transfers its high-potential electrons to a flavoprotein, which
transfers them, one at a time, to adrenodoxin, a nonheme iron protein. Adrenodoxin transfers one electron to reduce the
ferric (Fe3+) form of P450 to the ferrous (Fe2+) form (Figure 26.26). Without the addition of this electron, P450 will not
bind oxygen. Recall that only the ferrous form of hemoglobin binds oxygen (Section 10.2.1). The binding of O2 to the
heme is followed by the acceptance of a second electron from adrenodoxin. The acceptance of this second electron leads
to cleavage of the O O bond. One of the oxygen atoms is then protonated and released as water. The remaining oxygen
atom forms a highly reactive ferryl (Fe O) intermediate. This intermediate abstracts a hydrogen atom from the
substrate RH to form R . This transient free radical captures the OH group from the iron atom to form ROH, the
hydroxylated product, returning the iron atom to the ferric state.
26.4.3. The Cytochrome P450 System Is Widespread and Performs a Protective
Function
The cytochrome P450 system, which in mammals is located primarily in the endoplasmic reticulum of the liver and
small intestine, is also important in the detoxification of foreign substances (xenobiotic compounds) by oxidative
metabolism. For example, the hydroxylation of phenobarbital, a barbiturate, increases its solubility and facilitates its
excretion. Likewise, polycyclic aromatic hydrocarbons are hydroxylated by P450, providing sites for conjugation with
highly polar units (e.g., glucuronate or sulfate), which markedly increase the solubility of the modified aromatic
molecule. One of the most relevant functions of the cytochrome P450 system to human beings is its role in drug
metabolism. Drugs such as caffeine and ibuprofen are oxidatively metabolized by these monooxygenases. Indeed, the
duration of action of many medications depends on their rate of inactivation by the P450 system. Despite its general
protective role in the removal of foreign chemicals, the action of the P450 system is not always beneficial. Some of the
most powerful carcinogens are generated from harmless compounds by the P450 system in vivo in the process of
metabolic activation. In plants, the cytochrome P450 system plays a role in the synthesis of toxic compounds as well as
the pigments of flowers.
The cytochrome P450 system is a ubiquitous superfamily of monooxygenases that is present in plants, animals,
and prokaryotes. The human genome encodes more than 50 members of the family, whereas the genome of the
plant Arabidopsis encodes more than 250 members. All members of this large family arose by gene duplication followed
by subsequent divergence that generated a range of substrate specificity. Indeed, the specificity of these enzymes is
encoded in delimited regions of the primary structure, and the substrate specificity of closely related members is often
defined by a few critical residues or even a single amino acid.
26.4.4. Pregnenolone, a Precursor for Many Other Steroids, Is Formed from
Cholesterol by Cleavage of Its Side Chain
Steroid hormones contain 21 or fewer carbon atoms, whereas cholesterol contains 27. Thus, the first stage in the
synthesis of steroid hormones is the removal of a six-carbon unit from the side chain of cholesterol to form
pregnenolone. The side chain of cholesterol is hydroxylated at C-20 and then at C-22, and the bond between these
carbon atoms is subsequently cleaved by desmolase. Three molecules of NADPH and three molecules of O2 are
consumed in this remarkable six-electron oxidation.
Adrenocorticotropic hormone (ACTH, or corticotropin), a polypeptide synthesized by the anterior pituitary gland,
stimulates the conversion of cholesterol into pregnenolone, the precursor of all steroid hormones.
26.4.5. The Synthesis of Progesterone and Corticosteroids from Pregnenolone
Progesterone is synthesized from pregnenolone in two steps. The 3-hydroxyl group of pregnenolone is oxidized to a 3keto group, and the ∆ 5 double bond is isomerized to a ∆ 4 double bond (Figure 26.27). Cortisol, the major
glucocorticoid, is synthesized from progesterone by hydroxylations at C-17, C-21, and C-11; C-17 must be hydroxylated
before C-21 is, whereas C-11 can be hydroxylated at any stage. The enzymes catalyzing these hydroxylations are highly
specific, as shown by some inherited disorders. The initial step in the synthesis of aldosterone, the major
mineralocorticoid, is the hydroxylation of progesterone at C-21. The resulting deoxycorticosterone is hydroxylated at C11. The oxidation of the C-18 angular methyl group to an aldehyde then yields aldosterone.
26.4.6. The Synthesis of Androgens and Estrogens from Pregnenolone
Androgens and estrogens also are synthesized from pregnenolone through the intermediate progesterone. Androgens
contain 19 carbon atoms. The synthesis of androgens (Figure 26.28) starts with the hydroxylation of progesterone at C17. The side chain consisting of C-20 and C-21 is then cleaved to yield androstenedione, an androgen. Testosterone,
another androgen, is formed by the reduction of the 17-keto group of androstenedione. Testosterone, through its actions
in the brain, is paramount in the development of male sexual behavior. It is also important for maintenance of the testes
and development of muscle mass. Owing to the latter activity, testosterone is referred to as an anabolic steroid.
Testosterone is reduced by 5a-reductase to yield dihydrotestosterone (DHT), a powerful embryonic androgen that
instigates the development and differentiation of the male phenotype. Estrogens are synthesized from androgens by the
loss of the C-19 angular methyl group and the formation of an aromatic A ring. Estrone, an estrogen, is derived from
androstenedione, whereas estradiol, another estrogen, is formed from testosterone.
