cholesterol

Question 1

describe the structure and basic properties of cholesterol.

Answer 1

steroid

27 carbon atoms

cyclic rings with hydrophobic tail

planar

very hydrophobic (only composed of carbon and hydrogen atoms) apart from -OH group at position 3

Question 2

cholesterol is a vital component of what and why is this?

Answer 2

cell membranes

can increase and decrease membrane stiffness depending on temperature and nature of the membrane

Question 3

what is dietary cholesterol uptake in humans limited to (per day)?

Answer 3

500mg

Question 4

given the need for cholesterol as a membrane component, how are physiological requirements for cholesterol supplied?

Answer 4

by liver through synthesis of cholesterol from acetyl CoA

Question 5

briefly outline the 3 steps of cholesterol synthesis from acetyl CoA.

Answer 5

1- synthesis of isopentenyl pyrophosphate (cytoplasm)

2- condensation of 6 molecules of isopentenyl pyrophosphate to form squalene (cytoplasmic reactions)

3- cyclisation and demethylation of squalene by monoxygenases to give cholesterol (ER reaction)

Question 6

describe the first step of cholesterol synthesis.

Answer 6

synthesis of building block isopentenyl pyrophosphate (cytoplasm)

1- two molecules of acetyl CoA condense to form acetoacyl CoA (4C), catalysed by β-ketothiolase

2- acetoacyl CoA and another acetyl CoA molecule condense to form 3-hydroxy-3-methyl glutaryl CoA (HMG-CoA), catalysed by HMG-CoA synthase

3- HMG-CoA is reduced to mevalonate by HMG-CoA reductase, using NADPH for reducing power
HMG-CoA reductase is under intense negative feedback control by cholesterol (end product of pathway), bile salts (generated from cholesterol) and mevalonate

4- mevalonate undergoes sequential phosporylation at hydroxyl groups at position 3 (1 phosphate group) and 5 (2 phosphate groups) (requires ATP)

5- the phosphorylated mevalonate is decarboxylated to form 3-isopentenyl pyrophosphate -building block for cholesterol synthesis

Question 7

describe the second step of cholesterol synthesis.

Answer 7

condensation of 6 isopentenyl pyrophosphate to form squalene (cytoplasm)

1- dimethylalleyl pyrophosphate produced from isopentyl pyrophosphate via an isomerisation reaction

2- dimethylallyl pyrophosphate and another isopentenyl pyrophosphate molecule undergo condensation to form geranyl pyrophosphate (geranyl transferase enzyme)

3- geranyl pyrophosphate and another isopentenyl pyrophosphate molecule undergo condensation to form 15C farnesyl pyrophosphate (geranyl transferase enzyme)

4- 2 molecules of farnesyl pyrophosphate condense to form squalene (30C) and 2 pyrophosphate molecules (squalene synthetase enzyme, NADPH)

Question 8

describe the third step of cholesterol synthesis.

Answer 8

cyclisation and demethylation of squalene by monooxygenases (ER)

1- squalene reduced to squalene epoxide in the presence of NADPH and oxygen
squaline epoxide has a different C=C bond distribution, priming the molecule for carbon ring fusion

2- squalene epoxide forms lanosterol (catalysed by squalene epoxide lanosterol cyclase)

3- a series of 1,2-methyl group and hydride shifts along the chain of of the squalene molecule results in the formation of the 4 rings

4- lanosterol is reduced and 3 methyl units removed to generate cholesterol (19 step process)

Question 9

what was required in this biosynthetic pathway (cholesterol synthesis)?

Answer 9

ATP for phosphorylation of mevalonate

NADPH as reducing power

Question 10

give 3 examples of things that cholesterol forms the basic building blocks for.

Answer 10

bile salts

steroid hormones

vitamin D

Question 11

what are bile salts key for and what properties do they have that enable them to do this?

Answer 11

emulsifying dietary fats

have hydrophobic and hydrophilic faces

Question 12

what is the relationship between cholesterol and bile salts?

