Steroid Biosynthesis

Question 1

Exogenous Cholestrol Metabolism Pathway

Answer 1

Food is digested and broken down into its constituent parts; fatty acids, glycerol and cholesterol.
> Enterocytes in the gut pick up these constitiuents and gut sobule vitamins. On the Rough Endoplasmic Reticulum, Apoprotein B48 is made and it binds to triglycerides and cholesterol esters that are being made from the fatty acids and glycerol in the Smooth Endoplasmic Reticulum and it enzymatically combines lipids, cholesterol and ApoB48 to form Nascent Chylomicrons. The ApoB48 acts as a scaffold for the structure.
> When the Nascent Chylomicronn gets to the blood plasma, they momentarily interact with the circulating HDLs which donates 2 Apolipoproteins - ApoE and ApoCII - to form a Mature Chylomicron. ApoE allows the chylomicron to bind to a receptor in the liver and be internalised. ApoCII is a co-factor for the Lipoprotein Lipase (LPL) enzyme.
> When digesting the meal, through the release of incretins and glucose, the pancreatic beta cells will be stimulated and start secreting insulin. Insulin and ApoCII trigger various cell types within the tissue to start upregulating and releasing LPL from the epithelium of capillaries of these tissues e.g. skeletal muscle, cardiac tissue and adipose tissue.
> LPL acts to breakdown the triglycerides to turn back into free fatty acids and glycerol depleting the triglyceride core of the chylomicron. The free fatty acids are absorbed by the surrounding tissue and can be used to produce energy. Glycerol is released, diffuses in the blood back to the liver, and is used to make more glucose during gluconeogenesis and recycled. Now they are known as Remnant Chylomicrons.
> HDL then takes back ApoCII.
> Remnant Chylomicron is taken up by the liver due to ApoE allowing it to be recognised. It is endocytosed into the liver and broken down by lysosomes. Cholesterol esters are broken down into free cholesterol, triglycerides into free fatty acids and glycerol, any remaining lipoproteins are broken down into amino acids and recycled.

Question 2

Endogenous Cholesterol Metabolism Pathway

Answer 2

Starts in the liver. Hepatocytes synthesise VLDLs which are made up of ApoB100, triglycerides, phospholipids and cholesterol. ApoB100 provides a scaffold for the structure but it also acts as a ligand which the liver uses to endocytose the particle later on. This Nascent VLDL floats through the blood.
> In the blood, the Nascent VLDL interacts momentarily with HDL, receiving 2 apolipoproteins - ApoE and ApoCII - this creates a Mature VLDL.
> ApoCII stimulates LPL, so the Mature VLDL travels through the bloodstream and gives away fatty acids. The extrahepatic tissues use the fatty acids as an energy source and the glycerol is cycled back to the liver for gluconeogenesis.
> As the triglycerides are broken down, the portion of cholesterol increases and the particle density increases with it. Simultaneously, the ApoCII is given back to HDL. This turns the VLDL into an IDL.
> The IDL has 2 possible routes: 1. use the ApoE to bind to the liver and be taken up by hepatocytes and be internalised and recycled. OR 2. give back ApoE to the HDL and turn into an LDL. The LDL floats through the bloodstream and returns to the liver where it can be taken in via receptor-mediated endocytosis using ApoB100 as a ligand for LDL receptors. It is then broken down and recycled. If there is a build up in LDLs in the bloodstream, some may move into the vessel wall and become oxidised causing atherosclerosis, this can lead to plaque build up and can cause Coronary Artery Disease.

Question 3

Major Physiological Functions of Cholesterol

Answer 3

• Major component off membrane - decreases membrane fluidity by adding rigidity and restricting phospholipid tails from moving much, decreases physical permeability to charged/polar compounds and is associated with the formation of lipid rafts.
• Precursor for the production of bile salts - uptake of fats and fat-soluble vitamins in GIT.
• Precursor for all steroid hormones - glucocorticoids, mineralocorticoids, sex steroids.

