Lecture 33: Cholesterol and Steroid Biosynthesis/Metabolism

Describe the structure of cholesterol.

• 4 fused rings (steroid nucleus)

• rings are planar and relatively rigid

• hydrocarbon chain 

• weakly polar head on C3

• double bond at C-5 and C-6

True or False: Cholesterol intercalates between hydrocarbon chains and moderates extremes of fluidity.

true

Which areas of the body synthesize cholesterol?

• all cells especially…

  • liver

  • intestine

  • adrenal cortex

  • reproductive tissues

What are the sources of liver cholesterol?

• diet (via chylomicron remnants)

• de novo synthesis

• uptake (via HDL and LDL)

What are the major routes by which cholesterol leaves the liver?

• secretion of VLDL

• free cholesterol secreted in the bile

• conversion to bile acids/salts

What are all of the carbon atoms in cholesterol from?

acetate (via acetyl-CoA)

On cholesterol, what are the reactive sites for esterification and oxidation-reduction reactions?

• OH group

• C=C groups

• C17 (side chain)

Where does cholesterol synthesis occur?

in the smooth ER

What is the overview of cholesterol biosynthesis?

1. acetate condensation to form mevalonate

2. conversion to activated isoprene

3. six 5-carbon isoprene are condensed to form squalene

4. squalene is cyclized to form lanosterol

Describe the condensation of 3 acetyl-CoA to HMG-CoA.

1. acetyl-CoA is converted to acetoacetyl-CoA by thiolase; CoA-SH is given off

2. acetoacetyl-CoA is converted to HMG-CoA by HMG-CoA synthase; CoA-SH is given off

Where does condensation of 3 acetyl-CoA to HMG-CoA take place?

in the cytoplasm

Where does the energy to drive cholesterol synthesis come from?

acetyl CoA and ATP hydrolysis

Describe the reduction of HMG-CoA to mevalonate.

1. HMG-CoA is converted to mevalonate to HMG-CoA reductase with the help of 2 NAPDH and 2 H⁺; 2 NADP⁺ and CoA-SH is given off

• extra notes:

  • rate-limiting, committed step

Where does the reduction of HMG-CoA to mevalonate occur?

integral membrane protein in smooth ER

What is the target for statin drugs?

reduction of HMG-CoA to mevalonate step

What do statin drugs do?

lower cholesterol

Describe the pathway of activated isoprene formation.

1. mevalonate is converted to 5-phosphomevalonate by mevalonate 5-phosphotransferase with the help of ATP

2. 5-phosphomevalonate is converted to 5-pyrophosphomevalonate by phosphomevalonate kinase with the help of ATP

3. 5-pyrophosphomevalonate is converted to 3-phospho-5-pyrophosphomevalonate by pyro phosphomevalonate decarboxylase with the help of ATP; CO₂ and P_i is given off

4. 3-phospho-5-pyrophoshpomevalonate is converted to delta³-isopentenyl pyrophosphate (IPP)

5. IPP is isomerized to dimethylallyl pyrophosphate (DMAP)

Describe the formation of squalene.

1. DMAP and IPP are converted to geranyl pyrophosphate by prenyl transferase [head-to-tail condensation]; PP_i is given off

2. geranyl pyrophosphate and delta³-isopentenyl pyrophosphate (IPP) are converted to farnesyl pyrophosphate by prenyl transferase [head-to-tail]; PP_i is given off

3. farnesyl pyrophosphate is converted to squalene by squalene synthase with the help of NADPH and H⁺ [head-to-head]; NADP⁺ and 2 PP_i are given off; production of cyclopropyl intermediates

Successive condensations of how many activated isoprene units generate squalene?

6 activated isoprenes

True or False: Intermediates like geranyl and farnseyl pyrophosphates participate in many other synthetic reactions.

true

Does DMAP act as the electrophile or nucelphile in the prenyl transferase reaction?

electrophile

Describe squalene cyclization.

• substrate(s): squalene

• coactivator(s): NADPH, H⁺, and O₂

• enzyme: squalene monooxygenase

• product(s): squalene 2,3-epoxide, H₂O, and NADP⁺

What is important to know about squalene monooxygenase?

