Metabolism 7- Cholesterol

Question 1

Describe the characteristics of cholesterol

Answer 1

Cholesterol is a steroid.
It increases or decreases membrane stiffness, depending on temperature and nature of membrane.
Changes interactions with cytoskeleton

Question 2

How does cholesterol act as a bidirectional regulator of membrane fluidity

Answer 2

At high temperatures, it stabilizes the membrane and raises its melting point, whereas at low temperatures it intercalates between the phospholipids and prevents them from clustering together and stiffening

Question 3

Where is cholesterol derived from

Answer 3

Dietary cholesterol uptake in humans is limited to around 0.5g/day.

All physiological requirements for cholesterol are supplied by the liver through de novo synthesis of cholesterol from acetyl-CoA.

Question 4

Describe simply the synthesis of cholesterol

Answer 4

Synthesis of isopentenyl pyrophosphate, an activated isoprene unit which serves as a key building block (cytoplasm).
Condensation of six molecules of isopentenyl pyrophosphate to form squalene (cytoplasm).
Cyclisation and demethylation of squalene by monooxygenases to give cholsterol (ER).

Question 5

Describe reaction 1 of cholesterol biosynthesis

Answer 5

Condensation of 2 Acetyl-CoA molecules to form Acetoacetyl CoA.
Catalysed by beta thiolase.
CoA group released

Question 6

Describe reaction 2 of cholesterol biosynthesis

Answer 6

Condensation of another Acetyl-CoA molecule to form HMG-CoA. Catalysed by HMG-CoA synthase. H20 added, CoA group released. AcetoacetylCoA + AcetylCoA.

Question 7

Describe reaction 3 of cholesterol biosynthesis

Answer 7

HMG-Co-A is reduced to generate mevalonate. Catalysed by HMG-CoA reductase.
2H+ +2NADPH — 2NADP+ + CoA

Question 8

Describe the following reactions of cholesterol biosynthesis

Answer 8

Mevalonate undergoes 3 sequential phosphorylation at the hydroxyl groups at position 3 and 5, followed by decarboxylation to form 3-Isopentenyl pyrophosphate.
This activated isoprene unit is a useful building block for further synthesis.

Question 9

Describe the first phosphorylation reaction

Answer 9

Mevalonate — 5-phospho-mevalonate. ADP — ATP.

Catalysed by mevalonate kinase

Question 10

Describe the second phosphorylation reaction

Answer 10

5-phoshpo-mevalonate — 5-phyrophospho-mevalonate. ADP — ATP. Catalysed by phosphor mevalonate kinase.

Question 11

Describe the third phosphorylation reaction

Answer 11

5-pyrophosphate mevalonate undergoes a phosphorylation reaction catalysed by kinase. The intermediate is then decarboxylated to form 3-Isopentenyl pyrophosphate. Catalysed by phospomevalonate decarboxylase.

Question 12

At what positions ae the phosphate groups added

Answer 12

5C,5C and 3C

Question 13

describe the isomerisation reaction of 3-isopentyl pyrophosphate

Answer 13

Dimethylallyl pyrophosphate can be produced from isopentyl PP. Catalysed by Isopentenyl
Pyrophosphate isomerase

Question 14

Describe the subsequent condensation reaction with isopentyl pyrophosphate

Answer 14

Dimethylallyl pyrophosphate can condense with a unit of Isopentenyl-PP to form the C10 compound Geranyl-PP.2 phosphate groups released (PPi). Catalysed by Gernayl transferase

Question 15

Describe the following reaction from this to form a 15C compound

Answer 15

A third isopentenyl-PP molecule is added to form the C15 intermediate farnesyl-PP (condensation reaction).Catalysed by farnesyl transferase

Question 16

Describe the reaction in which squalene is produced

Answer 16

Two farnesyl-PP molecules condense to form C30 squalene plus 2 molecules of pyrophosphate.
(PPi). NADPH — NADP+ + H+.
Catalysed by squalene synthesase

Question 17

describe the first reaction in which squalene is cyclised to cholesterol

Answer 17

Squalene is first reduced in the presence of oxygen and NADPH to fm squalene epoxide which has a different C=C bond distribution priming the molecule for carbon ring fusion.
In : NADPH + H+ + O2 — NADP+ + H20
Catalysed by squalene monooxygenase.

Question 18

Describe the second reaction in which squalene is cyclised to cholesterol

Answer 18

The enzyme squalene epoxide lanosterol-cyclase catalyses the formation of Lanosterol. A series of 1,2-methyl group and hydride shifts along the chain of the squalene molecule result in the formation of the four rings.

Question 19

How is cholesterol then produced rom lanosterol

Answer 19

Lanosterol is subsequently reduced and three methyl units removed (demethylated) to generate cholesterol.
Requires 19 steps. HCOOH AND 2CO2 produced.

Question 20

How does cholesterol act as the basis of steroid hormones

Answer 20

The precursor pregnenolone is generated from cholesterol by the action of the enzyme desmolase.

All 5 classes of steroid hormones come from pregnenolone:

Question 21

What are the 5 classes of steroid hormones

Answer 21

Glucocorticoids, Mineralcorticoids, Androgens, Estrogens, Progestagens.

Question 22

Describe the synthesis of Vitamin D from cholesterol

Answer 22

7-Dehydrocholesterol (provitamin D3) iundergoes photolysis by the ultraviolet light of sunlight to previtamin D3, which spontaneously isomerizes to vitamin D3
Vitamin D3 (cholecalciferol) is converted into calcitriol (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.
Calcitriol plays a key role in Calcium metabolism.