Question 4

Cholesterol Biosynthesis

Answer 4

• Body can make between 800-1000mg of cholesterol everday. Most cells are capable of synthesis but pre-dominantly made by the liver, intestines, adrenals and reproductive organs.
• The liver synthesises cholesterol from Acetyl CoA in the smooth endoplasmic reticulum and the cytosol.
• Acetyl CoA -HMG-CoA Synthase-> HMG-CoA -HMG-CoA Reductase-> Mevalonate -> Lanosterol -> Cholesterol.
• The rate-limiting step is HMG-CoA Reductase which can be blocked by the use of statins or negative feedback inhibition by increased cholesterol concentration.
• HMG = hydroxymethyglutaryl

Question 5

Distribution of Cholesterol

Answer 5

• Membrane Lipids - regulates membrane fluidity
• Lipoproteins
• Cytosolic lipid droplets as cholesterol esters in Steroid Secreting Endocrine Cells

Question 6

LDL Uptake in the Liver via Receptor-mediated Endocytosis

Answer 6

LDL goes to receptor > receptor is covered in Clathrin > Clathrin-coated pit/vesicle is formed > Clathrin uncoats > Uncoated vesicle with LDL inside is attacked by lysosome > Contents of LDL are broken down into Fatty acids, Amino acids and Cholesterol > Cholesterol is brought into steroid synthesis pathway.

Question 7

Steroidogenic Acute Regulatory Protein (StAR)

Answer 7

predominantly mediates the rate-limiting step in steroid biosynthesis, i.e., the transport of the substrate of all steroid hormones, cholesterol, from the outer to the inner mitochondrial membrane

Question 8

Desmolase

Answer 8

Cleaves the side chain off of Cholesterol (27C) and creates the 21C molecule Pregnenolone - the precursor to all steroid hormones.

Question 9

Where does enzymatic processing occur in the cell?

Answer 9

Mitochondria and Smooth Endoplasmic Reticulum.

Question 10

Which type of enzymes are involved in Steroidogenesis?

Answer 10

Cytochrome P-450 Enzymes

Question 11

Where in the Adrenal gland are mineralocorticoids produced?

Answer 11

Zona Glomerulosa

Question 12

Where in the adrenal glands are glucocorticoids made?

Answer 12

Zona Fasciculata

Question 13

Where in the adrenal glands are androgens made?

Answer 13

Zona Reticularis

Question 14

Biosynthesis of Androstenediol in the Adrenal Glands

Answer 14

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER; 17alpha-Hydroxylase-> 17 alpha-Pregnenolone -SER; 17,20-Desmolase-> Dehydroepiandrosterone (DHEA) <> Androstenediol

Question 15

Biosynthesis of Testosterone in the Adrenal glands

Answer 15

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER; 3beta-hydroxysteroid dehydrogenase-> Progesterone -SER; 17 alpha-Hydroxylase-> 17 alpha-hydroxyprogesterone -SER; 17,20-Desmolase-> Androstenedione <> Testosterone

Question 16

Biosynthesis of Aldosterone in the Adrenal Glands

Answer 16

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER;21alpha-Hydroxylase-> 11-Deoxy-Corticosterone -Mitochondria; 11 beta-hydroxylase-> Corticosterone -Mitochondria: Aldosterone Synthase-> Aldosterone

Question 17

Biosynthesis of Progesterone in the Adrenal Glands

Answer 17

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER; 3beta-hydroxysteroid dehydrogenase-> Progesterone

Question 18

Biosynthesis of Cortisol in the Adrenal Glands

Answer 18

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER;21alpha-Hydroxylase-> 11-Deoxy-Corticosterone -Mitochondria; 11 beta-hydroxylase-> Corticosterone -SER; 17 alpha-hydroxylase-> Cortisol

Question 19

Biosynthesis of Dihydrotestosterone in the Testes

Answer 19

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER; 3beta-hydroxysteroid dehydrogenase-> Progesterone -SER; 17 alpha-Hydroxylase-> 17 alpha-hydroxyprogesterone -SER; 17,20-Desmolase-> Androstenedione -SER; 17beta-hydroxysteroid dehydrogenase-> Testosterone -SER; 5 alpha-Reductase-> Dihydrotestosterone

Mainly found in testosterone target cells e.g. prostate. Binds more strongly to antigen receptor than testosterone, promotes various transcriptions and different gene programs. In the prostate, it promotes development and growth. And it regulates testosterone target genes in other tissues as well.