• mixed function oxidase (uses O₂ as a substrate to form the product and H₂O)

• cytochrome P-450 monooxygenase

Describe cytochrome P-450 monooxygenases.

• large family of heme-containing enzymes that add an oxygen atom to various substrates

• used for:

  • biosynthetic reactions

  • xenobiotic detoxification/drug metabolism

• RH + NADPH + H⁺ + O₂ → ROH + NADP⁺ + H₂O

Where does steroid hormone biosynthesis occur?

the mitochondria

Where does xenobiotic detoxification/drug metabolism occur?

in the smooth ER of heptaocytes

True or False: Mitochondria of cells in the adrenal gland are specialized for steroid hormone synthesis.

true

Describe the cyclization of squalene 2,3-epoxide to cholesterol.

1. squalene 2,3-epoxide is converted to lanosterol by cyclase

2. multistep pathway to form cholesterol from lanosterol

Instead of cholesterol, what do plants and fungi produce?

stigmasterol and ergosterol, respectively

What is the difference between the structure of lanosterol and cholesterol?

cholesterol has 3 less methyl groups

Which non-steroidal lipids are derived from isoprenoids?

• visual pigments

• carotenoids

• vitamins A, E, and K

• membrane protein anchors (prenylation)

• quinone electron carriers

• dolichols

• plant hormones

• rubber and many other natural products

What are the various ways of feedback control of cholesterol synthesis?

• sterol-dependent regulation of gene expression

• sterol-accelerated enzyme degradation

• sterol-independent phosphorylation/dephosphorylation

• hormonal regulation

What senses how much cholesterol is in the membrane?

SREBP-SCAP

True or False: HMG CoA reductase is a sterol-sensing protein.

true

What inhibits HMG-CoA reductase activity?

• AMPK (low levels of ATP)

• glucagon

• oxysterol

What activates HMG-CoA reductase activity?

insulin

What stimulates proteolysis of HMG-CoA reductase?

oxysterol

What does intracellular cholesterol activate?

activates cholesteryl esters

Other than HMG-CoA reductase, what does oxysterol inhibit?

receptor-mediated endocytosis of LDL-cholesterol

What are the two ways that cells can get cholesterol?

• synthesize cholesterol themes

• take up LDL particles from the blood

Describe the mechanism for how LDL cholesterol enters the cell.

1. LDL receptor is synthesized in rough ER moves to the PM via the golgi

2. LDL receptor binds apoB-100 on LDL—initiating endocytosis

3. LDL is internalized in the endosome

4. LDL receptor is segregated into vesicles and recycled to the surface

5. endosome with LDL fuses with the lysosome

6. lytic enzymes in lysosome degrade apoB-100 and cholesteryl esters—releasing amino acids, fatty acids, and cholesterol

How do statin reduce LDL levels?

inhibiting cellular cholesterol synthesis by statins force cells to upregulate their LDL receptors; this increases their rate of LDL uptake—lowering circulating LDL levels

How is cholesterol degraded?

the ring system is eliminated in the feces as unmodified cholesterol, bile acids, or reduced cholesterol by bacteria (cholestanol and coprostanol)

True or False: Humans cannot degrade cholesterol to CO₂ and H₂O.

true

What are bile acids?

amphipathic detergent molecules that serve to emulsify fats—forming mixed micelles essential for fat digestion and absorption

How are bile acids synthesized?

• hydroxyl groups are added to cholesterol

• cholesterol side chain is shorted

• COO^- group is added at the end of a side chain

• double bond is reduced 

Are bile acids or bile salts more effective?

bile salts

True or False: Bile salts provide the only significant mechanism for cholesterol excretion, both as a metabolic product of cholesterol and as a solubilizer of cholesterol in the bile.

true

What is cholyl CoA converted to?

taurine (taurocholic acid) and glycine (glycocholic acid)

Bile acids are efficiently recycled via what?

the entero-hepatic circulation

How many grams of bile salts does the liver synthesize per day?

0.2-0.6 per day

How much of the bile is recycled every day?

95%

True or False: Secondary bile salts are reconjugated but not rehydroxylated.

true

What deconjugates and dehydroxylates bile salts?

gut bacteria

How much of the bile salts are reabsorbed and returned to the liver for recyling?