26.4.7. Vitamin D Is Derived from Cholesterol by the Ring-Splitting Activity of Light
Cholesterol is also the precursor of vitamin D, which plays an essential role in the control of calcium and phosphorus
metabolism. 7-Dehydrocholesterol (provitamin D ) is photolyzed by the ultraviolet light of sunlight to previtamin D ,
3
3
which spontaneously isomerizes to vitamin D (Figure 26.29). Vitamin D3 (cholecalciferol) is converted into calcitriol
3
(1,25-dihydroxycholecalciferol), the active hormone, by hydroxylation reactions in the liver and kidneys. Although not a
steroid, vitamin D acts in an analogous fashion. It binds to a receptor, structurally similar to the steroid receptors, to form
a complex that functions as a transcription factor, regulating gene expression.
Vitamin D deficiency in childhood produces rickets, a disease characterized by inadequate calcification of
cartilage and bone. Rickets was so common in seventeenth-century England that it was called the "children's
disease of the English." The 7-dehydrocholesterol in the skin of these children was not photolyzed to previtamin D3,
because there was little sunlight for many months of the year. Furthermore, their diets provided little vitamin D, because
most naturally occurring foods have a low content of this vitamin. Fish-liver oils are a notable exception. Cod-liver oil,
abhorred by generations of children because of its unpleasant taste, was used in the past as a rich source of vitamin D.
Today, the most reliable dietary sources of vitamin D are fortified foods. Milk, for example, is fortified to a level of 400
international units per quart (10 µg per quart). The recommended daily intake of vitamin D is 400 international units,
irrespective of age. In adults, vitamin D deficiency leads to softening and weakening of bones, a condition called
osteomalacia. The occurrence of osteomalacia in Bedouin Arab women who are clothed so that only their eyes are
exposed to sunlight is a striking reminder that vitamin D is needed by adults as well as by children.
26.4.8. Isopentenyl Pyrophosphate Is a Precursor for a Wide Variety of Biomolecules
Before this chapter ends, we will revisit isopentenyl pyrophosphate, the activated precursor of cholesterol. The
combination of isopentenyl pyrophosphate (C5) units to form squalene (C30) exemplifies a fundamental mechanism for
the assembly of carbon skeletons of biomolecules. A remarkable array of compounds is formed from isopentenyl
pyrophosphate, the basic five-carbon building block. The fragrances of many plants arise from volatile C10 and C15
compounds, which are called terpenes. For example, myrcene (C10H16) from bay leaves consists of two isoprene units,
as does limonene (C10H15) from lemon oil (Figure 26.30). Zingiberene (C15H24), from the oil of ginger, is made up of
three isoprene units. Some terpenes, such as gera-niol from geraniums and menthol from peppermint oil, are alcohols;
others, such as citronellal, are aldehydes. We shall see later (Chapter 32) how specialized sets of 7-TM receptors are
responsible for the diverse and delightful odor and taste sensations that these molecules induce.
We have already encountered several molecules that contain isoprenoid side chains. The C30 hydrocarbon side chain of
vitamin K , an important molecule in clotting (Section 10.5.7), is built from 6 isoprene (C5) units. Coenzyme Q in the
2
10
mitochondrial respiratory chain (Section 18.3) has a side chain made up of 10 isoprene units. Yet another example is the
phytol side chain of chlorophyll (Section 19.2), which is formed from 4 isoprene units. Many proteins are targeted to
membranes by the covalent attachment of a farnesyl (C15) or a geranylgeranyl (C20) unit to the carboxyl-terminal
cysteine residue of the protein (Section 12.5.3). The attachment of isoprenoid side chains confers hydrophobic character.
Isoprenoids can delight by their color as well as by their fragrance. The color of tomatoes and carrots comes from
carotenoids. These compounds absorb light because they contain extended networks of single and double bonds and are
important pigments in photosynthesis (Section 19.5.2). Their C40 carbon skeletons are built by the successive addition of
C5 units to form geranylgeranyl pyrophosphate, a C intermediate, which then condenses tail-to-tail with another
20
molecule of geranylgeranyl pyrophosphate.
Phytoene, the C40 condensation product, is dehydrogenated to yield lycopene. Cyclization of both ends of lycopene gives
β-carotene, which is the precursor of retinal, the chromophore in all known visual pigments (Section 32.3.2). These
examples illustrate the fundamental role of isopentenyl pyrophosphate in the assembly of extended carbon skeletons of
biomolecules. It is evident that isoprenoids are ubiquitous in nature and have diverse significant roles, including the
enhancement of the sensuality of life.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.23. Synthesis of Bile Salts. Pathways for the formation of bile salts from cholesterol.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.24. Biosynthetic Relations of Classes of Steroid Hormones and Cholesterol.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.25. Cholesterol Carbon Numbering. The numbering scheme for the carbon atoms in cholesterol and other
steroids.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.26. Cytochrome P450 Mechanism. These enzyme-bind O2 and use one oxygen atom to hydroxylate their
substrates.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.27. Pathways for the Formation of Progesterone, Cortisol, and Aldosterone.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.28. Pathways for the Formation for Androgens and Estrogens.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.29. Vitamin D Synthesis. The pathway for the conversion of 7-dehydrocholesterol into vitamin D3 and then
into calcitriol, the active hormone.
III. Synthesizing the Molecules of Life
26. The Biosynthesis of Membrane Lipids and Steroids
26.4. Important Derivatives of Cholesterol Include Bile Salts and Steroid Hormones
Figure 26.30. Three Isoprenoids from Familiar Sources.