Question 20

Biosynthesis of Estradiol in the Testes

Answer 20

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER; 3beta-hydroxysteroid dehydrogenase-> Progesterone -SER; 17 alpha-Hydroxylase-> 17 alpha-hydroxyprogesterone -SER; 17,20-Desmolase-> Androstenedione -SER; 17beta-hydroxysteroid dehydrogenase-> Testosterone -SER; Aromatase-> Estradiol.

Question 21

Leydig Cells

Answer 21

Produces Testosterone. Controlled by binding and activation of Luteinising Hormone Receptor by Luteinising Hormone. It Activates the intracellular signalling cascade and induces synthesis and transcription of enzymes which can breakdown cholesterol into testosterone. Testosterone is then diffused out of the cell as it is lipophilic, it goes to the bloodstream or the Sertoli cells.

Question 22

Sertoli Cells

Answer 22

Stimulated by Follicle Stimulating Hormone. Protein Kinase A induces new protein synthesis - Inhibins (hormones which feedback negatively at the level of the pituitary to inhibit further production of FSH), Antigen Binding Proteins (ABPs; go into the lumen of the seminiferous tubules and binds to testosterone which came from the Leydig cells, keeps it in high concentrations around developing sperm which is essential for its development), Growth Factors and Aromatase (to create Estradiol).

Question 23

Biosynthesis of Estradiol in the Ovary

Answer 23

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER; 3beta-hydroxysteroid dehydrogenase-> Progesterone -SER; 17 alpha-Hydroxylase-> 17 alpha-hydroxyprogesterone -SER; 17,20-Desmolase-> Androstenedione -SER; 17beta-hydroxysteroid dehydrogenase-> Testosterone -SER; Aromatase-> Estradiol

Question 24

Biosynthesis of Estriol

Answer 24

Cholesterol -Mitochondria; Desmolase-> Pregnenolone -SER; 3beta-hydroxysteroid dehydrogenase-> Progesterone -SER; 17 alpha-Hydroxylase-> 17 alpha-hydroxyprogesterone -SER; 17,20-Desmolase-> Androstenedione -SER; Aromatase-> Estrone -LIVER-> Estriol

Question 25

Granulosa Cell

Answer 25

• Immediately surrounding the ovum.
• In the follicular phase, Follicle Stimulating Hormone initiates cascade. G-Protein Coupled Receptor is activated and activates Adenylyl Cyclase > cAMP. The Androstenedione received from Theca Cells can be made into oestrogens by Aromatase > Estrone > Estradiol.
• It also has a pathway initiated by Luteinising Hormone or LDLs which turns Cholesterol to Pregnenolone to Progesterone, however in Luteal Phase, it cannot go further than Progesterone and it has to be transported out of the cell to either the bloodstream of Theca Cells.

Question 26

Theca Cell

Answer 26

• Found a bit further away from the ovum than Granulosa cells.
• Activated by Luteinising Hormone during Follicular Phase. LH activates a G-Coupled Protein which activates Adenylyl Cyclase which activates cAMP, this facilitates turning Cholesterol to Pregnenolone. Pregnenolone then turns into Progesterone, which turns into 17alpha-Progesterone which turns into Androstenediol which becomes Testosterone. There is no Aromatase in Theca Cells so oestrogens aren’t made.
• In the Follicular Phase, Androstenediol is diffused across to the Granulosa cells.
• In the Luteal Phase, the cells is stimulated by LDLs and creates Progesterone. It also receives Progesterone from Granulosa Cells which causes an overload and there is a high level of circulating Progesterone in the bloodstream.

Question 27

Compare the Structural Differences between Cortisol, Aldosterone, Testosterone, Progesterone and Estradiol

Answer 27

• Cortisol - C21H30O5 - 2 carbonyl groups, 2 hydroxyl groups, 1 hydroxymethyl group, 2 methyl groups.
• Aldosterone - C21H28O5 - 3 carbonyl groups, 1 hydroxyl group, 1 hydroxymethyl group and 1 methyl group.
• Testosterone - C19H28O2 - 1 carbonyl group, 1 hydroxyl group and 2 methyl groups.
• Progesterone - C21H30O2 - 2 carbonyl groups, 3 methyl groups.
• Estradiol - C18H24O2 - aromatic ring, 2 hydroxyl groups and a methyl group.