Answer 12

bile salts are the major breakdown product of cholesterol

cholesterol can be converted by a series of reactions into the primary bile salt glycocholate and also taurocholate

glycocholate and taurocholate have OH groups which give them hydrophilic properties

Question 13

what is the relationship between cholesterol and steroid hormones?

Answer 13

precursor (pregnenolone) generated from cholesterol through action of desmolase enzyme

all 5 classes of steroid hormones come from pregnenolone

• progestagens
• glucocorticoids (derived from progestagens)
• mineralocorticoids (derived from progestagens)
• androgens (derived from progestagens)
• estrogens (derived from androgens)

Question 14

what is vitamin D?

Answer 14

collective term for a group of steroids which are vital for the intestinal absorption of ions needed for bone development (i.e calcium, magnesium and phosphate)

Question 15

what is our main source of vitamin D in the western world?

Answer 15

activity of UV light on 7-dehydrocholesterol in the epidermis

bulk of western diet is low in vitamin D

Question 16

what is calcitriol?

Answer 16

most active vitamin D metabolite

key role in calcium metabolism

Question 17

how does calcitriol work?

Answer 17

functions as a steroid hormone

binds to vitamin D response elements (VDREs) in the promoter of target genes

this induces key genes in bone metabolism

Question 18

what does a deficiency of Vitamin D₃ in childhood lead to?

Answer 18

rickets

defect of bone development in children

Question 19

how does the action of UV light on 7-dehydrocholesterol in the epidermis produce calcitriol?

Answer 19

7-dehydrocholesterol —UV light—> previtamin D₃ ——–> vitamin D₃ ——–> calcitriol

Question 20

what is familial hypercholesterolaemia (FH)?

Answer 20

monogenic dominant trait

defective cholesterol transportation

Question 21

what symptoms do heterozygotes with the FH gene have?

Answer 21

cholesterol levels 2-3x higher than normal

susceptible to atherosclerosis in middle age

Question 22

what symptoms do homozygotes with the FH gene have?

Answer 22

severely affected

serum cholesterol levels 5x higher than normal

severe atherosclerosis and coronary infarction may be observed in adolescence

Question 23

what physical signs of homozygous FH may be present and why?

Answer 23

orange-yellow xanthomas superfically on the knees, wrists and hands

arise from scavenging of plasma LDL-derived cholesterol by macrophages of the skin

Question 24

why are homozygotes with the FH gene at risk of coronary infarction?

Answer 24

deposition of LDL-derived cholesterol in coronary arteries

occlusion occurs

risk of myocardial infarction

Question 25

what are the mechanisms responsible for FH?

Answer 25

cholesterol in the form of LDL is taken up by a specific receptor molecule on the cell surface (LDL receptor/LDLR)

fibroblasts lack functional LDLRs - mutations can occur in several domains (ligand binding, EGFP homology, O-linked carbohydrate, TM, cytoplasmic)

either receptor expression, LDL binding or LDLR endocytosis and recycling is affected by mutations, manifesting in FH

many LDLR mutations are documented

Question 26

what are the 2 main strategies to control hypercholesterolaemia?

Answer 26

inhibition of de novo cholesterol synthesis by the liver (HMG-CoA reductase inhibitors)

reduction of dietary cholesterol absorption by the intestines (resins)

Question 27

how do resins work to prevent hypercholesterolaemia? give an example of a resin.

Answer 27

reduce cholesterol absorption by intestines

e.g cholestyramine (brand names: Questran, Prevalite)

bind or sequester bile acid-cholesterol complexes, preventing their reabsorption

can lower LDL by 15-30%

can raise HDL by 3-5%

Question 28

how do HMG-CoA-Reductase inhibitors work to prevent hypercholesterolaemia? give an example of an HMG-CoA-Reductase inhibitor.

Answer 28

(also known as statins)

inhibit de novo cholesterol synthesis by the liver

e.g Lipitor, Crestor

lovastatin molecules act as a competitive inhibitor of HMG-CoA-Reductase

the OH/COO⁻/OH area resembles the enzyme substrate 3-HMG