12-32 g per day

What does cholestyramine do?

• bile acid sequestrant

• inhibits bile salts from being reabsorbed

What does Ezetimibe do?

inhibits cholesterol

What does Resin do?

inhibits bile acids

What does Niacin do?

inhibits the conversion of cholesterol to VLDL

Cholesterol is the precursor of 5 classes of steroid hormones. Name those classes.

• glucocorticoids

• mineralocorticoids

• androgens

• estrogens

• progestins

What does cortisol (glucocorticoid) do?

• affects protein and carbohydrate metabolism

• suppresses immune system response

• inflammation

• allergic responses

What does aldosterone (mineralocoricoid) do?

regulates reabsorption of Na⁺, Cl^-, HCO^-₃ in the kidney

True or False: Steroid hormones are transported in the blood bound to albumin or specific steroid carrier proteins.

true

Where are glucocorticoids and mineralocorticoids produced?

adrenal cortex

Where are sex hormones produced?

gonads

What does synthesis from cholesterol involve?

shortening of hydrocarbon chains and hydroxylation of rings (NADPH and O₂ are required) to form pregnenolone

What is the common precursor in the steroid hormone biosynthesis pathway?

pregnenalone

How does pregnenolone form progesterone?

oxidation and isomerization

How is aldosterone produced?

1. cholesterol

2. pregnenolone

3. progesterone

4. corticosterone

5. aldosterone

How is cortisol produced?

1. cholesterol

2. pregnenolone

3. progesterone

4. cortisol

How is estradiol produced?

1. cholesterol

2. pregnenolone

3. progesterone

4. testosterone

5. estradiol

True or False: In steroid hormone biosynthesis, further hydroxylation and modifications require various mixed function oxidases (cytochrome P450 monooxygenases).

true

Describe aromatase.

• key enzyme in the production of estrogens

• cytochrome P450 monooxygenase

• used as a breast and uterine anti-cancer drug

Which organ is generally responsible for the metabolic inactivation of steroid hormones?

the liver

What does the degradation and excretion of steroid hormones typically include?

• reduction of double bonds

• hydroxylation

• conjugation with glucuronic acid

What do modifications to steroids make them more?

more soluble

Modified/conjugated steroids are typically filtered through where and eliminated where?

filtered through the kidneys and eliminated in the urine

Modified/conjugated steroids are typically secreted into what and then eliminated where?

secreted into the bile and eliminated in the feces

List the 4 types of congenital adrenal hyperplasias (CAHs).

• 3-beta-hydroxysteroid dehydrogenase deficiency

• 17-alpha-hydroxylase deficiency

• 21-alpha-hydroxylase deficiency

• 11-beta1-hydroxylase deficiency

Describe 3-beta-hydroxysteroid dehydrogenase deficiency.

• pregnenolone cannot be converted to progesterone

• virtually no glucocorticoids, mineralocorticoids, active androgens, or estrogens

• salt excretion in the urine

• patients have female-like genitalia

• autosomal recessive with incidence of 1:10,000

Describe 17-alpha-hydroxylase deficiency.

• progesterone cannot be converted into 17-alpha-hydroxyprogesterone

• virtually no sex hormones or cortisol are produced

• increased production of mineralocorticoids causes sodium and fluid retention, and therefore, hypertension

• patients have female-like genitalia

Describe 21-alpha-hydroxylase deficiency.

• progesterone cannot be converted into 11-deoxycorticosterone

• most common form of CAH (>90%)

• partial and virtually complete deficiencies are known

• mineralocorticoids and glucocorticoids are virtually absent (salt wasting classic form) or deficient (non-classic form)

• overproduction of androgens leads to masculinization of external genitalia in females and early virilization in males

Describe 11-beta1-hydroxylase deficiency.

• 11-deoxycorticosterone cannot be converted to corticosterone

• decrease in serum cortisol, aldosterone, and corticosterone

• increased production of deoxycorticosterone causes fluid retention

• hormone suppresses the renin/angiotensin system—causing low-renin hypertension

• overproduction of androgens causes masculinization and virilization as with 21-alpha-hydroxylase deficiency