Question 23

Describe the synthesis of bile salts from cholesterol

Answer 23

Cholesterol is converted by a series of reactions into the primary bile salt glycocholate and also taurocholate.

Question 24

Describe the role of cholesterol in cell signalling

Answer 24

Lipid rafts are fluctuating assemblies of cholesterol (red) and sphingolipids (dark blue), within a plasma membrane. They organize processes such as cellular signalling by localising key proteins such as cell surface receptors- creates domains for different signal receptors- prevents them from diffusing through the cell membrane.Cholesterol is also covalently attached to the N-terminal fragment of the hedgehog signalling protein (N-Hh) during its processing.
This limits its diffusion within tissues which is key to successful limb formation during embryogenesis.

Question 25

How are fats transported around the body

Answer 25

They are packaged into lipoproteins. Lipoproteins are composed of a phopholipid monolayer containing cholesterol and proteins known as apoproteins such as Apo A-I, Apo B-100, or Apo E.
Packed within the core of the lipoprotein are a mixture of cholesterol esters and triacylglycerols.
The apoproteins can bind to receptor molecules on cells.

Question 26

Describe the synthesis of cholesterolesters.

Answer 26

Cholesterol esters are synthesized in the plasma from cholesterol and the acyl chain of phosphatidylcholine (lecithin) via a reaction catalyzed by lecithin:cholesterol acyltransferase (LCAT). Lysophosphatidylcholine is also produced.
This makes cholesterol esters more hydrophobic than cholesterol and allows them to pack more tightly within the lipoprotein core.
Alternatively, the enzyme Acyl CoA-acyltransferase (ACAT) can generate cholesterol esters from a long chain fatty acyl CoA species. ACAT is an intracellular enzyme and acts on cholesterol taken up by endocytosis.

Question 27

What are the 5 types of lipoproteins and how are they classified

Answer 27

• Chylomicrons (CM)
• Very low density lipoproteins (VLDL)
• Intermediate density lipoproteins (IDL)
• Low density lipoproteins (LDL)
• High density lipoproteins (HDL)
Lipoproteins are categorized according to their density
Each type of lipoprotein has a varying apoprotein component which allows them to be recognized by different cell types.

Question 28

What is the role of each lipoprotein

Answer 28

CM- Dietary fat transport
VLDL- Endogenous fat transport
IDL- precursor for LDL
LDL- Cholesterol transport
HDL- reverse cholesterol transport.

Question 29

Under what conditions do lipoproteins change density

Answer 29

As more and more triglycerides are removed from VLDL theybecomes intermediate-density lipoprotein (IDL).
When their cholesterol content becomes greater than the content of triglyceride, IDL become LDL.

Question 30

Describe the role of lipoprotein lipase

Answer 30

Chylomicrons (CMs) travel from the lacteals of the intestine to the thoracic duct and to the left subclavian vein where they enter the bloodstream.
Subsequently they encounter the enzyme Lipoprotein lipase which is located on the capillary endothelial cells lining a variety of tissues including adipose, heart and skeletal muscle.
Lipoprotein lipase catalyses the hydrolysis of the triacylglycerols in chylomicrons to glycerol and fatty acids.
The apoprotein C-II of the CM plays a key role in activating the lipoprotein lipase
Fatty acids undergo β-oxidation. Glycerol is returned to the liver for use in gluconeogenesis.

Question 31

Describe the role of HDL

Answer 31

are often referred to as “good cholesterol” as they function to take cholesterol from peripheral tissues back to the liver for use or disposal (reverse cholesterol transport). They help to lower total serum cholesterol. (e.g. dying cells, membranes undergoing turnover) and deliver the cholesterol to the liver for excretion.

Question 32

Describe the role of LDL

Answer 32

are often referred to as “bad cholesterol” as prolonged elevation of LDL levels leads to atherosclerosis (hardening of the arteries).
LDLs transport cholesterol synthesized in the liver to peripheral tissues with more than 40% of their weight made up of cholesterol esters.
The role of LDL is to transport cholesterol to peripheral tissues and regulate de novo cholesterol synthesis at these sites.

Question 33

LDL and Familial hypercholesterolaemia

Answer 33

Familial hypercholesterolaemia (FH) is inherited as a monogenic dominant trait.

Individuals who carry a single copy of a mutant gene (heterozygotes) have
cholesterol levels approximately 2-3 times higher than in normal people.
Susceptible to atherosclerosis in middle age.

Homozygotes who carry two copies of a mutant gene are severely affected. Their serum cholesterol levels are five times higher than in healthy individuals and severe atherosclerosis and coronary infarction may be observed in adolescence.

Question 34

Describe the structure of the LDL receptor

Answer 34

Top to bottom:
Ligand binding domain
GF Precursor (EGFP)  homology Domain
O-linked Carbohydrate domain
TM Domain
Cytoplasmic domain

Question 35

Describe the receptor mediated endocytosis of LDL

Answer 35

Binding to LDL receptor
Internalised in clathrin-coated vesicles.
Vesicles lose their coat and fuse with endosomes- in the acidic environment of the endosome, LDL dissociated from its receptors.
LDL ends up in lysosomes where it is degraded to release free cholesterol.
LDL receptors are returned to the plasma membrane via transport vesicles,