Lipoprotein Handling

Question 1

Name the two pathways by which lipoproteins can be formed?

Answer 1

Endogenous (made by liver in the form of VLDL) or exogenous (from dietary sources, transported in chylomicrons post-absorption in the intestines)

Question 2

Describe the endogenous pathway of lipoprotein synthesis

Answer 2

VLDLs transport endogenously synthesised TAG from the liver to the tissues

Question 3

Describe the exogenous pathway of lipoprotein synthesis

Answer 3

Chylomicrons transport dietary lipids, that have been absorbed by the intestine, to the tissues.

Question 4

Describe the general structure of lipoprotein

Answer 4

Cholesterol esters and TAG are located in the hydrophobic core and the outer layer contains phospholipids, free cholesterol and apoproteins

Question 5

When does endogenous fatty acid synthesis usually occur?

Answer 5

In the presence of excess energy intake, where fatty acids are combined with glycerol-3-phosphate to form TAG

Question 6

What is the most abundant form of lipid in the body?

Answer 6

TAG (90% of fatty acids go on to form this)

Question 7

Why does the transport of TAG require modification to become lipoprotein?

Answer 7

TAG is hydrophobic and therefore can’t be transported in aqueous blood so it is packaged into lipoproteins to enable transport

Question 8

What role do apoproteins play in lipoproteins?

Answer 8

Control the fate of the lipoproteins by interacting with cellular receptors as well as playing a role in activating and inhibiting enzymes

Question 9

Name the 5 different types of lipoprotein in order of density, from lowest to highest.

Answer 9

Chylomicron, VLDL, IDL, LDL, HDL

Question 10

What is the main component of chylomicron and what is it’s main apoprotein?

Answer 10

Main component = TAG.

Main apoprotein = B48

Question 11

What is the main component of VLDL and what is it’s main apoprotein?

Answer 11

Main component = TAG

Main apoprotein = B48

Question 12

What is the main component(s) of IDL and what is it’s main apoprotein?

Answer 12

Main components: TAG and cholesterol

Main apoprotein = B100

Question 13

What is the main component of LDL and what is it’s main apoprotein?

Answer 13

Main component: cholesterol

Main apoprotein: B100

Question 14

What is the main component of HDL and what is it’s main apoprotein(s)?

Answer 14

Main component: protein

Main apoproteins: AI, AII

Question 15

What is the role of the B100 apoprotein?

Answer 15

controls the metabolism of LDL

Question 16

What is the role of the B48 apoprotein?

Answer 16

controls chylomicrons

Question 17

What is the role of the ApoE apoprotein?

Answer 17

controls receptor binding of receptor particles

Question 18

What is the role of the ApoC apoprotein?

Answer 18

acts as enzyme inhibitor

Question 19

What is the function of chylomicrons?

Answer 19

Transport dietary TAG

Question 20

What is the function of VLDLs?

Answer 20

Transport TAG from liver to the periphery

Question 21

How are IDLs formed?

Answer 21

From the shrinking of VLDLs into a remnant particle (IDL)

Question 22

How are LDLs formed?

Answer 22

From the conversion of IDL particles to LDL particles in the liver

Question 23

What is the function of LDL?

Answer 23

Mainly contain esterified cholesterol which they deliver to the peripheral tissues

Question 24

What is the function of lipoprotein lipase?

Answer 24

Degrades TAG in chylomicrons and VLDL to release FFA and glycerol for uptake by adjacent cells.

Question 25

Describe lipoprotein lipase activity after a meal

Answer 25

LPL activity high in adipose tissue

Question 26

Describe lipoprotein lipase activity during starvation

Answer 26

LPL activity high in muscle so fatty acids derived from VLDL are mainly fed into oxidation (to release energy)

Question 27

Which enzyme converts IDL to LDL?

Answer 27

hepatic TAG lipase

Question 28

Why may cells take up LDL?

Answer 28

if intracellular cholesterol levels decline (only if reactions to make cholesterol cease)

Question 29

How do cells take up LDL?

Answer 29

LDL (apoB/E) receptor on the plasma membrane of the cell whereby binding causes cholesterol to be released and esterified within the cell

Question 30

Why is cholesterol essential?

Answer 30

Essential part of cell membranes and is a precursor for bile acids and steroids

Question 31

Which transcription factor regulates the expression of HMG-CoA reductase and the LDL receptor?

Answer 31

SREBP

Question 32

How may smoking/diabetes contribute to the formation of atheromatous plaques?

Answer 32

Oxidised LDL (free radical) generated which binds to scavenger receptor instead of LDL receptors in macrophages. Because these scavenger receptors take up cholesterol, and have no negative feedback system, the macrophages become laden with lipid and form foam cells which leads to the formation of fatty streaks in the arterial wall, leading to the formation of a plaque.

Question 33

How do statins work to reduce blood cholesterol levels?

Answer 33

In the liver HMG-CoA reductase is an enzyme involved in cholesterol sythesis which is packaged into VLDL, statins inhibit this enzyme to reduce blood cholesterol levels

Question 34

How does cholestryamine work to reduce blood cholesterol levels?

Answer 34

Binds bile acids in the gut to prevent enterohepatic circulation

Question 35

Where are HDLs formed?

Answer 35

Liver and intestine

Question 36

What is the role of HDL?

Answer 36

Scavenges free cholesterol from cell membranes via ABCA1 membrane transporter and esterifies this cholesterol. In addition, HDL transports cholesterol from the peripheral tissues to the liver for disposal as well as binding to scavenger receptors to transfer cholesterol to the cell membrane.